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Glossary
1a Afferent fiber
An afferent fiber is the axon of an afferent neuron. 1a afferent fibers are the largest sensory axons in vertebrate peripheral nerves.
A band
In striated muscle, the region of a sarcomere that spans the length of the thick filaments. It includes the H zone and regions on both ends where thick and thin filaments overlap. It appears dark in muscle sections prepared for microscopy. It takes its name from the fact that it is described as anisotropic because its refractive index in polarized light changes with the plane of polarization.
Absolute refractory period
In, e.g., a neuron, the time during and immediately after an action potential in which the voltage threshold is infinite. Thus, no depolarization can exceed threshold and no new action potentials can be initiated.
Absorbed chemical energy
See absorbed energy.
Absorbed energy
In the study of nutritional physiology, the chemical-bond energy of the compounds that an animal absorbs (assimilates) from its digestive tract. Also called assimilated energy.
Absorption
In the study of nutritional physiology, the entry of organic molecules into the living tissues of an animal from outside those tissues. Absorption includes the entry of materials from the lumen of the gut inasmuch as the lumen is continuous with the outside environment and therefore outside the animal. Also called assimilation.
Absorption coefficient
The dissolved concentration of a gas when the partial pressure of that gas in solution is 1 atm. See also Henry’s law.
Absorption efficiency
In the study of nutritional physiology, the fraction of molecules or the fraction of chemical-bond energy absorbed from the digestive tract expressed in relation to the amount ingested. Also called assimilation efficiency.
Absorption spectrum
The absorption of electromagnetic energy by a molecule or other object as a function of the wavelength of the energy.
Accessory heart
In an animal with two or more hearts, a heart other than the primary heart. Also called auxiliary heart.
Acclimation
A chronic response of an individual to a changed environment in cases in which the old and new environments differ in just one or two highly defined ways. A form of phenotypic plasticity. Acclimation is a laboratory phenomenon.
Acclimatization
A chronic response of an individual to a changed environment in cases in which the new and old environments are natural environments that can differ in numerous ways, such as winter and summer environments, or low and high altitudes. A form of phenotypic plasticity.
Acidosis
A state in which the pH of the body fluids is excessively acid.
Acoustico-lateralis system
A vertebrate sensory system in which the sensory receptors are hair cells and their derivatives. Includes auditory receptors, organs of balance and gravity detection, and the lateral line system of fish and amphibians.
Acrosomal reaction
The release of enzymes from the acrosome in the head of a sperm; occurs when the cell membrane of the sperm head binds to species-specific receptor sites on the zona pellucida surrounding the oocyte.
Actin
One of the contractile proteins of muscle cells. Globular G-actin monomers polymerize to form the filamentous F-actin of the thin myofilaments. Actin also contributes to motility in many other kinds of cells.
Action potential
A brief electrical signal of about 100 mV across the cell membrane of a neuron or other excitable cell. It is initiated by a depolarization above threshold and is propagated to the end of the axon or cell. Also called a nerve impulse.
Activation energy
The minimal amount of energy a molecule must gain to enter its transition state during a chemical reaction. Without entering its transition state, a molecule cannot react. Enzymes speed reactions by lowering the activation energy, making achievement of the transition state more likely.
Active (exhalation)
Exhalation driven by muscle action.
Active (ventilation)
See active change in lung volume.
Active change in lung volume
In the study of ventilation, a change in lung volume driven by muscle action. Contrast with passive change in lung volume.
Active evaporative cooling
Evaporative cooling that occurs because evaporation is accelerated by some physiological process other than the simple production of heat, such as panting or sweating. See also evaporative cooling.
Active site
A specific region of an enzyme molecule, at or near the surface of the molecule, where the enzyme binds with its substrate and where the conversion of substrate to product is catalyzed. Also called a substrate-binding site.
Active solute secretion
Solute secretion by active transport. See active transport.
Active transport
The transport of a solute across a membrane by a mechanism that is capable of using metabolic energy to cause solute molecules to move across the membrane. An active-transport mechanism is capable of making a solute move away from equilibrium (against its electrochemical gradient). Also called uphill transport.
Active zone
The region of a presynaptic ending where neurotransmitter molecules are released.
Acute response
A response exhibited by an animal during the first minutes or hours after an environmental change.
Adaptation
(1) In evolution, a genetically controlled trait that, through the process of natural selection, has come to be present at high frequency in a population because it confers a greater probability of survival and successful reproduction in the prevailing environment than available alternative states. (2) In a sensory receptor, a decrease in the frequency of action potentials in response to a stimulus, during prolonged exposure to the stimulus, even though the stimulus is maintained at a constant level. Tonic receptors adapt slowly, and phasic receptors adapt rapidly.
Adenohypophysis
The nonneural endocrine portion of the vertebrate pituitary gland. It is commonly called the anterior pituitary and includes three parts with variable representation in different species: the pars tuberalis, pars intermedia, and pars distalis.
Adequate stimulus
The kind of stimulus energy to which a receptor is most sensitive, or the kind of stimulus to which it normally responds.
Adipocyte
An animal cell specialized for the storage of fats or oils.
Adrenergic neuron
A neuron that synthesizes and releases norepinenphrine (noradrenaline) or epinephrine (adrenaline) as a neurotransmitter.
Adrenocorticotropic hormone (ACTH)
A tropic peptide hormone secreted by cells in the anterior pituitary gland (adenohypophysis); circulates in the general circulation and controls secretions of all three layers of the adrenal cortex.
Aerobic
Requiring O2.
Aerobic capacity
A synonym for V̇O2max.
Aerobic catabolic pathways
Catabolic pathways requiring O2. Usually refers to the Krebs cycle and electron transport.
Aerobic dive limit (ADL)
In diving mammals and birds, the length of the longest dive that can be undertaken without net accumulation of lactic acid above the resting level.
Aerobic expansibility
An animal’s peak rate of O2 consumption during locomotion expressed as a ratio of its resting rate of O2 consumption. Sometimes used to refer to the same calculation as aerobic scope.
Aerobic scope for activity
The difference between an animal’s peak rate of O2 consumption during locomotion and its resting rate of O2 consumption. Sometimes used to refer to the same calculation as aerobic expansibility.
Afferent
Going toward. Thus, for example, an afferent blood vessel carries blood toward an organ of interest. Contrast with efferent.
Afferent neuron
A neuron, normally sensory, that conducts signals from the periphery into the central nervous system.
Affinity
See enzyme–substrate affinity, oxygen affinity.
Aglomerular
Lacking a glomerulus.
Air capillaries
Minute, gas-filled, blind-ended channels that branch off from the principal respiratory tubules, the parabronchi, within the lungs of a bird and that act as the primary sites of respiratory gas exchange between the air and blood.
Air sacs
Expanded, gas-filled chambers connected to the breathing system in birds or insects. In birds, the air sacs (of which there are usually nine) act as bellows for ventilating the lungs.
Akt1
A signal transduction molecule that stimulates cells to survive and grow; activated by phosphatidylinositol 3-kinase (PI-3 K) when PI-3 K is activated by an extracellular signal. Also called protein kinase B.
Aldosterone
A hormone of vertebrates, secreted by the adrenal cortex or homologous interrenal tissue, that increases Na+ retention by the kidneys and other organs.
Alkalosis
A state in which the pH of the body fluids is excessively alkaline.
All-or-none
Occurring fully or not at all; not graded.
Allometric equation
Two variables, X and Y, are related in an allometric manner when Y = aXb (b 1). See Appendix F. Also called the power equation.
Allosteric activation
Activation of an enzyme by allosteric modulation. See allosteric modulation.
Allosteric inhibition
Inhibition of an enzyme by allosteric modulation. See allosteric modulation.
Allosteric modulation
(1) In relation to enzymes, modulation of the catalytic properties of an enzyme by the binding of nonsubstrate ligands to specific nonsubstrate-binding sites, called regulatory sites or allosteric sites; a type of cooperativity, commonly important in the regulation of enzyme-catalyzed reactions and pathways. (2) In relation to respiratory pigments, modulation of O2 binding by the binding of ligands other than O2. See also cooperativity.
Allosteric modulator
(1) In relation to enzymes, a nonsubstrate ligand of an enzyme that modulates the catalytic activity of the enzyme by binding to a specific regulatory site on the enzyme molecule. (2) In relation to respiratory pigments, a ligand other than O2 that modulates binding with O2.
Allosteric site
Alpha (α) motor neuron
A relatively large motor neuron in a vertebrate spinal cord that innervates extrafusal muscle fibers.
α–γ coactivation
Synaptic activation of α and γ motor neurons together, so that a centrally commanded contraction of a muscle does not inactivate muscle spindle receptors.
Alphastat hypothesis
The hypothesis that in poikilotherms the pH of body fluids is typically regulated at a relatively fixed difference from neutral pH—even as the body temperature changes and the neutral pH therefore also changes—as a way of maintaining a relatively constant state of electrical charge on proteins (particularly on protein imidazole groups).
Alveolar sacs
In the lungs of a mammal, the finest airways, which are lined with alveoli and end blindly.
Alveolar ventilation rate
In the lungs of a mammal, the rate at which air is brought into the alveoli and other respiratory airways (collectively) of the lungs during breathing.
Alveoli (singular alveolus)
(1) In the lungs of vertebrates, the blind-ended terminations of the branchings of the respiratory tract that form the surface for exchange of gases between the air and the blood. In mammalian lungs, each alveolus is a semispherical outpocketing measuring less than 0.5 mm in diameter. (2) In the mammary glands of mammals, the hollow glandular structures that secrete milk.
Amacrine cell
A retinal neuron that mediates lateral antagonistic effects and is sensitive to visual movements.
Ambient
Relating to the surroundings of an organism. Synonymous with environmental; for instance, the ambient temperature is the environmental temperature.
Amine hormones
Chemical signals derived from the amino acids tyrosine (catecholamines and iodothyronines) or tryptophan (melatonin).
Ammonotelic
Incorporating most nitrogen from the catabolism of nitrogenous compounds into ammonia. “Most” is defined differently by different authorities; a common approach is to categorize an animal as ammonotelic if 50% or more of the nitrogen released by catabolism is incorporated into ammonia.
Amniotic egg
A vertebrate egg in which the developing embryo is enclosed in the amnion, a sac (extra-embryonic membrane) that contains a bathing amniotic fluid. The amnion evolved and develops in conjunction with other extra-embryonic membranes that support the embryo, notably the chorion, allantois, and yolk sac. The internally developing eggs of mammals are amniotic eggs, as are the cleidoic eggs of birds and other reptiles. See also cleidoic egg.
Amphipathic molecule
A molecule that consists of a polar subpart and a nonpolar subpart.
Amplification
In a sequence of enzyme-catalyzed biochemical reactions, an increase in the number of reacting molecules resulting from the fact that one enzyme molecule can catalyze the formation of more than one product molecule.
Amylase
A digestive enzyme that breaks down starch or glycogen.
Anabolism
Constructive metabolism; the set of metabolic processes that build relatively large molecules from smaller molecular building blocks using energy.
Anadromous
Relating to an aquatic animal that undergoes most of its growth in seawater but enters freshwater to breed.
Anaerobe
An anaerobic organism. See anaerobic.
Anaerobic
Capable of functioning without O2.
Anaerobic glycolysis
The reactions that convert glucose to lactic acid.
Anastomose
To form an interconnected network of tubules, vessels, or similar structures by patterns of branching, reconnection, and rebranching.
Anatomical dead space
In the lungs of an animal, the sum total of the conducting airways; that is, that portion of the lungs that does not participate in the exchange of respiratory gases between air and blood.
Androgen
A masculinizing hormone such as testosterone.
Angiogenesis
The formation of new blood vessels (e.g., new capillaries) by sprouting of branches from existing vessels.
Angiotensin converting enzyme (ACE)
A membrane-bound enzyme produced by vascular endothelial cells that converts inactive angiotensin I to angiotensin II.
Angiotensin I
Formed by the action of renin, which catalyzes the cleavage of amino acids from angiotensinogen; an inactive component of the renin–angiotensin–aldosterone system.
Angiotensin II
Formed by the action of angiotensin-converting enzyme; an active component of the renin–angiotensin–aldosterone system; stimulates secretion of aldosterone from the adrenal cortex and vasopressin from the posterior pituitary, and also promotes thirst and systemic vasoconstriction
Angiotensinogen
A protein secreted by liver cells that is continuously present in the blood; the first molecule in the renin–angiotensin–aldosterone system.
Anhydrobiosis
Survival while dried as fully as possible by desiccation in air.
Animal physiology
The study of the functional properties of animals; the study of “how animals work.”
Anion
A negatively charged ion.
Annotation
In the study of genomics, the process of adding interpretive information to gene identities. For example, if a genomic analysis indicates the likely presence of a particular gene that is known in some organisms to code for a detoxification enzyme, stating that the gene is “involved in detoxification” would be an act of annotation.
Anoxia
In relation to the tissues of an animal, the state of being devoid of O2.
Antagonism
A type of influence of one substance (such as a hormone) in relation to another in which one opposes the action of the other on a target tissue.
Antagonist
An opponent. In biochemical reactions, a substance that opposes the action of another substance. See also antagonistic muscle pair.
Antagonistic muscle pair
Two muscles, or groups of muscles, that perform coordinated, opposing actions. When one muscle contracts to close the angle of a joint, its antagonist relaxes. To open the joint, the formerly relaxed muscle contracts and the formerly contracted muscle relaxes.
Antennal gland
The urine-producing structure of a decapod crustacean. Also called a green gland.
Anterior pituitary
The nonneural endocrine portion of the vertebrate pituitary gland. It includes three parts with variable representation in different species: the pars tuberalis, pars intermedia, and pars distalis. Also called the adenohypophysis.
Anti-Müllerian hormone
See Müllerian inhibitory substance.
Antidiuresis
The production of urine at a low rate. Such urine is usually concentrated as well as scanty.
Antidiuretic
Opposing diuresis; promoting antidiuresis.
Antidiuretic hormone (ADH)
A hormone that promotes antidiuresis. In mammals, also called vasopressin.
Antifreeze compound
A metabolically synthesized compound, added to a body fluid, that has the principal function of lowering the freezing point of that body fluid.
Antifreeze protein
A protein or glycoprotein that acts as an antifreeze compound. See antifreeze compound.
Antioxidants
A collective term referring to molecules that prevent or delay damaging oxidation of macromolecules (or other molecules) involved in normal function by reactive oxygen species. See also reactive oxygen species.
Antiporter
See countertransporter.
Aortic body
In mammals, a small organ juxtaposed to the systemic aortic arch that senses O2 in the arterial blood. Not found in humans.
Apical membrane
In an epithelial cell, the part of the cell membrane that faces toward the lumen or open space lined by the epithelium of which the cell is a part.
Apical surface
In an epithelium, the surface that faces toward the lumen or open space lined by the epithelium. Also called the mucosal surface.
Apnea
The absence of breathing.
Aquaporin
A chemically defined type of water channel, often highly specific for water as opposed to solutes. See also water channel.
Arachnid
A spider, tick, scorpion, or mite.
Archimedes’ principle
The principle that states that a body immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced.
Aromatase
The enzyme that converts androgens such as testosterone to estrogens such as estradiol.
Aromatization
The process of converting androgens such as testosterone to estrogens such as estradiol, catalyzed by aromatase.
Arousal
(1) In the study of hibernation, the emergence of an animal from hibernation, involving the rewarming of tissues to ordinary homeothermic temperatures. (2) In the study of sleep, the transition from sleep to wakefulness.
Arousal threshold
n the study of sleep, a measure of the difficulty of waking a sleeping individual at a particular time; a low threshold signifies that sleep terminates relatively easily.
Arteriole
A microscopically fine blood vessel with muscular walls that carries blood from arteries to capillaries in a microcirculatory bed of a vertebrate. Contraction and relaxation of the muscular walls controls the rate of blood flow to the capillaries supplied by the arteriole.
Artery
A macroscopic blood vessel that carries blood away from the heart.
Asexual reproduction
The formation of new individuals without the union of gametes from two different parents. The offspring are genetically identical to their parent.
Assimilated chemical energy
In the study of nutritional physiology, the chemical-bond energy of the compounds that an animal assimilates (absorbs) from its digestive tract. Also called absorbed energy.
Assimilation
In the study of nutritional physiology, synonymous with absorption.
Astrocyte
A type of glial cell (non-neuronal cell) of the vertebrate central nervous system that regulates extracellular ion concentrations and metabolically supports neurons. Important in the blood–brain barrier.
Atmosphere
A unit of measure for pressure. An atmosphere or, more technically speaking, a normal atmosphere is the average pressure exerted by Earth’s atmosphere at sea level. It is quantitatively specified by international agreement to be 1.01325 × 105 pascal at 15°C. It is also specified to be equal to the pressure exerted by a column of mercury 760 mm high under standard gravitational acceleration.
ATP synthase
A mitochondrial enzyme that synthesizes ATP using energy drawn from the mitochondrial proton electrochemical gradient.
ATPase
An enzyme capable of hydrolyzing adenosine triphosphate (ATP) and thereby releasing energy from ATP.
Atresia
In the ovary, the process by which an ovum or follicle stops developing and degenerates.
Atrial natriuretic peptide
A member of a family of peptide hormones secreted by specialized cells in the atria of the heart that stimulate the excretion of water and sodium.
Atrioventricular (A-V) node
In the mammaliam heart, a cluster of modified atrial muscle cells that initiate the relay of depolarization from the atria to the ventricles.
Atrioventricular bundle
A subpart of the conducting system in which depolarization spreads throughout the vertebrate heart. See conducting system.
Atrium
In the study of hearts, a heart chamber; in a mammalian heart, one of the two weakly muscular chambers into which blood flows as it enters the heart. In the book lungs of arachnids, the chamber into which the multiple page-like lamellae project.
Atrophy
A decrease in the mass of a tissue or organ by the loss of cells or of intracellular components of cells. Also called wasting.
Audition
Hearing; sound detection and perception.
Autocrine
A locally acting chemical signal that binds to receptors and exerts a regulatory effect on the same cell that secreted it.
Autonomic effector
An effector other than skeletal muscle; includes smooth and cardiac muscles and tissues of the viscera and exocrine glands.
Autonomic ganglia
A collection of peripheral (postganglionic) neurons in the autonomic nervous system; preganglionic neurons synapse onto these postganglionic neurons, which control autonomic effectors.
Autonomic nervous system (ANS)
The division of the nervous system that innervates and controls autonomic effectors and conveys sensory information from internal organs.
Autoreceptor
In synaptic processes, a receptor protein on a presynaptic terminal that is stimulated by neurotransmitter released by the same neuron.
Autotroph
An organism that can obtain the energy it needs to stay alive from sources other than the chemical bonds of organic compounds. Some autotrophs (photoautotrophs) use photon energy; others (chemoautotrophs) use energy from inorganic chemical reactions. Contrast with heterotroph. See also primary production.
Autoventilation
In an insect, ventilation of the treachae (breathing tubules) of the flight muscles by the flight-muscle contractions.
Auxiliary heart
See accessory heart.
Average daily metabolic rate (ADMR)
An animal’s rate of metabolism averaged over all hours of the 24-hour day.
Avogadro’s number
The number of molecules in a mole; also, the number of independent dissolved entities in an osmole. Equal to 6.022 × 1023.
Axis
In the study of endocrinology, two or more hormone-secreting tissues that together form an hierarchical control system.
Axodendritic synapse
A synapse of an axon onto the dendrite of another neuron.
Axon
A process of a neuron specialized for conveying action potentials (usually) away from the cell body. An axon may be myelinated (ensheathed in myelin) or unmyelinated.
Axon hillock
The region of a neuronal cell body from which an axon originates; often the site of impulse initiation.
Axon initial segment
The region of an axon close to the cell body: often the site if impulse initiation.
Axonal transport
The transport of materials within the cytoplasm of an axon; may be either anterograde (away from the cell body) or retrograde (toward the cell body).
Axosomatic synapse
A synapse of an axon onto the soma of another neuron.
Baculum
A rodlike structure of cartilage or bone that gives stiffness to the shaft of the penis of some species of mammals.
Basal ganglia
In the vertebrate brain, a collection of cell groups in the ventral forebrain that organize motor behavior. The caudate nucleus, putamen, and globus pallidus are major components of the basal ganglia; the subthalamic nucleus and substantia nigra (midbrain) are usually included.
Basal lamina
See basement membrane.
Basal membrane
In an epithelial cell, the part of the cell membrane that faces toward the underlying tissue on which the epithelium rests or to which the epithelium is attached.
Basal metabolic rate (BMR)
The metabolic rate of a homeothermic animal when it is in its thermoneutral zone, resting and fasting.
Basal surface
In an epithelium, the surface that faces toward the underlying tissue on which the epithelium rests or to which the epithelium is attached. Also called the serosal surface.
Basement membrane
A thin, permeable, noncellular, and nonliving sheet of matrix material on which an epithelium rests, or that surrounds a cell such as a muscle or fat cell. It is composed of glycoproteins and particular types of collagen and is secreted by cells; for example, the cells of an epithelium help secrete the basement membrane on which the epithelium rests. Also called a basal lamina.
Basilar membrane
A membranous tissue within the cochlea of the vertebrate ear that contains the auditory sensory hair cells and is vibrated by sound waves.
Basolateral membrane
In an epithelial cell, the basal cell membrane plus the lateral cell membranes between the basal membrane and the ring of tight junctions, in contradistinction to the apical membrane. See basal membrane.
Behavioral fever
In a poikilotherm, a modification of behavioral thermoregulation caused by bacterial infection, such that the behaviorally regulated body temperature is higher than usual.
Behavioral thermoregulation
(1) In poikilotherms, the maintenance of a relatively constant body temperature by behavioral means. (2) In homeotherms, the use of behaviors to assist in the maintenance of a relatively constant body temperature.
Bends
Bile salt
A type of emulsifying compound, produced by the biliary system in the liver of a vertebrate, that plays a key role in lipid digestion and absorption because of its ability to emulsify lipids within the gut lumen.
Biliary system
The system in the liver of a vertebrate that produces bile-containing bile salts.
Bimodal breather
See dual breather.
Biological clock
A physiological mechanism that gives an organism an endogenous capability to keep track of the passage of time.
Bioluminescence
Biochemical production of light by cells.
Bipolar cell
A type of neuron in the vertebrate retina that mediates the direct (straight-through) pathway connecting rods and cones to ganglion cells.
Bivalve mollusc
A clam, mussel, scallop, oyster, or related animal.
Black-body temperature
See radiant temperature.
Bladder
In the study of urine production, an expanded chamber where urine is temporarily stored prior to being excreted. The bladder may exert no effect on the urine other than storing it (e.g., mammals) or may modify urine composition and volume (e.g., amphibians).
Blastocyst
A hollow sphere of cells formed in early mammalian development that implants in the uterus. Its inner cell mass will form the embryo and extra-embryonic structures, and its outer trophoblast cells will contribute to formation of the placenta.
Blood
The fluid (including suspended cells) that is circulated within the circulatory system.
Blood plasma
The part of the blood that remains after blood cells are removed; the part of the blood other than cells.
Blood pressure
The extent to which the pressure of the blood exceeds the ambient pressure.
Blood–brain barrier
Phenomenologically, a tendency for many substances to exchange much less readily between the blood plasma and the extracellular tissue fluids in the brain than such exchanges in most other tissues. Structurally, a specialized morphology of blood capillaries and glial cells in the brain that interferes with passive transport between blood and extracellular tissue fluid.
Bohr effect
A decrease in the O2 affinity of a respiratory pigment (a shift of the oxygen equilibrium curve to the right) caused by a decrease in pH or an increase in CO2 partial pressure.
Bohr shift
Synonym for Bohr effect.
Bomb calorimeter
A device used to measure the energy content of organic materials.
Bone marrow
In vertebrates, spongy tissue within certain bones, responsible for producing red blood cells.
Book gills
Unusual gill structures consisting of many sheets of tissue arrayed like pages of a book; found in horseshoe crabs.
Book lungs
Lungs within which sheets of gas-exchange tissue alternate with sheetlike air spaces, like pages of a book slightly separated by air. Found in spiders, scorpions, and some other arachnids.
Bowman’s capsule
In a vertebrate kidney, the invaginated, blind end of a nephron; the site of formation of primary urine.
Brackish water
Water that is intermediate in salinity between seawater and freshwater. One criterion sometimes used is that water is considered brackish if its salinity is between 0.5 and 30 g/kg.
Bradycardia
A heart rate that is unusually low.
Brain
The anterior enlargement of the central nervous system in an animal with a cephalized nervous system.
Branchial
Relating to gills.
Breathing
The process by which an animal gains O2 from its environment and voids CO2 into its environment. The environment may be air (air breathing) or water (water breathing).
Brown adipose tissue (BAT)
A metabolically active form of adipose tissue, known only in mammals, that serves as the primary site of mammalian nonshivering thermogenesis and expresses a distinctive mitochondrial protein, uncoupling protein 1 (UCP1). Also called brown fat. Contrast with white adipose tissue.
Brown fat
Synonym for brown adipose tissue.
Brush border
An apical epithelial surface bearing microvilli.
Buccal pressure pump
The development of positive pressure within the buccal cavity of a vertebrate, used to force air into lungs or water across gills.
Buffer reaction
A chemical reaction in a solution that tends to stabilize the pH of the solution by removing H+ from the solution when H+ is added by some external process and by adding H+ to the solution when H+ is removed by an external process.
Bulk flow
Gross flow of a fluid (gas or liquid), such as occurs in a wind or water current.
Bulk solution
Solution that is not immediately next to a membrane or other surface.
Bundle branches
Subparts of the conducting system in which depolarization spreads throughout the vertebrate heart. See conducting system.
Burst exercise
Sudden, intense exercise.
Ca2+-ATPase
An ATPase enzyme that pumps Ca2+ across a membrane. See ATPase.
Caching behavior
The storage of food or other material for later use.
Caisson disease
Calmodulin (CaM)
An intracellular protein that binds calcium ions to form a Ca–CaM complex, which can then act as an intracellular messenger to affect other proteins.
Calorie (cal)
A unit of energy equal to the amount of heat required to raise the temperature of 1 g of water by 1°C, from 14.5°C to 15.5°C. Some people outside science distinguish the calorie from the kilocalorie (kcal; 1000 calories) by writing “calorie” (small “c”) to represent the calorie and “Calorie” (large “c”) to represent the kilocalorie. Sometimes, in fact, in writing outside science, “calorie” (small “c”) is used to refer to the kilocalorie; thus one must be cautious in interpreting uses of “calorie” outside science.
Calorigenic effect of ingested food
Synonym for specific dynamic action.
Camera eye
An eye that optically resembles a camera, with a lens that focuses an image on a retina of light-sensitive cells.
cAMP
See cyclic adenosine monophosphate.
Capacitance
An electrical term meaning the ability of a capacitor or a capacitor-like structure, such as a cell membrane, to store electrical charges. A cell membrane acts like a capacitor because of its electrically insulating properties. Capacitance (C, in farads) is a measure of the amount of charge stored per unit of voltage. See capacitor.
Capacitance coefficient (β)
In the study of respiratory gas exchange, the change in total gas concentration per unit of change in gas partial pressure in air, water, or a body fluid like blood. The capacitance coefficient sometimes differs from simple gas-solubility coefficients such as the absorption coefficient because in the case of the capacitance coefficient, the gas concentration includes bound gases, such as O2 bound to hemoglobin.
Capacitation
The final maturation of sperm that takes place in the female reproductive tract to make the sperm capable of rapid forward swimming and fertilization.
Capacitor
Two conducting plates separated by an insulating layer. If the plates are close enough to each other, charges on one plate can electrostatically attract or repel charges on the other plate, even if charges cannot cross the insulation. A capacitor can store electrical charge.
Capillary
A microscopically fine blood vessel, the wall of which consists of only a single layer of epithelial cells. Capillaries are the principal sites of exchange between blood and other tissues in a closed circulatory system.
Carbamate groups
Chemical structures formed when CO2 reversibly combines with amino groups of hemoglobin or other proteins.
Carbon dioxide dissociation curve
See carbon dioxide equilibrium curve.
Carbon dioxide equilibrium curve
In relation to the CO2-carrying properties of blood, a graph of the total carbon dioxide concentration as a function of the CO2 partial pressure of the blood. Also called the carbon dioxide dissociation curve.
Carbonic anhydrase (CA)
An enzyme that accelerates the conversion of CO2 and H2O into HCO3– and H+, or the reverse reaction.
Cardiac ganglion
In arthropods, a ganglion, located in the heart, that is composed of the pacemaker neurons and other neurons that stimulate rhythmic contractions of the heart muscle cells.
Cardiac muscle
Muscle that forms the wall of the heart. In vertebrates, cardiac muscle consists of branched, generally uninucleate, striated muscle cells that are connected by intercalated discs. The contractions of the cells are initiated by endogenously generated myogenic action potentials and may be modified by neural and hormonal factors. The cardiac muscle of invertebrates may consist of striated or nonstriated cells that may be endogenously active (myogenic) or controlled by neural input (neurogenic).
Cardiac output
The volume of blood pumped by a heart per unit of time, calculated as the stroke volume multiplied by the number of beats per unit of time (heart rate). In the case of the mammalian or avian heart, the cardiac output is specifically the output of the left ventricle into the systemic aorta unless stated otherwise.
Carotid bodies
In mammals, two small organs juxtaposed to the carotid arteries that sense O2 in the arterial blood.
Carrier
See transporter.
Carrier-mediated transport
Solute transport across a membrane that requires the reversible, noncovalent binding of solute molecules with a protein (called a transporter or carrier) in the membrane. There are two kinds of carrier-mediated transport: active transport (which employs metabolic energy) and facilitated diffusion (which does not).
Catabolism
Destructive metabolism; the set of metabolic processes by which complex chemical compounds are broken down to release energy, create smaller chemical building blocks, or prepare chemical constituents for elimination.
Catadromous
Relating to an aquatic animal that undergoes most of its growth in freshwater but enters seawater to breed.
Catalyst
A compound that facilitates a chemical reaction—in which covalent bonds are made or broken—without, in the end, being modified by the reaction. Enzymes are catalysts.
Catalytic rate constant (kcat)
The number of substrate molecules a unit of enzyme is capable of converting to product per unit of time when the enzyme is saturated.
Catalytic vacuole
The part of an enzyme molecule that provides a suitable setting for catalysis during the conversion of substrate to product; includes, but is not limited to, the active site.
Catecholamines
Chemical signals derived from the amino acid tyrosine that serve as neurotransmitters and also as three amine hormones (epinephrine, norepinephrine, and dopamine).
Cation
A positively charged ion.
Cell body
The portion of a neuron that contains the cell nucleus; also called the soma or perikaryon.
Cell membrane
The membrane that encloses an animal cell and forms the outer boundary of the cell. Also called the plasma membrane.
Cell signal transduction
The processes by which cells alter their intracellular activities in response to extracellular signals.
Cell theory
The theory, developed in the nineteenth century, that organisms are composed of cells, which act as structural, functional, and developmental units of organization.
Cell-volume regulation
The process by which the volume or size of a cell is kept stable.
Cellular oscillator
A neuron that can generate a rhythmic change in membrane potential or activity without needing synaptic input to do so; one possible basis of a central pattern generator.
Central circulation
The heart and the veins and arteries that connect immediately to the heart.
Central control
Control of motor output to produce rhythmic movements, for which the pattern of activity depends on the CNS, without requiring temporally patterned sensory input. Contrast with peripheral control.
Central nervous system (CNS)
The consolidated integrative part of an animal’s nervous system; in vertebrates, consists of the brain and spinal cord.
Central pattern generator (CPG)
A neural circuit (or a single neuron) that generates a behaviorally significant pattern of motor output in space and time without requiring temporally patterned sensory input.
Centralization
Over the course of evolution, the tendency of animal groups to concentrate integrative neural functions into a central nervous system.
Cephalization
The concentration of structures of the nervous system toward the anterior end of an animal, a trend underlying the evolution of anterior brains in many animal groups.
Cephalopod mollusc
An octopus, squid, cuttlefish, or related animal.
Cerebellar cortex
The outer layer of the cerebellum of the vertebrate hindbrain; involved in motor coordination and learning.
Cerebellum
A prominent structure of the vertebrate hindbrain, concerned with motor coordination, posture, and balance. Consists of the cerebellar cortex and deep cerebellar nuclei.
Cerebral cortex
The outer part of the cerebral hemispheres of the vertebrate forebrain; greatly enlarged in birds and mammals.
cGMP phosphodiesterase (PDE)
An enzyme that hydrolizes cyclic GMP to 5′-GMP.
cGMP
See cyclic guanosine monophosphate.
Channel
A membrane protein that aids the passive transport of a solute across a membrane without undergoing any sort of chemical binding with that solute. Channels participate particularly in passive transport of inorganic ions across membranes. See also water channel.
Channel-mediated water transport
Water transport by osmosis through a water channel. See water channel.
Chaperones
See molecular chaperones or chemical chaperones.
Chemical chaperones
Synonym for counteracting solutes.
Chemical energy (chemical-bond energy)
Energy that is liberated or required when atoms are rearranged into new configurations. Animals obtain the energy they need to stay alive by reconfiguring atoms in food molecules, thereby liberating chemical energy.
Chemical potential
Qualitatively speaking, the strength of the tendency of a chemical substance to undergo a physical or chemical change. Measures of chemical potential are useful for predicting the direction of change because chemical substances tend to go from high chemical potential to low chemical potential.
Chemical synapse
A synapse that for signal transmission employs a chemical neurotransmitter that is released presynaptically and acts on postsynaptic neurotransmitter receptors.
Chemical-bond energy
Energy that is liberated or required when chemical bonds are broken or made. Animals obtain the energy they need to stay alive by breaking chemical bonds in food molecules, thereby liberating chemical-bond energy.
Chemiosmotic hypothesis
A hypothesis concerning the biochemical relationship between electron transport and oxidative phosphorylation in mitochondria. According to the hypothesis, electron transport pumps protons into the mitochondrial intermembrane space, and the back-diffusion of the protons through ATP synthase results in ATP formation.
Chemoautotroph
An organism that can obtain the energy it needs to stay alive from energy-yielding inorganic chemical reactions.
Chemoreception
A sensory response to a chemical stimulus. Chemoreception includes taste (also termed the gustatory sense) and olfaction (the sense of smell), as well as other chemical sensitivities.
Chemosynthetic autotroph
Synonym for chemoautotroph.
Chloride cell
Chloride shift
In vertebrate blood, diffusion of Cl– from the blood plasma into the red blood cells in exchange for diffusion of HCO3– out of the cells, or the reverse process. Mediated by a membrane countertransporter often called the band 3 protein.
Chlorocruorin
A type of respiratory pigment found in certain marine annelid worms, formed by the combination of hemelike structures with protein. Undergoes reversible combination with O2 at the hemelike loci.
Cholinergic neuron
A neuron that synthesizes and releases acetylcholine as a neurotransmitter.
Chorionic gonadotropin (CG)
A hormone secreted by the embryonic placenta in horses and primates that maintains the function of the corpus luteum beyond the time when it would degenerate if pregnancy did not occur.
Chromatophore
(1) In most animals, a type of cell containing pigment granules that can undergo changes in dispersion, thereby altering their influence on the color of the skin or other structure; a color-change cell. (2) In cephalopod molluscs (e.g., squids), the same term is used to refer to a very different color-change structure, namely an organ composed of a pigment cell and many muscle cells that can change the size of the pigment cell.
Chronic response
A response expressed by an animal following days, weeks, or other prolonged exposure to new environmental conditions.
Ciliary photoreceptor
A photoreceptor cell in which the light-sensitive part is a modified cilium; characteristic of vertebrates. Contrast with rhabdomeric photoreceptor.
Circadian oscillator
The biological clock that times a circadian rhythm.
Circadian rhythm
An endogenous rhythm with a period of about a day (ca. 24 hours).
Circannual rhythm
An endogenous rhythm with a period of about a year.
Circatidal rhythm
An endogenous rhythm with a period approximating a tidal cycle (ca. 12.4 hours).
Circulation
The pressure-driven mass flow of blood through a system of tubular vessels or other passages that brings the blood to all parts of the body.
Circulatory system
The blood and the system of vessels or other passages through which it circulates.
Classical conditioning
A form of associative learning in which an animal learns the association between two stimuli.
Clearance
See plasma clearance.
Cleidoic egg
Metaphorically, a cleidoic egg is a “locked box” (cleido, “key”) (in contrast to an egg that exchanges nutrients with its surroundings throughout development). In a cleidoic egg—exemplified by the eggs of birds and other reptiles—everything needed by an embryo to develop from a single cell to a hatchling is present within the shell of the egg. The egg includes all nutrients required by the embryo, a method of gas exchange, and mechanisms to store or eliminate metabolic wastes. See also amniotic egg.
Clone
A new individual, produced by asexual reproduction, that is genetically identical to its parent, or a group of such individuals. Used as a verb, to clone means to make an exact genetic copy.
Closed circulatory system
A circulatory system in which the blood is enclosed within blood vessels throughout and is therefore distinct from the interstitial fluids. Contrast with open circulatory system.
Cochlea
A part of the inner ear of many vertebrates, coiled in mammals, that contains the auditory sensory hair cells.
Cochlear amplifier
An active physiological process that amplifies the movement of the basilar membrane in the cochlea in response to sound.
Cocurrent
Referring to two fluids flowing in the same direction. Also called concurrent.
Collecting duct
In a vertebrate kidney, a duct that receives fluid from multiple nephrons and carries it to where the fluid will exit the kidney. Activities of the collecting duct epithelium typically modify fluid composition and volume along the way.
Colligative properties
The properties of an aqueous solution that depend simply on the number of dissolved entities per unit of volume, rather than on the chemical nature of the dissolved entities. The three principal colligative properties in animal physiology are osmotic pressure, freezing point, and water vapor pressure.
Colloid osmotic pressure
The difference in osmotic pressure that arises between two solutions on either side of a cell membrane or epithelium because the two solutions differ in their concentrations of nonpermeating protein solutes. Because the solutes responsible for colloid osmotic pressure cannot cross the membrane or epithelium, the colloid osmotic pressure represents a “fixed” difference in osmotic pressure between the solutions. Also called oncotic pressure.
Commissure
A bundle of axons that connects the two sides of a bilaterally symmetrical central ganglion or bilateral regions of a central nervous system.
Compact myocardium
Myocardium in which the muscle cells are so closely packed together that the tissue is not perfused by the blood flowing through the lumens of the heart chambers, and thus the tissue must be perfused by a coronary circulation. The myocardium of a mammal is compact.
Comparative method
A method of analysis that seeks to identify adaptive traits or adaptive evolutionary trends by comparing how a particular function is carried out by related and unrelated species in similar and dissimilar environments.
Compass
A mechanism that indicates geographical direction on the Earth’s surface; most human-made compasses are magnetic.
Compass direction
North, south, east, west, or a combination of these; the direction of a movement or orientation as it could be described using a compass.
Compatible solute
A solute that, when concentrated enough to contribute significantly to the osmotic pressure of a fluid, has little or no effect on the structure and function of macromolecules with which the fluid is in contact.
Compensation
In the study of acclimation or acclimatization, the return of a physiological property toward its value that existed prior to an environmental change even though the animal remains in the changed environment. Compensation is partial if the physiological property returns only partly toward its preceding value. It is complete if the physiological property returns to the value that existed prior to the environmental change.
Complete compensation
See compensation.
Complex cell
A neuron found in the mammalian primary visual cortex that has an orientation-selective receptive field but lacks distinct subparts excited or inhibited by reception of light. A complex cell responds to a bar or edge of a certain size and orientation, anywhere in the cell’s receptive field.
Compound eye
A multifaceted eye characteristic of arthropods, composed of many individual optical units called ommatidia.
Concentration gradient
Technically, the difference in the concentration of a solute between two places divided by the distance separating those two places. Often used more loosely to refer simply to a difference in concentration.
Concentric muscle contraction
An isotonic muscle contraction in which the muscle shortens while generating force.
Concurrent
See cocurrent.
Condensation
A change in the physical state of a compound from a gas to a liquid.
Conductance
A measure of how easily electrical current will flow through a conductive pathway. Contrast with electrical resistance. See also thermal conductance.
Conducting airways
In the lungs of an animal, the airways that do not participate in the exchange of respiratory gases between air and blood, but rather simply conduct air from one place to another.
Conducting system
A system of specialized muscle cells by which depolarization spreads throughout the vertebrate heart.
Conduction
(1) In the study of heat, the transfer of heat by intermolecular collisions through a material that is macroscopically motionless. (2) In the study of heart physiology, the process by which depolarization spreads through the vertebrate heart or any other myogenic heart. (3) In the study of electrical phenomena, the transmission of electrical currents or signals through a conductive pathway.
Cone
A type of photoreceptor in the vertebrate retina. Cones are smaller and less light sensitive than rods and are used for diurnal vision and color vision.
Conformity
A state in which an animal’s internal conditions match the external environmental conditions.
Connective
A bundle of neuronal axons in the central nervous system that connects central ganglia; found in the ganglionic nervous systems of arthropods, annelids, and molluscs.
Connexin
The protein that makes up connexons.
Connexon
The protein channel of a gap junction at which cytoplasmic continuity is established between two adjacent cells. Two connexons, positioned respectively in the cell membranes of the two cells, form the channel. Channels of this sort electrically couple cells (permitting current flow between them) and permit small molecules to move between cells.
Constitutive enzyme
An enzyme (or other protein) that is always expressed in a tissue. Contrast with inducible enzyme.
Consumed energy
Energy that was initially in the form of chemical energy or another form of high-grade energy but that has been converted to heat.
Consumption, of energy
Conversion of chemical-bond energy to heat or external work.
Contact chemoreceptor
A chemosensory cell (of a terrestrial animal) that is normally stimulated by chemicals that contact it in a liquid. Contrast with distance chemoreceptor.
Continuous breathing
In a terrestrial vertebrate, inhaling and exhaling continuously (without significant interruptions).
Contraction
The condition in which a muscle is activated to produce force. In a skeletal muscle, contraction can be shortening (isotonic), isometric (remaining the same length), or lengthening (isotonic).
Control system
A system that sets the level of a particular variable that is being controlled. To do so, it uses information from sensors to determine signals it sends to effectors that can modify the controlled variable. Control systems often (but not always) operate on negative feedback and are stabilizing.
Controlled variable
In speaking of a control system (e.g., home thermostat), the variable that the system controls (room temperature in the case of a home thermostat).
Convection
Always refers to mass flow, but has different specific meanings depending on context. (1) In the study of heat transfer, convection is the transfer of heat by the mass flow of a material substance (e.g., wind). (2) In the study of material transport, convection refers to (i) the flow of a fluid from place to place and (ii) the transport of molecules in the fluid from place to place by the fluid flow. Blood flow, for example, is a type of convection, and when the blood carries a material such as O2 from place to place, the material is said to undergo convective transport.
Convective gas transport
Transport of a gas by bulk flow (e.g., by a wind).
Convergence
Coming together. In neurophysiology, a pattern in which signals from many presynaptic neurons come together to affect a particular postsynaptic neuron. Contrast with divergence.
Cooperativity
A type of chemical behavior that occurs in protein molecules that have multiple ligand-binding sites, in which the binding of any one site to its ligand may facilitate or inhibit the binding of other sites on the same molecule to their ligands. The binding sites do not interact directly; instead, binding at one site induces protein conformational shifts that affect other sites at a distance.
Coronary artery
An artery that carries blood into the myocardium of a heart.
Coronary vein
A vein that carries blood out of the myocardium of a heart.
Corpus allatum (plural corpora allata)
One of a bilateral pair of organs in insects that serve both as neurohemal organs, where prothoracicotropic hormone (PTTH) is released from axon terminals of neurosecretory cells in the brain, and as nonneural endocrine tissue, which secretes juvenile hormone.
Corpus luteum (plural corpora lutea)
An endocrine structure in the vertebrate ovary formed by reorganization of the cells of an ovarian follicle that has undergone ovulation. In mammals, it secretes progesterone, estrogen, and inhibin. If fertilization occurs, it remains active during pregnancy; if not, it degenerates.
Corpus luteum of pregnancy
A corpus luteum that remains active for an extended period of time because pregnancy prevents it from degenerating. See corpus luteum.
Corpus luteum of the cycle
A corpus luteum that undergoes spontaneous degeneration after a brief period of being active. See corpus luteum.
Cortex
The outer layer of something (such as the cerebrum or the adrenal gland), as opposed to the medulla, or inner portion.
Corticotropin-releasing hormone (CRH)
A tropic peptide chemical signal secreted by hypothalamic parvocellular neurosecretory neurons; travels in hypothalamo–hypophysial portal vessels to the anterior pituitary gland and stimulates secretion of adrenocorticotropic hormone.
Cost of transport
For an animal that is undergoing directional locomotion, the energy cost of covering a unit of distance.
Costal suction pump
The development of negative pressure within the thorax of an air-breathing vertebrate by action of the costal muscles between the ribs, used to suck air into the lungs.
Costamere
In muscle fibers, a complex of protein molecules that serves as a region of attachment between the myofibrils, sarcolemma, and extracellular matrix. Costameres are organized into many parallel bands that circumscribe each muscle fiber at regular intervals coincident with the Z discs.
Cotransmitter
In neurons that synthesize and release more than one kind of neurotransmitter molecule, the second kind of neurotransmitter.
Cotransporter
A transporter protein that obligatorily carries two different solutes in the same direction simultaneously. Through the participation of a cotransporter, the diffusion of one solute in the direction of its electrochemical gradient can cause a second solute to move away from equilibrium. Also called a symporter.
Counteracting solutes
Osmolytes that act in teams of two or more to modify the osmotic pressures of body fluids without greatly perturbing macromolecules because the individual members of the teams have mutually offsetting effects on the macromolecules.
Countercurrent
Referring to two fluids flowing in opposite directions.
Countercurrent exchange
Exchange of heat, O2, or other substances by passive-transport processes (e.g., diffusion) between two closely juxtaposed fluid streams flowing in opposite directions (e.g., blood flowing in opposite directions in two blood vessels).
Countercurrent multiplication
A process that occurs in a system consisting of two juxtaposed fluid streams flowing in opposite directions, in which metabolic processes produce a difference (termed the single effect) between adjacent parts of the two streams. Because of the dynamics of such a system, the difference produced between adjacent parts of the two streams is multiplied to create a much larger difference between the two ends of the system.
Countertransporter
A transporter protein that obligatorily carries two different solutes in opposite directions simultaneously. Through the participation of a countertransporter, the diffusion of one solute in the direction of its electrochemical gradient can cause a second solute to move away from equilibrium. Also called an antiporter.
Coupling
In the study of cell energy transduction, the use of energy released in the electron-transport chain to synthesize adenosine triphosphate (ATP). It can be graded; i.e., the two processes—electron transport and ATP synthesis—can be tightly or loosely coupled.
Covalent bond
A chemical bond in which atoms fully share electrons. Covalent bonds are strong and, for the most part in biological systems, require enzyme catalysis to be made or broken.
Covalent modulation
Modulation of the catalytic properties of an enzyme, or the functional properties of another type of protein, by chemical reactions that make or break covalent bonds between a modulator and the enzyme or other modulated protein. The most common mode of covalent modulation is phosphorylation and dephosphorylation catalyzed by protein kinases and protein phosphatases. Also called covalent modification.
Cranial nerves
Peripheral nerves that connect to the brain.
Critical temperature
(1) In the study of poikilotherms, a body temperature (high or low) at which animals have little or no ability to increase their rate of O2 consumption above their resting rate—making them incapable of much physical activity. (2) In the study of homeotherms, an ambient temperature that defines the upper or lower limit of the thermoneutral zone; see also lower-critical temperature and upper-critical temperature.
Cross-bridge
The head of a myosin molecule interacting with actin molecules to produce muscle contraction by repeated oarlike power strokes. Each myosin molecule has two heads, and each head has an actin-binding site and an enzymatic site that binds and hydrolyzes ATP to liberate energy to fuel each power stroke.
Cross-current exchange
A type of exchange between two fluid streams, most commonly found in breathing organs in which O2 and CO2 are exchanged between air and blood. During cross-current exchange of this sort, the total blood flow is broken up into multiple separate streams, each of which “crosses” the flow of air at a particular point, thereby exchanging respiratory gases with the air at just one point along the path of airflow. The streams then coalesce to reestablish a unified total blood flow.
Cryptobiosis
Latent life. A resting state in which there is little or no metabolism, so that there are few, if any, signs that the organism is still alive.
Cryptonephridial complex
A specialized, close association between the Malpighian tubules and rectum that occurs in certain types of insects (e.g., mealworms) and plays a role in the production of concentrated urine and the absorption of water vapor from the atmosphere.
Current, electric
The flow of electrical charge.
Cutaneous
Relating to the skin.
Cuticle
The exoskeleton of an arthropod, or the material of which the exoskeleton is composed.
Cyclic adenosine monophosphate (cyclic AMP, cAMP)
A second messenger produced intracellularly in response to several neurotransmitters and hormones.
Cyclic guanosine monophosphate (cyclic GMP, cGMP)
An intracellular messenger in rod and cone photoreceptors and a second messenger in some neurotransmitter actions.
Cytochrome P450 enzymes
A set of detoxification enzymes that are upregulated when an animal is exposed to toxins.
Cytokines
Peptide regulatory molecules that are involved in cell development and differentiation and in immune responses.
Cytoskeleton
Intracellular structural support elements (e.g., microtubules, intermediate filaments, actin microfilaments) composed primarily of fibrous protein polymers.
Cytosol
See intracellular fluids.
D218O method
See doubly labeled water method.
Daily torpor
In mammals and birds, a form of controlled hypothermia in which the body temperature is able to approximate ambient temperature for part (but only part) of each 24-hour day, generally on many consecutive days.
Dale’s principle
The idea that a neuron releases the same kind of neurotransmitter at all its endings. With the discovery of cotransmitters, Dale’s principle may be revised to say “the same kind(s) of neurotransmitters.”
Dalton (Da)
A unit of measure for atomic and molecular mass, equal to 1/12 the mass of an atom of the most abundant carbon isotope, 12C. Thus the mass of a 12C atom is 12 daltons.
Dark adaptation
Increase in light sensitivity of a photoreceptor or visual system after a period in the dark.
Dark current
The ionic current, carried mainly by Na+ ions, that flows into the outer segments of vertebrate photoreceptors in the dark. Light absorption leads to closing of the Na+ channels, turning off the dark current and hyperpolarizing the photoreceptor.
Data logger
A small microcomputer with a large amount of memory that can be placed in or on an animal to collect and store time-labeled information on physiological or behavioral variables. After the data logger is recovered from the animal, the information it has stored is off-loaded to a computer.
Deamination
Removal of nitrogen-containing amino groups from amino acids or proteins.
Decapod crustacean
A lobster, crayfish, crab, shrimp, or related animal.
Decompression sickness
A pathological state that arises after diving when bubbles are formed within body fluids because the reduction in pressure during surfacing allows gases (especially N2) present at high dissolved partial pressures to come out of solution. Also called the bends, caisson disease.
Decremental spread
Spread of a signal in such a way that signal strength decreases with distance. In electrophysiology, passive voltage changes (synaptic potentials, receptor potentials, etc.) spread decrementally, in contrast to propagated action potentials. Also called electrotonic conduction, passive spread.
Deep cerebellar nuclei
Collections of neurons under the cerebellar cortex and receiving output from it.
Definitive urine
The final urine that is excreted by an animal. Contrast with primary urine.
Degradation of energy
Conversion of energy from a high-grade form to heat.
Degrade
In the study of energy, the process of converting chemical-bond energy or other forms of high-grade energy to heat.
Delayed implantation
Embronic diapause in placental mammals. A programmed state of arrested embryonic development that occurs after an embryo has arrived in the uterus but before it implants in the uterine wall.
Denaturation
A change in the tertiary (three-dimensional) structure of an intact protein that renders the protein nonfunctional. May be reversible or irreversible. The primary structure is not altered during denaturation.
Dendrite
The receptive element of most neurons, which receives synaptic input from other neurons. Most neurons have many, multiply branching dendrites, in contrast to one sparsely branching axon.
Dendritic
Branching in a way that resembles the branching of a tree. Also: Of or having to do with a dendrite.
Dendritic spine
A knoblike protrusion on a dendrite of a neuron that is typically the postsynaptic site of a single synapse. Common in mammalian central neurons.
Dense bodies
Structures in the cytoplasm and on the inner cell membrane of smooth muscle cells to which actin filaments attach.
Deoxy-
A prefix referring to a molecule that is capable of reversible combination with O2 that is in a state of not being so combined (e.g., deoxyhemoglobin).
Deoxygenation
The release of O2 from a combined state with a respiratory pigment such as hemoglobin. Not equivalent to reduction.
Depolarization
A decrease in amplitude of the inside-negative electrical potential of a cell membrane toward zero. More generally, any increase in the inside positivity of a cell membrane, even if it exceeds zero.
Desmosome
A “spot weld” type of junction between two adjacent cells in which protein filaments intermingle across the space between the cells, thereby strengthening and stabilizing the morphological arrangement of the cells.
Developmental physiology
The study of functional properties in successive stages of the development of an individual animal. It includes studies of function in early developmental stages as well as in adults.
Developmental polyphenism
Diacylglycerol (DAG)
A molecule derived from membrane phospholipids that acts as a second messenger in cell signaling.
Diapause
A programmed state of suspended development or suspended animation in the life history of an animal. See also embryonic diapause, delayed implantation.
Diaphragm
A sheet of muscular and connective tissue that completely separates the thoracic and abdominal cavities, found only in mammals.
Diastole
The period of relaxation during each beating cycle of a heart.
Diastolic pressure
During the beating cycle of the heart, the blood pressure prevailing during periods of heart relaxation.
Diencephalon
Part of the vertebrate forebrain that contains the thalamus and hypothalamus.
Diet-induced thermogenesis (DIT)
A chronic increase in metabolic rate induced in certain types of animals by chronic overeating.
Diffusion
See simple diffusion.
Diffusion lung
A lung within which the air is still, so that O2 and CO2 must travel the full length of the lung passages by diffusion.
Digestion
The process of splitting up ingested food molecules into smaller chemical components that an animal is capable of distributing to the tissues of its body. Most commonly carried out by hydrolytic enzymes.
Dihydropyridine receptor (DHPR)
The voltage-sensitive molecule in the transverse tubules of vertebrate striated muscle fibers that links excitation to the release of Ca2+ ions from the sarcoplasmic reticulum. It serves as a Ca2+ channel in cardiac muscle and as a voltage sensor in skeletal muscle.
Dipnoan
A lungfish. A member of the sarcopterygian class of bony fish, believed to be among the closest living relatives of the ancestors of amphibians. There are three genera of dipnoans: Protopterus in Africa, Neoceratodus in Australia, and Lepidosiren in South America.
Direct acting hormones
A category of anterior pituitary chemical signals that exert their effects on nonendocrine target cells; not tropic. Contrast with tropic hormones.
Direct calorimetry
Measurement of metabolic rates by quantifying heat and external work.
Direct measurement
A measurement procedure that quantifies a property by measuring exactly what the definition of the property specifies. Contrast with indirect measurement.
Disaccharidase
A digestive enzyme that breaks a disaccharide sugar molecule into two single sugar molecules.
Disc
A flat structure. Retinal rods have membrane-bound discs that are the site of phototransduction of rhodopsin.
Distance chemoreceptor
A chemosensory cell (of a terrestrial animal) that is normally stimulated by chemicals that are airborne over a considerable distance. Distance chemoreceptors are usually more sensitive than contact chemoreceptors. Contrast with contact chemoreceptor.
Diuresis
The production of urine at a high rate. Such urine is usually dilute as well as abundant.
Diuretic
Promoting diuresis.
Diurnal
Active in the daytime.
Divergence
Spreading apart. In neurophysiology, a pattern in which signals from a particular presynaptic neuron synaptically excite or inhibit many postsynaptic neurons. Contrast with convergence.
Diving bradycardia
Slowing of the heart rate during diving.
DNA microarray
A grid of numerous, diverse DNA spots that, by hybridizing with messenger RNA molecules (mRNAs) in a mix of mRNAs, reveals which mRNAs are present in cells or tissues—thereby revealing which genes are being transcribed. A major tool in transcription profiling. Also called a DNA microchip or gene chip.
Donnan equilibrium
A complex multi-ionic equilibrium state that tends to be reached by the interacting diffusion of multiple permeating ions and water across a cell membrane or epithelium when there is a set of nonpermeating ions (ions that cannot cross the membrane or epithelium) that are more abundant on one side than on the other. Because of their content of nonpermeating anionic proteins and nucleic acids, animal cells would approach Donnan equilibrium if it were not for the fact that living processes hold them away from any sort of equilibrium.
Dorsal aorta
In the circulatory system of a fish, the major vessel that conveys blood from the gills to the systemic tissues.
Dorsal root ganglion
A type of peripheral ganglion found at the dorsal root of a spinal nerve, containing cell bodies of the sensory neurons in that nerve.
Doubly labeled water method
A method used to measure the metabolic rate of a free-living animal in which water labeled with unusual isotopes of hydrogen and oxygen is injected into the animal. It estimates the animal’s rate of CO2 production. Also called the D218O method.
Downregulation
A downward shift in the catalytic activity of an enzyme, the rate of functioning of a biochemical pathway, or the rate of some other similar process brought about in a controlled manner by a regulatory system.
Dry heat transfer
Heat transfer by conduction, convection, or thermal radiation; heat transfer that does not involve evaporation or condensation of water.
Dual breather
An animal that simultaneously possesses the ability to breathe from air and from water. Also called bimodal breather.
Dynamic viscosity
See viscosity.
Dystrophin
A cytoskeletal protein in muscle fibers that connects actin filaments of the cytoskeleton to a complex of proteins in the sarcolemma.
Eccentric muscle contraction
An isotonic muscle contraction in which the muscle lengthens as it exerts force while resisting stretch. Also called a lengthening contraction.
Ecdysis
The process of shedding the outer body covering. In arthropods, the shedding of the old cuticle (exoskeleton) from one instar as the animal makes the transition to the next instar. Also called molting.
Ecdysone
The steroid prohormone secreted by the thoracic glands of arthropods that stimulates ecdysis, or molting. Also called molting hormone.
Ectotherm
See poikilotherm.
Effector
A tissue, organ, or cell that carries out functions under the direction of the nervous system or another physiological control system (e.g., the endocrine system).
Efferent
Going away. Thus, for example, an efferent blood vessel carries blood away from an organ of interest. Contrast with afferent.
Efferent neuron
A neuron that conveys signals from the central nervous system to the periphery, usually exerting motor control.
Efficiency of energy transformation
In any process that transforms high-grade energy from one form to another, the output of high-grade energy expressed as a ratio of the input of high-grade energy. See also absorption efficiency, gross growth efficiency, net growth efficiency.
Elasmobranch fish
Sharks, skates, and rays.
Electric current
The flow of electric charges. In animals, electric current is carried by ions, unlike the flow of electrons in manmade circuits.
Electrical energy
Energy that a system possesses by virtue of the separation of positive and negative electrical charges.
Electrical gradient
Technically, the difference in electrical potential (voltage) between two places divided by the distance separating those two places. Often used more loosely to refer simply to a difference in electrical potential.
Electrical synapse
A synapse at which current spreads directly from cell to cell through a low-resistance gap junction.
Electrocardiogram (EKG, ECG)
A recording as a function of time of differences in electrical potential set up in extracellular body fluids by the depolarization and repolarization of the myocardium during heart contraction and relaxation. These differences can be detected on the surface of the body and recorded from there.
Electrochemical equilibrium
A term that is synonymous with equilibrium, but emphasizes that the equilibrium state for an ion or other charged solute depends on both electrical and chemical effects. A charged solute is at electrochemical equilibrium across a membrane when the effect of concentration on its diffusion and the effect of the electrical potential difference on its diffusion sum to zero.
Electroencephalogram (EEG)
A record of gross electrical activity in the brain, usually recorded using multiple electrodes placed on the skin of the head or on the brain surface. In sleep studies, it is used to measure the stages of sleep during each sleep period.
Electrogenic pump
An active-transport process that pumps net charge across a membrane, acting to generate an electric current across the membrane and to produce a voltage difference across the membrane. Contrast with electroneutral pump.
Electrolyte
An inorganic ion in a body fluid.
Electron-transport chain
A series of compounds in the mitochondria (each compound capable of reversible oxidation and reduction) that passes electrons removed from food molecules to O2, capturing released energy for use in producing ATP.
Electroneutral pump
An active-transport process that pumps charges across a membrane such that no difference of charge is created across the membrane; it therefore is not a current source. Contrast with electrogenic pump.
Electroreceptor
A sensory receptor cell that responds to environmental electrical stimuli (e.g., in weakly electric fish).
Electrotonic conduction
See decremental spread.
Embryonic diapause
A programmed state of arrested or profoundly slowed embryonic development. See also delayed implantation.
Empirical
Based on data rather than merely reasoning.
Endocrine cell
A nonneural epithelial cell or a neuron that secretes a hormone or neurohormone.
Endocrine gland
A gland or tissue without ducts that secretes a hormone into the blood. Nonneural endocrine cells may form glands that are discrete (with all cells grouped together), diffuse (with cells scattered within other tissues), or intermediate between discrete and diffuse. Contrast with exocrine gland.
Endogenous rhythm
A rhythmic pattern of physiological or behavioral activity, the rhythmicity of which arises as an intrinsic property of an animal’s cells (e.g., in the nervous system) without need of external timing information.
Endometrium
In the mammalian uterus, the inner tissue layer that during pregnancy helps form the placenta.
Endopeptidase
A type of protein-digesting enzyme.
Endothelium
The epithelium that lines the heart and the lumen of blood vessels in vertebrates.
Endotherm
An animal in which the body temperature is elevated by metabolically produced heat.
Endothermy
Warming of the body of an animal by its metabolic heat production.
Endurance exercise
Exercise that consists of many repetitions of relatively low-intensity muscular actions over long periods of time. Exercise that emphasizes aerobic catabolism as the source of ATP. Also termed endurance training. Contrast with resistance exercise.
Energy
The ability to maintain or increase order in a system.
Energy degradation
Energy metabolism
The set of processes by which energy is acquired, transformed, channeled into useful functions, and dissipated by cells or organisms.
Enteric division
One of three divisions of the autonomic nervous system; exerts largely autonomous control over the gut.
Entrainment
The process of synchronizing an endogenous rhythm to an environmental rhythm.
Environment
An organism’s surroundings, including other organisms as well as the abiotic conditions that prevail in the surroundings, such as temperature. Sometimes called the external environment to distinguish it from the internal environment. See also internal environment.
Environmental
See ambient.
Enzyme
A molecule—usually a protein—that catalyzes a chemical reaction in which covalent bonds are made or broken.
Enzyme–substrate affinity
The proclivity of an enzyme to form a complex with its substrate when the enzyme and substrate meet.
Epicuticle
The outermost layer of the exoskeleton of an arthropod, where lipids that protect against desiccation are deposited in insects and arachnids.
Epididymis
A tubular structure, juxtaposed to the testis (testicle), where sperm are stored in anticipation of ejaculation.
Epigenetic mark
A change in a gene or its immediate biochemical environment other than a change in the DNA sequence—such as methylation of cytosine residues—that modifies expression of the gene and that, when the gene replicates, also replicates so that the resulting gene copies also have their expression modified in the same way. The gene is said to be marked or tagged.
Epigenetics
The study of modifications of gene expression that are transmitted when genes replicate despite there being no change in the DNA sequence. See also epigenetic mark.
Epigenome
The sum total of all epigenetic marks in an animal. See epigenetic marks.
Epithelial endocrine cells
synthesize and secrete hormones; also called nonneural endocrine cells
Epithelial glands
endocrine structures made of epithelial endocrine cells; also called nonneural glands;
Epithelium
A sheet of cells that lines a cavity or covers an organ or body surface, thereby forming a boundary between functionally different regions of the body or between an animal and its external environment.
Epitoke
In polychaete annelid worms, a specialized reproductive stage that is formed by either transformation of an ordinary individual or budding from an ordinary individual.
Equilibrium
The state toward which an isolated system changes; that is, the state toward which a system moves—internally—when it has no inputs or outputs of energy or matter. A system is at equilibrium when internal changes have brought it to an internally stable state from which further net change is impossible without system inputs or outputs. The state of equilibrium is a state of minimal capacity to do work under locally prevailing conditions. See also electrochemical equilibrium.
Equilibrium potential
The membrane potential at which an ion species is at electrochemical equilibrium, with concentration-diffusion forces offset by electrical forces so that there is no net flux of that ion species across the membrane.
Erythrocyte
See red blood cell.
Erythropoiesis
Production of red blood cells.
Erythropoietin
A hormone that stimulates production of red blood cells.
Essential amino acid
A standard amino acid that an animal cannot synthesize and that thus must be obtained from food, microbial symbionts, or other sources besides biosynthesis by the animal.
Essential fatty acid
A type of fatty acid that an animal cannot synthesize from scratch and that necessitates acquisition of precursors from food, microbial symbionts, or other sources besides biosynthesis by the animal. The omega-3 and omega-6 fatty acids are the major types.
Essential
In the study of nutrition, a required material—such as an amino acid, fatty acid, vitamin, or mineral element—that an animal must obtain from outside sources, i.e., sources other than biosynthesis by the animal.
Estivation
(1) In mammals and birds, a form of controlled hypothermia in which the body temperature is able to approximate ambient temperature continuously for two or more consecutive days during summer. (2) In other animals, a nonspecific term referring to a resting or dormant condition during the summer or during drought.
Estrogen
A feminizing hormone such as estradiol. Estrogens, which may be secreted by the ovary, placenta, testis, and possibly the adrenal cortex, are essential for female secondary sexual characteristics and reproduction.
Estrous cycle
A cycle of behavioral readiness to copulate (correlated with ovulation) in most female mammals.
Estrus
A stage of the estrous cycle around the time of ovulation during which a female uses behaviors to indicate that she is ready to conceive offspring. Also called heat.
Estuary
A body of water along a seacoast that is partially enclosed by land and that receives inputs of both freshwater and seawater; it is intermediate in salinity between freshwater and seawater.
Euryhaline
Referring to aquatic animals able to live over a wide range of environmental salinities. Contrast with stenohaline.
Eurythermal
Referring to poikilotherms, able to live over a broad range of body temperatures. Contrast with stenothermal.
Euthermia
The state of having a usual or normal body temperature. Specifically, in the study of mammals or birds that undergo controlled hypothermia, the state of having a fully homeothermic body temperature (e.g., about 37°C in a placental mammal).
Evaporation
A change in the physical state of a compound from a liquid to a gas; most commonly used to refer to water. Evaporation can in principle occur at any temperature.
Evaporative cooling
Removal of heat by the evaporation of water. The evaporative cooling caused by the evaporation of a gram of water equals the latent heat of vaporization of water per gram.
Evolution
A change in gene frequencies over time in a population. Evolution can result in adaptation, or it can be nonadaptive.
Excess postexercise oxygen consumption (EPOC)
An elevation of the actual O2 uptake by breathing above the theoretical O2 requirement of rest when an animal is resting immediately after exercise; “breathing hard” after exercise. Also called oxygen debt.
Excitable cells
Cells that can generate action potentials because their cell membranes contain voltage-gated channels, notably neurons and muscle cells.
Excitation
Process tending to produce action potentials.
Excitation–contraction coupling
In a muscle cell, linkage of the electrical excitation of the cell membrane with contractile activity by facilitation of the availability of Ca2+ in the cytoplasm. Key events occur at the transverse tubules and sarcoplasmic reticulum. Also called E-C coupling.
Excitatory postsynaptic potential (EPSP)
A voltage change in a postsynaptic cell—normally a depolarization—that tends to excite the cell.
Excitatory
Resulting in an increase in activity or probability of activity. In a neuron, depolarization is excitatory because it increases the likelihood of generation of action potentials. Contrast with inhibitory.
Exocrine gland
A gland with ducts in which secretions exit the gland by way of the ducts, rather than being secreted into the blood. Examples include salivary glands and sweat glands. Contrast with endocrine gland.
Exopeptidase
A type of protein-digesting enzyme.
Expiratory reserve volume
The volume of air that remains in the lungs following a resting exhalation, not including the portion that cannot be exhaled under any conditions.
Exponential
Referring to a type of relationship between two variables in which the dependent variable goes up in multiplicative steps as the independent variable goes up in additive steps. See Appendix F.
Expression
Synthesis of the protein (or other functional product) encoded by a gene.
Expression profiling
Sometimes used (but not in this book) as a synonym for transcription profiling. See transcription profiling.
External ear
The portion of the ear external to the eardrum or tympanic membrane.
External environment
See environment.
External respiration
Breathing.
External work
Mechanical work by an animal that involves applying forces to objects outside the animal’s body. Locomotion is the principal example; other examples would be a squirrel chewing into a nut, or a mole pushing soil aside.
Exteroceptor
A sensory receptor cell that is activated by stimuli from outside the body.
Extracellular digestion
Digestion that takes place outside of cells, such as in the lumen of the digestive tract.
Extracellular fluids
The aqueous solutions (body fluids) outside cells. In animals with closed circulatory systems, subdivided into blood plasma and interstitial fluids.
Extrafusal muscle fiber
In vertebrate skeletal muscle, an “ordinary” muscle fiber that is not associated with a muscle-spindle stretch receptor.
Extrarenal salt excretion
Excretion of inorganic ions by structures other than the kidneys, such as the gills in marine teleost fish and salt glands in marine birds.
Facilitated diffusion
Passive transport of a solute across a membrane mediated by the noncovalent and reversible binding of solute molecules to a solute-specific transporter (carrier) protein in the membrane. Facilitated diffusion is the principal mode of passive transport of polar organic solutes, such as glucose and amino acids, across membranes.
Fast glycolytic (FG) muscle fibers
Muscle fibers that are poised to make ATP principally by anaerobic catabolism, develop contractile tension rapidly, have relatively high peak power outputs, and fatigue relatively rapidly.
Fast oxidative glycolytic (FOG) muscle fibers
Muscle fibers that produce ATP principally by aerobic catabolism, develop contractile tension at a rate intermediate between the rates of slow oxidative (SO) and fast glycolytic (FG) fibers, have intermediate peak power outputs, and exhibit intermediate resistance to fatigue.
Fasting
(1) In life histories, failure to eat, often for prolonged periods of time, because of features intrinsic to an animal’s life history. Often distinguished from starvation, which refers to extrinsically imposed food deprivation. (2) In metabolism studies, not eating for a sufficient period of time so as to end the specific dynamic action of the last meal. See specific dynamic action.
Feedback
See negative feedback and positive feedback.
Feed-forward
A concept in control theory that is in certain respects the opposite of feedback. During feedback, deviations of a controlled property from a set-point level are detected, and the control system responds by either diminishing the deviations (negative feedback) or enhancing them (positive feedback). During feed-forward control, however, a system is driven to change by an input external to itself, not by responses to deviations of its own performance from a set point. Contrast with negative feedback, positive feedback.
Fenestrations
Minute, physical openings—pores—in the walls of blood capillaries. Some fenestrations are formed by tiny gaps (about 4 nm in diameter) between cells in the capillary endothelium; others are formed in some tissues by holes through the capillary endothelial cells. Water and ions can freely cross the capillary endothelium through fenestrations by osmosis and diffusion.
Fermentation
Enzyme-catalyzed reactions that occur without O2, such as reactions that accomplish the anaerobic breakdown of compounds to liberate energy for metabolic use.
Fermenter
An animal that maintains a symbiosis with major populations of anaerobic, fermenting microbes (e.g., rumen microbes) that perform nutritional functions for the animal.
Fermenting microbes
Anaerobic microbes that carry out fermentation. See fermentation.
Fever
A regulated elevation of body temperature to a level higher than usual.
Fiber
See muscle fiber.
Fick equation
An equation used to predict the rate of diffusion. See Equation 5.1.
Field metabolic rate (FMR)
The average daily metabolic rate of an animal when living free in its natural environment.
Filter feeding
Feeding on objects suspended in water that are very small by comparison to the feeding animal, when the mechanism of food collection is some sort of sieving. If the mechanism is of another sort or is unknown, the feeding should be termed suspension feeding.
Filtrate
See ultrafiltrate.
Filtration rate
See glomerular filtration rate.
Fixed act
A simple all-or-none behavioral response to a stimulus. Contrast with reflex.
Flexion reflex
A reflex response that flexes or withdraws a limb from a painful or noxious stimulus.
Fluid
A gas or liquid.
Fluid compartment
A defined subpart of the body fluids, often distributed throughout the body rather than occupying a discrete physical location. The simplest subdivision of the body fluids recognizes three fluid compartments: blood plasma, interstitial fluids, and intracellular fluids.
Fluid mosaic model
A theory of the nature of cell membranes. According to this theory, a cell membrane consists of a mosaic of protein and lipid molecules, all of which move about in directions parallel to the membrane faces because of the fluid state of the lipid matrix.
Fluidity
In reference to the phospholipids in a cell membrane, the ability of individual phospholipid molecules to diffuse through the population of all such molecules in a membrane leaflet because the molecules are not covalently bonded to one another. Fluidity is quantitatively variable. Diffusion is relatively rapid in some membranes, which are said to exhibit high fluidity.
Fluorescence
A phenomenon associated with bioluminescence in which preexisting light (e.g., from a bioluminescent mechanism) is absorbed and re-emitted at longer wavelengths. Fluorescence does not produce light de novo.
Follicle
(1) In secretory tissues such as the thyroid gland, a globe-shaped, hollow structure enclosed by an epithelium. (2) In the ovary, an oocyte and the layers of somatic cells surrounding it.
Follicle-stimulating hormone (FSH)
A tropic peptide chemical signal secreted by cells in the anterior pituitary gland (adenohypophysis); travels in the general circulation to the gonads; stimulates sperm production in males and follicle development and secretions in females; a gonadotropin.
Follicular phase
In a female mammal, the phase of the menstrual or estrous cycle prior to ovulation, during which primary oocytes and associated somatic follicle cells develop, mature, and secrete hormones.
Foodstuff
A material in the body, such as carbohydrate or lipid, that an animal uses as a source of energy or chemical building blocks. Foodstuffs are often derived directly from foods but differ from foods in being already in the body.
Force–velocity relationship
Also called load–velocity relationship, shows that the speed at which a muscle contracts decreases as the load against which it exerts force increases.
Forebrain
The anterior portion of the brain, consisting in vertebrates of the telencephalon and diencephalon.
Foregut
In a vertebrate, the esophagus and stomach.
Foregut fermenter
An animal that has a specialized foregut chamber housing communities of fermenting microbes that assist with the breakdown of food materials and that often provide biosynthetic capabilities the animal lacks. The microbial communities commonly include bacteria, protists, yeasts, and fungi.
Fossorial
Living underground. Used usually to distinguish species that live underground but belong to phylogenetic groups that generally live aboveground.
Fourier’s law of heat flow
Synonym for the linear heat-transfer equation.
Fovea
A central region in a vertebrate retina specialized for high-resolution processing of visual information.
Frank–Starling mechanism
An important intrinsic control mechanism of the vertebrate heart in which stretching of the cardiac muscle tends to increase the force of its contraction by an effect exerted at the level of individual muscle cells.
Free-running rhythm
An endogenous rhythm that is not entrained by an environmental rhythm.
Freezing-intolerant
An animal exposed to freezing temperatures that dies if it freezes.
Freezing point
The highest temperature at which a liquid can turn to a solid and freeze. A colligative property.
Freezing-point depression (ΔFP)
The difference (sign ignored) between the freezing point of a solution and the freezing point of pure water. For example, if the freezing point of a solution is –1.6°C, its freezing-point depression is 1.6°C. A colligative property.
Freezing-tolerant
An animal that can survive freezing of its extracellular body fluids.
G protein
A protein involved in signal transduction that is activated by binding with guanosine triphosphate (GTP). Some G proteins occur in cell membranes and are typically trimers; others occur intracellularly and are typically monomers.
G protein–coupled receptor
A membrane receptor protein that, when it binds to its specific extracellular signal ligand, relays a signal into the cell by activating G proteins in the cell membrane.
Gamete
A reproductive cell, also called a germ cell; an egg (ovum) produced by an ovary or a sperm produced by a testis.
Gametogenesis
The formation of haploid eggs or sperm (gametes) through the process of meiosis. It is called specifically oogenesis when referring to processes in the ovary and spermatogenesis in the testis.
Gamma (γ) motor neuron
A small motor neuron in a vertebrate spinal cord that innervates an intrafusal muscle fiber.
Ganglion (plural ganglia)
A discrete collection of neuronal cell bodies. In arthropod nervous systems, most ganglia are segmental components of the central nervous system; in vertebrates, ganglia are components of the peripheral nervous system.
Ganglion cell
An output cell of the vertebrate retina, with an axon extending in the optic nerve to visual processing areas of the brain.
Gap junction
A region where the cell membranes of adjacent cells are unusually close to each other and share channels (formed by adjoining connexons in vertebrates) that establish cytoplasmic continuity between the cells.
Gas tension
See partial pressure.
Gated channel
A channel that “opens” and “closes” to facilitate or inhibit solute passage. Some gated channels “open” and “close” in response to changes in ligand binding; others do so in response to voltage changes or other changes.
Gene deletion
Synonym for gene knockout. See knockout animal.
Gene expression profiling
Gene family
A group of genes that are evolutionarily related; genes related by common descent from ancestral genes.
Gene knockout
See knockout animal.
Gene lineage
In the study of evolution, a set of genes that are related by descent from a common ancestral gene; also a representation of the family tree of such a set of genes.
Genetic drift
Changes in gene frequencies within a population over time resulting primarily from chance.
Genome
The full set of genetic material of an organism.
Genome-wide association study (GWAS)
A study that compares the complete DNA of individuals with a particular condition to the DNA of individuals without the condition, with the objective of identifying the genes that play roles in causing the condition.
Genomic imprinting
An epigenetic phenomenon in which, in a given individual, an allele inherited from the individual’s father is expressed exclusively (or predominantly) relative to the allele of the same gene inherited from the individual’s mother, or vice versa—because of epigenetic marks. At present, genomic imprinting is known to occur only in mammals, insects, and flowering plants.
Genomics
The study of the genomes (the full sets of genetic material) of organisms.
Germ cell
See gamete.
Ghrelin
A peptide hormone secreted by cells in the stomach and upper intestine that stimulates hunger.
Giant axons
Axons that are much bigger than other axons around them. Diameters vary from 1mm in squid to 4 μm in fruitflies.
Gills
In the most general sense used by physiologists, structures specialized for external respiration that project from the body into the ambient medium and are thereby surrounded by the environmental medium. Contrast with lungs.
Glial cells
Cells in an animal’s neural tissue (e.g., brain) other than neurons. Glial cells are considered support cells, ensheathing neuronal processes or regulating the metabolism of neurons. They may play secondary roles in signaling and integration. Also called neuroglia.
Globins
A family of structurally similar proteins believed to be evolutionarily related by common descent from ancestral protein forms. Proteins of this family occur, for example, in hemoglobin O2-transport pigments.
Glomerular filtration rate (GFR)
The rate at which all the nephrons in the kidneys of a vertebrate collectively produce primary urine by ultrafiltration.
Glomerulus
A minute anastomosing cluster of blood capillaries associated with a nephron in the kidney of a vertebrate, serving as the site of formation of primary urine by ultrafiltration. The term is also sometimes used to refer not only to such a cluster of capillaries, but also to the Bowman’s capsule with which it is associated.
Glucagon
A hormone secreted by alpha cells of the endocrine pancreas in response to low plasma levels of glucose and/or high plasma levels of amino acids; increases plasma glucose by stimulating glycogenolysis, gluconeogenesis, and breakdown of triglyceride molecules.
Glucocorticoids
Steroid hormones, such as cortisol and corticosterone, that are released from the cortex of the adrenal gland and regulate carbohydrate, lipid, and protein metabolism.
Gluconeogenesis
Production of glucose or glycogen from noncarbohydrate molecules such as lactic acid or amino acids.
Glycolysis
The reactions that convert glucose to pyruvic acid.
Goldman equation
An equation that describes membrane potential in terms of the concentrations of and membrane permeabilities to more than one ion species.
Gonadotropin
A hormone that stimulates the gonads (ovaries or testes) to produce gametes and secrete hormones, and also supports and maintains the gonadal tissue.
Gonadotropin-releasing hormone (GnRH)
In a vertebrate, a hypothalamic hormone that controls gonadotropin secretion from the anterior pituitary gland.
Graded potential
A voltage change that is variable in amplitude—that is, not all-or-none like an action potential. Examples include synaptic potentials and receptor potentials.
Granular cells
Granulosa cells
Somatic cells surrounding the primary oocyte in an ovarian follicle.
Gray matter
A histological region of a vertebrate central nervous system that contains neuronal cell bodies, dendrites, and synapses as well as axons. See also white matter.
Green gland
See antennal gland.
Grid cell
A neuron, commonly in the entorhinal cortex of the brain, that generates action potentials when an animal is at one of several gridlike locations in its environment. The loci at which grid cells are active form an invisible regular hexagonal grid that can provide a distance calibration for a map representation, in the brain, of the animal’s environment.
Gross growth efficiency
In a growing animal, the chemical-bond energy of new biomass added by growth expressed as a ratio of the animal’s ingested energy over the same time period.
Growth hormone (GH)
A direct acting hormone secreted by cells of the anterior pituitary gland (adenohypophysis); acts at many target tissues to promote lean body mass by stimulating protein synthesis and breakdown of triglyceride molecules.
Gular fluttering
Rapid up-and-down oscillation of the floor of the mouth cavity of a bird or other reptile to enhance the rate of evaporative cooling by increasing air flow over moist membranes in the mouth.
Gustatory
Having to do with taste.
Gut microbiome
Populations of microbes, consisting of many species of bacteria and other heterotrophic microbes, living in the gut lumen of an animal.
Gut motility
See motility.
20-Hydroxyecdysone (20E)
The physiologically active form of the molting hormone ecdysone; stimulates the epidermis to replace the cuticle in successive molts; a gonadotropin in adults. See ecdysone.
H zone
In striated muscle, a region at the center of a sarcomere that contains only thick filaments; shortens during contraction. Also called H band.
Habituation
A simple, nonassociative form of learning characterized by a learned decrease in a behavioral response with repeated presentations of a nonthreatening stimulus.
Hagen-Poiseuille equation
See Poiseuille equation.
Hair bundle
The tuft of microvilli (stereocilia) at the apical end of a hair cell. It responds to mechanical stimulation.
Hair cell
A sensory epithelial cell in a vertebrate acoustico-lateralis system that transduces displacement of its apical stereocilia into an electrical signal.
Haldane effect
A shift of the carbon dioxide equilibrium curve of the blood caused by the oxygenation and deoxygenation of the blood respiratory pigment. In blood that exhibits a Haldane effect, the total carbon dioxide concentration is higher when the blood is deoxygenated than when it is oxygenated.
Half-life
The time required to reduce something by one-half; applies, for example, to the concentrations of substances such as hormones, stored voltages, and radioactivity.
Half-saturation constant (Km)
See Michaelis constant.
Halophyte
A plant that roots in saline soils, often characterized by high salt concentrations in its tissue fluids.
Head direction cell
A neuron in the brain that generates action potentials when the animal’s head is pointing in a particular direction. Head direction cells provide spatial information about the animal’s bearings in its environment. They are presumably used with place cells and grid cells in spatial navigation. See grid cell and place cell.
Heart
A discrete, localized structure specialized for pumping blood. It may be neurogenic or myogenic and associated with an open or a closed circulatory system.
Heat
The energy that matter possesses by virtue of the ceaseless, random motions that all of the atoms and molecules of which it is composed undergo on an atomic-molecular scale of distance. Also called molecular kinetic energy.
Heat increment of feeding
Synonym for specific dynamic action.
Heat-shock proteins
An evolutionarily related group of proteins that are principally inducible—being expressed in the aftermath of heat stress or other stress—and that function as molecular chaperones. Heat-shock proteins use ATP to assist in the repair of stress-damaged proteins by preventing the damaged proteins from aggregating with one another and by promoting molecular folding patterns that restore them to correct three-dimensional conformations.
Hematocrit
In blood, the sum total volume of all cells expressed as a percentage of blood volume.
Heme
A particular metalloporphyrin containing iron in the ferrous state: ferrous protoporphyrin IX. It is the O2-binding site in all hemoglobins.
Hemerythrin
A type of respiratory pigment found in scattered groups of animals in three or four phyla, consisting of an iron-based metalloprotein. Undergoes reversible combination with O2 at iron-containing loci.
Hemocyanin
A type of respiratory pigment found in arthropods and molluscs, consisting of a copper-based metalloprotein. Undergoes reversible combination with O2 at copper-containing loci.
Hemoglobin
A type of respiratory pigment formed by the combination of heme (ferrous protoporphyrin IX) with a globin protein. Undergoes reversible combination with O2 at the heme loci.
Hemolymph
A synonym for blood in an animal that has an open circulatory system. The term emphasizes that the blood in such animals includes all extracellular fluids, and thus that there is no distinction between the fluid that is in the blood vessels at any one time and the interstitial fluid between tissue cells.
Henry’s law
A law that relates partial pressure and concentration for gases dissolved in an aqueous solution (or in another liquid solution). One way to express this law is that C = AP, where C is dissolved concentration, P is partial pressure in atmospheres, and A is the absorption coefficient (dissolved concentration when partial pressure is 1 atm).
Hepatopancreas
An organ in crustaceans that connects with the stomach and functions in secretion of digestive enzymes; absorption of nutrient molecules; storage of lipids, glycogen, and Ca2+; and sequestration of foreign compounds.
Hermaphrodite
An individual that possesses both ovaries and testes. An animal may be a simultaneous hermaphrodite, in which both types of gonads produce gametes at the same time, or a sequential hermaphrodite, in which only one type of gonad produces gametes at a time.
Heterothermy
The property of exhibiting different thermal relations from time to time or place to place. There are two types of heterothermy: (1) In regional heterothermy, some regions of an individual animal’s body exhibit different thermal relations than other regions at the same time. (2) In temporal heterothermy, an individual exhibits one type of thermal relation at certain times and another type of thermal relation at other times (e.g., hibernation at some times and homeothermy at others).
Heterotroph
An organism that obtains the energy it needs to stay alive by breaking up organic compounds that it obtains from other organisms, thereby releasing the chemical-bond energy of those organic compounds. Contrast with autotroph.
Hibernaculum (plural hibernacula)
The place where an animal resides while in hibernation.
Hibernation
(1) In mammals and birds, usually refers to a form of controlled hypothermia in which the body temperature is able to approximate ambient temperature continuously for two or more consecutive days during winter. (2) In other animals, a nonspecific term referring to a resting or dormant condition during winter.
High-grade energy
Energy in a form that can do physiological work. Chemical, electrical, and mechanical energy are forms of high-grade energy.
High-throughput method
An analytical method that is carried out by computer programs and robots without much direct human attention, and thus can process samples at a relatively high rate.
Hill coefficient
A measure of the degree of cooperativity among O2-binding sites in a molecule of a respiratory pigment.
Hindbrain
Posterior portion of a vertebrate brain, consisting of the cerebellum, pons, and medulla.
Hindgut
The posterior part of the gut. In a person, the large intestines and rectum. In an insect, the part of the gut posterior to the connections of the Malpighian tubules.
Hindgut fermenter
An animal that has a specialized hindgut chamber housing communities of fermenting microbes that assist with the breakdown of food materials and that often provide biosynthetic capabilities the animal lacks.
Histones
Basic proteins with which DNA (acidic) is complexed in the cell nucleus.
Hodgkin cycle
The cycle that explains the rising phase of an action potential: Depolarization opens voltage-gated Na+ channels, increasing membrane permeability to Na+. The resulting inflow of Na+ further depolarizes the membrane, opening more Na+ channels.
Homeosmotic animal
Synonym for osmoregulator.
Homeostasis
Internal constancy and the physiological regulatory systems that automatically make adjustments to maintain it. In the words of Walter Cannon, who coined the term, “the coordinated physiological processes which maintain most of the [constant] states in the organism.”
Homeotherm
An animal that thermoregulates by physiological means (rather than simply by behavior).
Homeoviscous adaptation
The maintenance of a relatively constant lipid fluidity regardless of tissue temperature.
Homing
The ability of an animal to return to its home site after being displaced.
Homologous
Relating to features of organisms that are similar because of common evolutionary descent.
Horizontal cell
A neuron in the vertebrate retina that is part of the lateral pathway, mediating center–surround antagonistic effects in retinal neuron receptive fields.
Hormonal axis
A hormonal sequence in which one hormone stimulates the secretion of a second hormone that may in turn stimulate secretion of a third hormone. Known hormonal axes consist of two or three hormones in sequence.
Hormonal modulation
The influence of endocrine signals on the rate of hormone secretion by means of negative and positive feedback, synergism, antagonism, and permissiveness
Hormone
A chemical substance, released by nonneural endocrine cells or by neurons, that is carried in the blood to distant target cells, where it exerts regulatory influences on their function. There are three main chemical classes of hormones: steroids, peptides or proteins, and amines.
Hourglass timer
See interval timer.
Humidic
Restricted to humid, water-rich terrestrial microenvironments; unable to live steadily in the open air.
Humidity
A general term referring loosely to the concentration of gaseous water in a gas. See also water vapor pressure, relative humidity, and saturation deficit—all of which represent ways to express humidity.
Hybrid oscillator
A neural circuit of central pattern generation that contains both cellular oscillator and network oscillator elements.
Hydrolytic enzyme
An enzyme that catalyzes the break-up of large organic molecules into smaller parts by bond-splitting reactions with H2O.
Hydrophilic
Dissolving readily in water (“water loving”).
Hydrophobic
Not dissolving readily in water (“water hating”); typically lipid-soluble.
Hydrostatic pressure
The sort of pressure that is developed in a fluid (gas or liquid) when forces are applied that tend to increase the amount of matter per unit of volume. Hydrostatic pressure is what is meant by everyday scientific uses of the word pressure. A tire pump, for instance, produces a hydrostatic pressure in a tire.
Hyper-hyposmotic regulator
An aquatic animal that maintains a blood osmotic pressure that is (1) higher than the osmotic pressure of the water in which it lives when the ambient osmotic pressure is low, but (2) lower than the osmotic pressure of the water in which it lives when the ambient osmotic pressure is high. Such an animal may have a stable blood osmotic pressure over a wide range of ambient osmotic pressures.
Hyper-isosmotic regulator
An aquatic animal that maintains a blood osmotic pressure that is (1) higher than the osmotic pressure of the water in which it lives when the ambient osmotic pressure is low, but (2) the same as the osmotic pressure of the water in which it lives when the ambient osmotic pressure is high.
Hyperbolic kinetics
A type of saturation kinetics in which the velocity of a chemical reaction increases in a smooth, strictly asymptotic way toward its maximum.
Hyperosmotic
Having a higher osmotic pressure. Said of a solution in comparison to another, specific solution. Hyperosmotic is a relative term that is meaningless unless the comparison solution is specified.
Hyperosmotic regulator
An aquatic animal that maintains a blood osmotic pressure higher than the osmotic pressure of the water in which it lives.
Hyperpolarization
A voltage change that makes a cell membrane potential more inside-negative (normally moves it further from zero).
Hyperthermia
The state of having a body temperature that is higher than the temperature considered to be normal or usual.
Hypertrophy
The addition of structural components to cells in a way that increases the size of a tissue or organ. In muscle, the addition of contractile proteins to skeletal muscle fibers.
Hyposmotic
Having a lower osmotic pressure. Said of a solution in comparison to another, specific solution. Hyposmotic is a relative term that is meaningless unless the comparison solution is specified. The word is a contraction of hypo-osmotic.
Hyposmotic regulator
An aquatic animal that maintains a blood osmotic pressure lower than the osmotic pressure of the water in which it lives.
Hypothalamo–hypophysial portal system
A system of blood vessels in a vertebrate that connects capillaries in the hypothalamus to capillaries in the anterior pituitary; provides a direct pathway by which hypothalamic hormones can reach specific populations of cells in the adenohypophysis (anterior pituitary).
Hypothermia
The state of having a body temperature that is lower than the temperature considered to be normal or usual.
Hypoxia
Referring to the tissues of an animal, the state of having an unusually low level of O2.
Hypoxia-inducible factor (HIF)
A transcription factor that increases in concentration in the cytosol of a cell when the cell experiences hypoxia and that enters the nucleus, where it activates genes coding for hypoxia responses.
I band
In striated muscle, a region of two sequentially adjacent sarcomeres that includes only thin filaments and is bisected by the Z disc; shortens during contraction. It appears light in muscle sections prepared for microscopy; takes its name from the fact that it is described as isotropic because its refractive index in polarized light changes only minimally with the plane of polarization.
Ice-nucleating agent
A dissolved or undissolved substance that promotes freezing (i.e., limits supercooling).
Ideal gas law
See universal gas law.
Imidazole group
A type of chemical group, found on the amino acid histidine and some other compounds, that is often extremely important in buffering body fluids because of its particular chemical buffering properties.
Implantation
During pregnancy in a female placental mammal, the entry of an early embryo into the cellular matrix of the inner uterine epithelium (endometrium).
Imprinting (in molecular genetics)
See genomic imprinting.
In phase
Occurring at approximately the same phase of a cycle as something else. Two events that occur at approximately the same phase of a cycle are said to be in phase.
Inactivation (in ion transport)
The closing of an ion channel in response to a stimulus such as membrane depolarization. This occurs in a time-dependent manner.
Indirect calorimetry
Measurement of metabolic rates by quantifying respiratory gas exchange or some other property besides heat and external work.
Indirect measurement
A measurement procedure that quantifies a property by measuring something other than what the definition of the property specifies. Contrast with direct measurement.
Induced ovulation
Ovulation (release of an egg from the ovaries of a female) that results from, and is dependent on, stimuli generated by the actual act of copulation. Contrast with spontaneous ovulation.
Induced ovulator
A species in which a female ovulates as a direct response to copulation.
Inducible enzyme
An enzyme (or other protein) that is expressed only when “induced” by the presence of a molecule or condition that serves as an inducing agent. An inducible enzyme disappears from a cell or tissue when its inducing agent is absent, but is expressed when the cell or tissue is exposed to its inducing agent. Contrast with constitutive enzyme.
Infrabranchial sinus
In a crustacean, a ventral sinus where blood collects after perfusing the systemic tissues, just prior to return to the gills.
Ingested chemical energy
Synonym for ingested energy.
Ingested energy
The energy present in the chemical bonds of an animal’s food.
Inhibition
Decreasing the probability or frequency of action potentials.
Inhibitory postsynaptic potential (IPSP)
A voltage change in a postsynaptic cell—normally a hyperpolarization—that tends to inhibit the cell.
Inhibitory
Resulting in a decrease in activity or probability of activity. In a neuron, hyperpolarization is inhibitory because it decreases the likelihood of generation of action potentials. Contrast with excitatory.
Inka cells
Endocrine cells associated with the tracheae of insects that secrete two peptides: pre-ecdysis triggering hormone and ecdysis triggering hormone.
Innate behavior
Behavior that has a strong genetic basis or results from genetic preprogramming.
Inner ear
In a vertebrate, the cochlea and the semicircular canals of the vestibular organ.
Inner hair cells
In the organ of Corti of the cochlea, a single row of hair cells that transduce sound vibrations into voltage changes that excite auditory sensory neurons.
Inner segment
The portion of a retinal rod or cone that does not contain specialized light-sensitive membranes.
Innervate
To provide neural input.
Inorganic ion
A non-carbon atom (e.g., Na+) or group of atoms (e.g., SO42–) that bears a net negative or positive charge.
Inositol trisphosphate (IP3)
A molecule derived from membrane phospholipids that acts as a second messenger in cell signaling.
Insensible water loss
In humans, water loss across the skin without sweating; a synonym for transpirational water loss in people.
Insertion
The movement of channel or transporter proteins from inactive intracellular locations into the membrane where they are active.
Inspiratory reserve volume
The volume of air that can be taken into the lungs following a resting inhalation, up to the maximum lung inflation that can be achieved.
Instar
A period between ecdyses, or molts, in the arthropod life cycle.
Insulation
The resistance to dry heat transfer through a material or between an animal and its environment. See also resistance (meaning 2).
Integral membrane protein
A protein that is part of a cell membrane and cannot be removed without extraction procedures that take the membrane apart.
Integration
The coordination of input signals, as by summing, to provide a harmonious control of output. Cellular integration refers to the integration of signals within a cell, and physiological integration refers to the integration of sensory, central nervous system, and endocrine signals for harmonious control of effectors in the body.
Integument
The outer body covering of an animal, such as the skin of a vertebrate or the exoskeleton of an arthropod.
Intensity difference
In sensory processes, intensity refers to the amount of a stimulus, as opposed to the kind of a stimulus (modality or quality).
Intercalated disc
An intercellular contact between adjacent cardiac muscle fibers of vertebrates that contains desmosomes and gap junctions.
Intercellular fluids
See interstitial fluids.
Intercellular
Between cells.
Intercostal muscles
Sheetlike muscles that run between adjacent ribs, the contraction of which expands or contracts the volume of the rib cage.
Intermittent breathing
Breathing in which breaths or sets of breaths are regularly interrupted by extended periods of apnea.
Internal effectors
Effectors other than skeletal muscle, such as glands, cardiac muscle, and smooth muscle, which are controlled by the autonomic nervous system.
Internal environment
The environment of the cells within an animal’s body. The set of conditions—temperature, pH, Na+ concentration, and so forth—existing in the body fluids of an animal and therefore experienced by cells within the animal’s body. Also called the milieu intérieur.
Internal work
Mechanical work performed inside the body, such as the beating of the heart or peristalsis in the gut.
Interneuron
A neuron that is confined to the central nervous system and is therefore neither a sensory neuron nor a motor neuron.
Internode
The region of a myelinated axon that lies between two nodes of Ranvier and is covered by a myelin sheath.
Interoceptor
A sensory receptor cell that is activated by stimuli within the body and thus monitors some aspect of the internal state.
Interspecific enzyme homologs
Different molecular forms of a single enzyme occurring in two or more related species; called homologs because they are assumed to be related by evolutionary descent.
Interstitial cells
See Leydig cells.
Interstitial fluids
The fluids between cells in tissues. More specifically, in animals with closed circulatory systems, the fluids between cells in tissues other than blood; that is, the interstitial fluids are the extracellular fluids other than the blood plasma. Sometimes called intercellular fluids or tissue fluids.
Interval timer
A biological clock that times an interval shorter than a day but appears to be noncyclic, having to be restarted each time it operates, like a kitchen timer or an hourglass. Also called an “hourglass” timer.
Intracellular digestion
Digestion that takes place within cells.
Intracellular fluids
The aqueous solutions inside cells. Also called the cytosol.
Intracellular membrane
A membrane, such as the endoplasmic reticulum, found inside a cell. Typically has a phospholipid-bilayer structure similar to that of the outer cell (plasma) membrane. Also called a subcellular membrane.
Intracrine
A peptide signaling molecule that regulates intracellular functions and is known to act as a hormone, paracrine, or autocrine in different contexts. It may be retained within the cell that synthesized it or internalized from the extracellular space.
Intrafusal muscle fiber
A specialized muscle fiber associated with a vertebrate muscle-spindle stretch receptor. Muscle stretching or activation of the intrafusal muscle fiber by a gamma motor neuron can activate the stretch receptor.
Intraluminal digestive enzyme
A digestive enzyme that is secreted into the lumen of the gut or another body cavity, where it mixes with food materials and digests them.
Inulin
A polysaccharide widely used in studies of plasma clearance. See plasma clearance.
Iodothyronines
The amine hormones thyroxine and triiodothyronine, derived from the amino acid tyrosine and synthesized in the thyroid gland.
Ion
An atom or group of atoms that bears a net negative or positive charge.
Ionic conformity
A state in which the concentration of an inorganic ion in the blood plasma matches, and varies with, the concentration of that ion in the external environment.
Ionic hypothesis
The organizing principle that the distributions of ions across the membrane of a cell, and the permeability of the membrane to ions, determine membrane potentials and cellular electrical events.
Ionic regulation
The maintenance of a constant or nearly constant concentration of an inorganic ion in the blood plasma regardless of the concentration of that ion in the external environment.
Ionotropic receptor (in synaptic function)
A neurotransmitter receptor molecule that changes the membrane permeability of the postsynaptic cell to particular ions when it binds neurotransmitter molecules; usually a ligand-gated channel.
Ionotropic transduction (in sensory function)
A kind of sensory transduction in which a sensory receptor molecule is itself an ion channel, changing ion flow into the cell in direct response to a sensory stimulus. It is analogous to ionotropic synaptic action, and stands in contrast to metabotropic (sensory) transduction. See also metabotropic transduction.
Islets of Langerhans
Clusters of endocrine cells distributed among the exocrine-gland tissue of the pancreas. They contain specific endocrine cells that secrete specific hormones: β (or B) cells secrete insulin, α (or A) cells glucagon, and δ (or D) cells somatostatin.
Isoform
In relation to a protein that exists in multiple molecular forms, any one of those molecular forms.
Isolated system
A defined part of the material universe that (at least as a thought exercise) cannot exchange either matter or energy with its surroundings.
Isometric contraction
A contraction in which a muscle does not shorten significantly as it exerts force (tension) against a load it cannot move.
Isosmotic
Having the same osmotic pressure. Said of a solution in comparison to another, specific solution. Isosmotic is a relative term that is meaningless unless the comparison solution is specified.
Isosmotic line
A line, entered on a graph of internal versus external osmotic pressure, that connects all the points where internal osmotic pressure equals external osmotic pressure.
Isotonic contraction
A contraction in which a muscle changes its length as it exerts force (tension) against a load. The tension during contraction remains constant and equal to the force exerted by the load.
Isozymes
Various molecular forms of a single kind of enzyme synthesized by one species. Also called isoenzymes.
Iteroparity
A type of reproductive life history in which individuals are physiologically capable of two or more separate bouts of reproduction during their lives. Contrast with semelparity.
Iteroparous
Characterized by iteroparity.
Juvenile hormone
In insects, a lipid-soluble hormone secreted by the nonneural endocrine cells of the corpora allata that stimulates its target tissues to maintain immature characteristics.
Juxtaglomerular apparatus
An assembly of cells in a vertebrate nephron where the juxtaglomerular cells are found. See juxtaglomerular cells.
Juxtaglomerular cells
Specialized smooth muscle cells in the arterioles associated with the glomerulus of a vertebrate nephron that secrete renin in response to low blood pressure. Also called granular cells.
Kairomone
A chemical signal released by a member of one species in the course of its activities that is detected and exploited by a member of another species. Its detection confers a benefit on the detecting species that is detrimental to the releasing species.
kcat
Kidney
An organ that regulates the composition and volume of the blood and other extracellular body fluids by producing and eliminating from the body an aqueous solution (urine) derived from the blood or other extracellular fluids.
Km
See Michaelis constant.
Knockout animal
An animal that has been genetically engineered to lack functional copies of a gene of interest.
Labeled lines
The principle that sensory neurons encode the modality or quality of a sensory stimulus by having different sensory receptor cells respond to different kinds of stimuli, so that the CNS can decode the stimulus by monitoring which axons (“lines”) deliver action potentials.
Lactase persistence
In human biology, continued synthesis (expression) in adulthood of the digestive enzyme lactase, permitting milk sugar—lactose—to be digested in adulthood.
Lactation
In mammals, the process of producing milk and providing the milk to nursing offspring.
Lacunae
In an open circulatory system, minute spaces between nonvascular cells that serve as passageways for blood flow.
Laminar flow
Flow of a fluid without turbulence. When a fluid flows without turbulence over a surface or through a tube, the flow can be envisioned as occurring in a series of thin layers (laminae) of fluid at progressively greater distances from the surface or tube walls. The layer juxtaposed to the surface or tube wall does not flow at all, and layers at increasing distances from it flow faster and faster.
Landmark
A recognizable natural or artificial feature used for navigation, a feature that stands out from its near environment and may be visible from a long distance.
Laplace’s law
The tension developed within the walls of a hollow structure exposed to a particular difference in pressure between inside and outside is directly related to the radius of the structure.
Larva
A free-living developmental stage that is very different in appearance from the adult stage of the same species.
Latch state
A characteristic state of smooth muscle in which dephosphorylated myosin heads remain attached to actin and maintain tension for long periods of time.
Latent heat of vaporization
The heat that must be provided to convert a material from a liquid to a gas at constant temperature (called latent because although heat is provided, the temperature of the material does not rise). It is expressed per unit of mass of the material under study.
Lateral geniculate nucleus (LGN)
A region of the thalamus in the diencephalon of the vertebrate brain that receives axons of retinal ganglion cells and relays visual information to the primary visual cortex.
Lateral pathways
In the retina, neural pathways that extend parallel to the retinal surface, rather than from the external to the internal side. Contrast with straight-through pathways.
Leaflet
One of the two phospholipid layers within a cell membrane.
Leak channel
A channel in a cell membrane that is normally open and mediates the membrane permeability (primarily to K+ ions) that underlies the resting membrane potential.
Left-to-right shunting
In the study of blood flow through the central circulation of an air-breathing fish, amphibian, or nonavian reptile, diversion of pulmonary venous blood directly back to the lungs, bypassing the systemic circuit.
Length constant (λ)
The distance along a cell over which a change in passively spreading electrical potential decays in amplitude to 37% of its amplitude at the origin.
Length–tension relationship
The relationship between the length of a whole muscle or a sarcomere prior to stimulation and the tension developed during isometric contraction. Maximum tension develops when there is optimal overlap of thick and thin filaments to allow effective cross-bridge action.
Lengthening contraction
An isotonic contraction in which a muscle lengthens as it is activated and exerts force. Also called an eccentric contraction.
Leptin
A peptide hormone secreted by white adipose tissue (white fat) that in some animals tends to inhibit feeding.
Leydig cells
Testosterone-secreting cells located in the connective tissue between the seminiferous tubules of the vertebrate testis; also called interstitial cells.
LH surge
In mammalian reproduction, a sharp increase in a female’s blood concentration of luteinizing hormone, serving to cause ovulation.
Ligand
Any molecule that selectively binds noncovalently to a structurally and chemically complementary site on a specific protein. The substrate of an enzyme, for example, is a ligand of that enzyme.
Ligand-gated channel
A gated channel that opens to allow diffusion of a solute as a result of binding by a neurotransmitter (or other specific signaling molecule) to a receptor site on the channel protein.
Linear heat-transfer equation
An equation that relates heat loss from an animal to the difference between body temperature and ambient temperature by means of a proportionality coefficient. See Equation 10.9.
Linear velocity
In a moving fluid, the linear distance covered per unit of time.
Lipase
A type of lipid-digesting enzyme.
Load compensation
The ability to control the shortening of a muscle somewhat independently of the load against which it shortens.
Load
The force against which a contracting muscle exerts an opposing force, the latter being referred to as muscle tension.
Load–velocity relationship
The principle that the velocity of shortening of a muscle during isotonic contraction decreases as load increases.
Loading
Oxygenation of a respiratory pigment.
Long-term potentiation (LTP)
A stable, long-lasting increase in the amplitude of the response of a neuron after it has been stimulated repeatedly by presynaptic input at a high frequency.
Loop (circuit)
A loop circuit is one that feeds back onto itself; e.g., locus A sends a signal to B, which sends a signal back to A.
Loop of Henle
A portion of a nephron tubule shaped like a hairpin, found in the nephrons of mammals and some of the nephrons of birds, that is the site of countercurrent multiplication, the process responsible for production of urine hyperosmotic to the blood plasma.
Low-grade energy
Energy in a form that cannot do any physiological work (i.e., heat).
Lower-critical temperature
In a homeotherm, the ambient temperature that represents the lower limit of the thermoneutral zone. See also thermoneutral zone.
Luciferase
An enzyme that catalyzes oxidation of a luciferin, resulting in light production, during bioluminescence. Many different chemical forms of luciferase are known.
Luciferin
A compound capable of light emission during bioluminescence. Light emission occurs when luciferin is oxidized by an enzyme luciferase. Many different chemical forms of luciferin are known.
Lumen
The open central cavity or core of a “hollow” organ or tissue; for example, the open central core of a blood vessel.
Luminal
Related to the lumen.
Lungfish
See dipnoan.
Lungs
In the most general sense used by physiologists, structures specialized for external respiration that are invaginated into the body and thereby contain the environmental medium. Contrast with gills.
Luteal phase
In a female mammal, the ovarian phase of the menstrual or estrous cycle that follows ovulation, during which the corpus luteum forms, is functional, and then degenerates.
Luteinizing hormone (LH)
A tropic peptide chemical signal secreted by cells in the anterior pituitary gland (adenohypophysis); travels in the general circulation to the gonads; promotes secretion of sex hormones.
Lymph
In an animal with a closed circulatory system, the interstitial fluids. Lymph is often used in a more restrictive sense to refer specifically to excess volumes of interstitial fluid that are collected from the tissues and returned to the blood by the lymphatic vascular system.
Lymphatic vascular system
An elaborate system of vessels that approximately parallels the blood vascular system in vertebrates and serves to remove excess interstitial fluids (lymph) from tissues throughout the body, returning the fluids ultimately to the blood plasma.
M line
In muscle fibers, a web of accessory proteins at the center of a sarcomere that anchors the thick filaments and titin.
Macula
In the vertebrate ear, a sensory area in the vestibular organs containing hair cells that monitor tilt and acceleration of the head.
Magnetic compass
A mechanism by which an animal uses Earth’s magnetic field to determine compass direction in navigation.
Magnetoreception
A mechanism by which specialized animal cells are able to detect Earth’s magnetic field by direct or indirect means.
Magnocellular neurons
Large neurosecretory neurons with cell bodies in the supraoptic and paraventricular nuclei of the mammalian hypothalamus; synthesize oxytocin and vasopressins and secrete them into the general circulation from axon terminals in the pars nervosa
Maintenance
The set of processes that maintain the status quo in an animal’s body, including circulation, breathing, and tissue repair.
Malpighian tubules
Fine tubules that initiate urine formation in insects. They empty their product into the gut at the junction of the midgut and hindgut.
Mantle
In a mollusc, a sheet of tissue that secretes the shell (in those molluscs with a shell) and that encloses an external body cavity, the mantle cavity. In squids, the mantle is muscular and pressurizes fluid for jet propulsion.
Mantle cavity
In molluscs, an external body cavity formed where a sheetlike outfolding of the dorsal body wall, the mantle, overhangs or surrounds all or part of the rest of the body. The gills typically are suspended in the mantle cavity.
Navigation in which an animal has information about where it is in the world (the “map”) and about the direction it should take to reach a destination (the “compass”).
Maps
A map is a geometrical representation of space and spatial relationships. In animal navigation a map is an internally stored, geographical representation of a part of the world around an animal. A map may also refer to the topological representation in the brain of the body surface (somatotopic map) or the outside world (retinotopic map, auditory map).
Mark
See epigenetic mark.
Mass
An object’s mass is a measure of the amount of matter in the object. In physics, this is defined and measured in terms of inertia relative to a standard.
Mass action
The inherent tendency for the reactants and products of a chemical reaction to shift in their concentrations, by way of the reaction, until an equilibrium state—defined by particular concentration ratios—is achieved. Mass action impels reactions toward equilibrium, although other processes may operate against equilibrium and therefore prevent an equilibrium from actually being established.
Maximal aerobic power
A synonym for V̇O2max.
Maximal exercise
Exercise that requires an individual’s maximal rate of O2 consumption.
Maximum reaction velocity (Vmax)
The greatest rate at which an enzyme-catalyzed reaction can convert substrate to product with the number of active enzyme molecules that are present.
Mean pressure
During the beating cycle of the heart, the average blood pressure (averaged over the full beating cycle).
Mechanical energy
Energy of organized motion in which many molecules move simultaneously in the same direction. The energy of motion of a moving arm provides an example. Mechanical energy and heat are the two forms of kinetic energy.
Mechano growth factor
A locally acting peptide, produced by muscle fibers when they are stimulated to produce mechanical activity, that stimulates muscle satellite cells to proliferate and become incorporated into the muscle fibers.
Mechanoreceptor
A sensory receptor cell specialized to respond to mechanical stimulation.
Medulla
The interior tissue of a kidney. The interior tissue of the adrenal gland.
Melanocyte-stimulating hormone (MSH)
A direct acting hormone secreted by cells of the anterior pituitary gland (adenohypophysis); derived from proopiomelanocortin; promotes skin darkening in amphibians, reptiles (except birds), and fish.
Melatonin
An amine hormone derived from the amino acid tryptophan; synthesized in and secreted from the pineal gland; influences circadian and seasonal rhythms; promotes sleep.
Membrane-associated digestive enzyme
A digestive enzyme that is positioned in the apical membranes of epithelial cells lining the gut or another body cavity. Food materials in the gut lumen must make physical contact with the gut epithelium to be digested by this sort of enzyme.
Membrane fluidity
See fluidity.
Membrane potential
The potential difference (voltage) across a cell membrane or other selectively permeable membrane.
Membrane resistance (Rm)
The electrical resistance of a membrane per unit of area. Many cell membranes have an Rm of about 1000 ohm × cm2 (1000 Ω × cm2). (The unit is ohm × cm2 rather than ohm/ cm2 because membrane resistance decreases as area increases, as more resistances are added in parallel.)
Menarche
First menstruation.
Menstrual cycle
The cycle in which oocytes mature and are ovulated periodically in females of some primate species; one phase of each cycle is characterized by menstruation, the shedding of the uterine lining in a blood-tinged discharge from the vagina.
Mesencephalon
The midbrain region of the vertebrate brain.
Mesic
Moderately moist. “Mesic animals” are terrestrial animals that live in moderately moist environments.
Metabolic acidosis or alkalosis
Acidosis or alkalosis that is caused by abnormal excretion or retention of bicarbonate (HCO3–) in the body fluids. Contrast with respiratory acidosis or alkalosis.
Metabolic depression
A reduction in the ATP needs of an animal (or a specific tissue) to below the level ordinarily associated with rest in a way that does not present an immediate physiological threat to life.
Metabolic rate
An animal’s rate of energy consumption; the rate at which it converts chemical-bond energy to heat and external work.
Metabolic scaling
The regular allometric or quasi-allometric relationship that typically exists between metabolic rate and body weight within sets of related species. For example, small-bodied mammals have higher rates of metabolism per unit of body weight than large-bodied species do. This relationship, which is approximately allometric, represents a case of metabolic scaling.
Metabolic water
Water that is formed by chemical reaction within the body. For example, when glucose is oxidized, one of the products is H2O that did not previously exist. Also called oxidation water. Contrast with preformed water. See also net metabolic water production.
Metabolism
The set of processes by which cells and organisms acquire, rearrange, and void commodities (e.g., elements or energy) in ways that sustain life.
Metabolism–size relation
See metabolic scaling.
Metabolite
An organic molecule of relatively low molecular weight (e.g., glucose, an amino acid, or lactic acid) that is currently being processed by metabolism.
Metabolomics
The study of all the organic compounds in cells or tissues other than macromolecules coded by the genome. The molecules encompassed by metabolomics are generally of relatively low molecular weight (roughly <1500 daltons).
Metabotropic receptor (in synaptic function)
A neurotransmitter receptor that acts via signal transduction to alter a metabolic function of the postsynaptic cell, often by stimulating production of a second messenger.
Metabotropic transduction (in sensory function)
Sensory transduction by means of a signal transduction cascade, rather than by direct ionotropic action. The sensory receptor molecule is a G protein that activates a second messenger, ultimately producing a receptor potential. See also ionotropic transduction.
Metalloprotein
A protein that includes one or more metal atoms in its structure.
Metamorphosis
The process of changing from one body form to another, such as changing from a larva to an adult.
Metencephalon
The anterior part of the vertebrate hindbrain, containing the cerebellum and pons.
Methylation
Covalent bonding of a methyl group (—CH3) to another molecule.
Methylome
In the study of epigenetics, the sum total of an animal’s methylated DNA sites.
Michaelis constant (Km)
The half-saturation constant of an enzyme-catalyzed reaction that exhibits hyperbolic kinetics—that is, the concentration of substrate at which the reaction velocity is half of the maximal velocity. This constant is a measure of enzyme–substrate affinity.
Michaelis–Menten equation
An equation that describes the relation between reaction velocity and substrate concentration in an enzyme-catalyzed reaction that exhibits hyperbolic kinetics. See Equation 2.2.
Microarray
See DNA microarray.
Microbiome
See gut microbiome.
Microcirculatory bed
In a closed circulatory system, one of the systems of microscopically fine blood vessels that connect arteries and veins, weaving among ordinary tissue cells. In vertebrate systemic tissues, microcirculatory beds consist of arterioles, capillaries, and venules.
Microclimate
The set of climatic conditions (temperature, humidity, wind speed, and so forth) prevailing in a subpart of a larger environmental system.
Microenvironment
A place within a larger environment in which the physical and chemical conditions differ significantly from the average conditions characterizing the larger environment.
Microvilli
Microscopic finger-shaped projections from the apical membranes of certain types of epithelial cells. Microvilli occur most commonly in epithelia that are active in secretion or reabsorption.
Midbrain
Middle region of the vertebrate brain, between the forebrain and the hindbrain.
Middle ear
The portion of the vertebrate ear between the tympanic membrane and the cochlea.
Midgut
In a person, the small intestines. More generally, the intitial part of the intestines.
Migration
A seasonal or other periodic movement of animals from one geographic region to another.
Milieu intérieur
See internal environment.
Milk ejection (or let-down)
In a lactating female mammal, release of milk from the mammary glands under positive pressure.
Miniature EPSP (mEPSP)
A small excitatory postsynaptic potential at a neuromuscular junction or postsynaptic neuron produced by presynaptic release of a single quantal packet of neurotransmitter.
Minute volume
The amount of air, water, or blood pumped per minute in a breathing system or circulatory system.
Mirror neuron
A neuron in the primate brain that is activated when the animal performs a particular movement, or watches another animal perform the same movement.
Mitochondria-rich cell
A type of epithelial cell specialized for ion transport, found particularly in the gill epithelia of fish but also in some other epithelia of aquatic animals. Characterized by an abundance of mitochondria and other signs of secretory or absorptive activity. Also called a chloride cell.
Mixed venous blood
The blood in the great veins leading back to the heart, formed by mixing of the venous blood coming from the various regions and organs of the body.
Modality
The subjective sensation of a particular sense, such as sight, taste, or hearing.
Molal
A unit of measure of chemical concentration; specifically, the molal concentration is the number of moles mixed with a kilogram of water to make a solution.
Molar
A unit of measure of chemical concentration; specifically, the molar concentration is the number of moles dissolved in a liter of solution.
Mole (mol)
A set of 6.022 × 1023 items (i.e., an Avogadro’s number of items). For example, a mole of a chemical compound contains 6.022 × 1023 molecules of the compound. The mass in grams of a mole of a chemical compound is identical to the formula mass of the compound. Thus, if you have 18 g of water, a compound that has a formula mass of 18, you have one mole of water, or 6.022 × 1023 water molecules. See also Avogadro’s number, osmole.
Molecular chaperones
Proteins that use energy from ATP to guide the folding of other proteins into correct three-dimensional configurations. Molecular chaperones ensure that target proteins assume correct tertiary structures during initial synthesis, and they sometimes are able to guide reversibly denatured proteins back to functional conformations, thereby preventing permanent denaturation.
Molecular kinetic energy
See heat.
Molting hormone
See ecdysone.
Molting
See ecdysis.
Molts
Processes that cast off body coverings, such as feathers, fur, and the cuticle (by ecdysis in hemimetabolous and holometabolous insects).
Monounsaturated fatty acid
Motility
Any sort of muscular activity by the gut, such as peristalsis or segmentation.
Motor cortex
The region of the mammalian cerebral cortex that lies anterior to the central sulcus and is concerned with motor behavior; includes the primary motor cortex and associated motor areas in the frontal lobe.
Motor neuron
A neuron that conveys motor signals from the central nervous system to the periphery to control an effector such as skeletal muscle.
Motor unit
A motor neuron and all the muscle fibers it innervates.
Mucosal surface
See apical surface.
Müllerian inhibitory substance
A glycoprotein hormone secreted by Sertoli cells in the developing testes of genetic males that causes degeneration of the Müllerian ducts, which would otherwise give rise to the uterus and oviducts. Also called anti-Müllerian hormone, Müllerian inhibitory hormone.
Multiterminal innervation
A pattern of innervation in which a single axon branches near its end to make many synaptic contacts along the length of a muscle fiber.
Multiunit smooth muscles
A muscle made up of unstriated muscle cells that are not connected to neighboring cells by gap junctions; receive input from the autonomic nervous system, as well as endocrine and paracrine stimuli. Contrast with single-unit smooth muscle.
Muscarinic
Action of acetylcholine that is mimicked by muscarine, or receptors mediating the action. Contrast with nicotinic.
Muscle
A group of muscle cells (muscle fibers) and associated tissues. It may be smooth, skeletal, or cardiac.
Muscle cells
contractile cells that constitute muscle tissues.
Muscle fiber
A term typically used to refer to skeletal or cardiac muscle cells, but sometimes also applied to smooth muscle cells. This term, now used broadly, was initially introduced to emphasize the long, fibrous appearance of certain muscle cells, such as vertebrate skeletal muscle cells (fibers), that are multinucleate because they are formed during development by the fusion of two or more embryonic myoblasts.
Muscle spindle
A stretch receptor that is arranged in parallel with the tension-producing fibers of a muscle and sends action potentials to the central nervous system when the muscle is stretched.
Myelencephalon
Posterior part of the vertebrate hindbrain.
Myelin
An insulating sheath around an axon, composed of multiple wrappings of glial cell membranes, that increases the velocity of propagation of action potentials.
Myoblasts
uninucleate embryonic muscle cells that fuse to form multinucleate muscle fibers
Myocardium
The muscle tissue of a heart.
Myofibril
A longitudinal component of a striated muscle cell that consists of a series of sarcomeres and extends the length of the cell. In cross section, a muscle cell consists of multiple myofibrils, each surrounded by a sleeve of sarcoplasmic reticulum.
Myofilament
Either of the two types of longitudinal components of sarcomeres. Thick myofilaments consist of polymerized myosin molecules, and thin myofilaments consist of polymerized actin molecules.
Myogenic heart
A heart in which the electrical impulse to contract during each beating cycle originates in muscle cells or modified muscle cells. Contrast with neurogenic heart.
Myoglobin
Any type of hemoglobin found in the cytoplasm of muscle.
Myometrium
In the mammalian uterus, the outer tissue layer, consisting of smooth muscle.
Myosin
A molecular motor found in many types of cells that converts chemical energy of ATP into mechanical energy of motion. In muscle cells, it functions as a contractile protein. Myosin monomers polymerize to form thick myofilaments.
Myostatin
An inhibitory growth factor that regulates the growth of skeletal muscles by tempering hyperplasia in developing muscles and hypertrophy of adult muscle fibers.
Na+–K+-ATPase
A ubiquitous and extremely important transporter protein that directly cleaves ATP molecules to release energy (it is an ATPase) and uses the energy to transport Na+ and K+ ions in a 3:2 ratio (a case of primary active transport). The ion gradients created are often used as energy sources for secondary active transport.
Na+–K+ pump
Active transport of Na+ out of a cell and K+ into a cell by Na+–K+-ATPase.
Natriuretic
Promoting loss of sodium in the urine.
The act of moving on a particular course or toward a specific destination using sensory cues to determine direction and position.
Nebulin
In a sarcomere, a large inelastic protein that extends along the thin filament from the Z disc to the margin of the H zone.
Needle biopsy
A procedure that uses a specially constructed needle to insert into a tissue and extract a small sample for study.
Negative feedback
A process by which the deviations of a property from a specific set-point level are opposed, thereby tending to keep the property at the set-point level.
Nephridium (plural nephridia)
A term used to refer to a kidney in certain types of animals such as molluscs.
Nephron
One of the tubules that forms urine in the kidney of a vertebrate.
Nernst equation
An equation used to determine the equilibrium electrical potential for a particular ion, given the ion concentrations on both sides of a membrane.
Nerve
A collection of axons running together in the peripheral nervous system.
Nerve impulse
See action potential.
Nerve net
A simple, uncentralized, and unpolarized network of neurons, found in cnidarians and locally in many other groups and considered to be a primitive stage in the evolution of nervous systems.
Nervous system
An organized constellation of neurons and glial cells specialized for repeated conduction of electrical signals (action potentials) within and between cells. These signals pass from sensory receptors and neurons to other neurons and effectors. Nervous systems integrate the signals of convergent neurons, generate new signals, and modify the properties of neurons based on their interactions.
Net growth efficiency
In a growing animal, the chemical-bond energy of new biomass added by growth expressed as a ratio of the animal’s absorbed energy over the same time period.
Net metabolic water production
The production of metabolic water by a process minus the losses of water that are obligatory for that process to take place.
Network oscillator
A neural circuit that acts as a central pattern generator as a result of the synaptic interaction of its constituent neurons, rather than as a result of oscillator activity in single cells.
Neurogenic heart
A heart in which the electrical impulse to contract during each beating cycle originates in neurons. Contrast with myogenic heart.
Neuroglia
See glial cells.
Neurohemal organ
An organ made up of axon terminals of neurosecretory cells in association with a well-developed bed of capillaries or other circulatory specializations, in which the axon terminals store neurohormones and secrete them into the blood.
Neurohormone
A hormone secreted by a neuron (also called a neuroendocrine cell or neurosecretory cell) into the blood.
Neurohypophysis
The neuroendocrine portion of the vertebrate pituitary gland in which neurohormones are released from axon terminals. It is an extension of the brain, commonly called the posterior pituitary. See posterior pituitary.
Neuromuscular junction
The synaptic junction of a motor neuron and a muscle fiber.
Neuron
A nerve cell; the fundamental signaling unit of the nervous system, composed of a cell body and elongated processes—dendrites and axon—that carry electrical signals.
Neuron doctrine
The theory that the nervous system, like other organ systems, is composed of discrete cellular elements (neurons) that are its fundamental signaling elements.
Neuronal integration
The process by which a postsynaptic neuron sums the inputs from several presynaptic neurons to control its generation of action potentials.
Neuropil
A CNS region of small neural processes and synapses that is relatively devoid of neural cell bodies.
Neurosecretory cell
A neuron that synthesizes and releases hormones.
Neurosecretory glands
Endocrine structures made of neurosecretory cells.
Neurotransmitter
A molecule that is used as a chemical signal in synaptic transmission.
Neutral pH
The pH of pure water. The neutral pH varies with temperature; at any given temperature, pH values less that the neutral pH represent an acid solution, whereas pH values above the neutral pH represent an alkaline solution.
Newton’s law of cooling
A common, although not necessarily historically defensible, synonym for the linear heat transfer equation.
Nicotinic
Action of acetylcholine that is mimicked by nicotine, or receptors mediating the action. Contrast with muscarinic.
Nocturnal
Active in the nighttime.
Node of Ranvier
In the myelin sheath surrounding an axon, spaces between adjacent glial cells. These interruptions in the sheath allow propagation of action potentials by saltatory conduction.
Nonadaptive evolution
Evolution that occurs by processes other than natural selection and that therefore can produce traits that are not adaptations. See also evolution, genetic drift, pleiotropy.
Noncovalent bond
A chemical bond that does not involve covalent bonding. Because they are not covalent, noncovalent bonds are flexible rather than rigid, and they can be made and broken with relative ease, without enzyme catalysis. There are four types: hydrogen bonds, ionic bonds, van der Waals interactions, and hydrophobic bonds. Also called weak bonds.
Nonelectrogenic
See electroneutral pump.
Nonessential amino acid
A standard amino acid that an animal can synthesize for itself.
Nonneural endocrine cells
Cells that synthesize and secrete hormones; also called epithelial endocrine cells
Nonneural glands
Endocrine structures made of nonneural endocrine cells; also called epithelial glands
Nonpolar molecule
A molecule in which electrons are evenly distributed, the various regions of which are all therefore similar in charge.
Nonshivering thermogenesis (NST)
In mammals and some birds, elevation of heat production for thermoregulation by means other than shivering. The same mechanisms potentially function in body weight regulation by serving to get rid of the energy value of excess organic food molecules in the form of heat.
Nonspiking neuron
A neuron that transmits information without generating action potentials.
Norm of reaction
In phenotypic plasticity, the specific relations between environments and phenotypes. If environment A causes phenotype M to be expressed, and environment B causes phenotype N to be expressed, the relations A-leads-to-M and B-leads-to-N constitute the norm of reaction.
Nucleating agent
See ice-nucleating agent.
Nucleus (of nervous system)
A cluster of functionally related neuronal cell bodies in a vertebrate central nervous system.
Nutrition
The study of the chemical components of animal bodies and how animals are able to synthesize those chemical components from the chemical materials they collect from their environments.
Obscurin
A giant protein in skeletal muscle fibers that links the M-line to the sarcoplasmic reticulum; contributes to maintaining the structural organization of the muscle fiber.
Off-center cell
A neuron in the retina that is hyperpolarized in response to light in the center of its receptive field. Contrast with on-center cell.
Ohm’s law
The relationship of voltage (V), current (I), and resistance (R): V = IR.
Olfaction
The sense of smell; chemoreception of molecules released at a distance away from the animal. Among chemoreceptors, olfactory receptors are typically more sensitive than taste receptors and respond to distant or dilute chemical stimuli (odorants) that are usually airborne in terrestrial animals.
Oligodendrocyte
A type of ensheathing glial cell (non-neuron cell) in the vertebrate central nervous system.
Omega 3 and omega 6 fatty acids
Fatty acids characterized by double bonds at particular points in their molecular structures. If the first double bond encountered when scanning a fatty acid molecule from its methyl (—CH3) end is the third bond in the carbon-chain backbone of the molecule, the fatty acid is an omega 3 fatty acid. If the first double bond occurs at the sixth position, the fatty acid is of the omega 6 type.
On-center cell
A neuron in the retina that is depolarized in response to light in the center of its receptive field. Contrast with off-center cell.
Oncotic pressure
Synonym for colloid osmotic pressure.
Ontogeny
Individual development.
Oogenesis
The formation of haploid eggs through the process of meiosis in the ovary.
Oogonia
Diploid cells in the ovary that proliferate by mitosis, then go through meiosis to give rise to female gametes (ova). In elasmobranchs, birds, and most mammals, oogonia are generally thought to proliferate only during embryonic or fetal life and remain suspended in an early stage of meiosis until the female becomes reproductively mature.
Open circulatory system
A circulatory system in which the blood leaves discrete vessels and bathes at least some nonvascular tissues directly, meaning that blood and interstitial fluid are the same. Contrast with closed circulatory system.
Open system
A defined part of the material universe that is not isolated, meaning that it is capable of exchanging matter, energy, or both with its surroundings.
Operculum
In a fish, the external flap on each side of the head that covers the gills.
Opsin
The protein part of the photopigment rhodopsin, which is a G protein–coupled receptor molecule.
Optic flow
The pattern of apparent motion of objects and images in a visual field that is caused by the movement of an animal relative to its surroundings.
Optogenetics
A type of manipulative method for study of neuron function (or function of other excitable cells) in which light-gated (light-modulated) ion channels—obtained from microorganisms—are expressed (through genetic engineering) in neuronal membranes of animals. Light signals can then be used to modulate the membrane potential of the neuronal membranes—for example, by inducing depolarization on very short time scales.
Ordinary least squares regression
See Appendix D.
Organ of Corti
A region of the cochlea in the vertebrate ear containing the inner and outer hair cells that transduce sound vibrations into electrical signals.
Orientation
The way an organism positions itself in relation to environmental cues.
Ornithine–urea cycle
A cyclic metabolic pathway that produces the nitrogenous end product urea.
Oscillator
Something that oscillates—moves rhythmically around a central point. A pendulum is an oscillator. Theoretical descriptions of central pattern generators refer to them as oscillators.
Osmoconformer
An aquatic animal that allows its blood osmotic pressure to match and vary with the environmental osmotic pressure. Also called a poikilosmotic animal.
Osmoconformity
See osmotic conformity.
Osmolar (Osm)
unit of measure of osmotic pressure. A 1-osmolar solution is defined to be a solution that behaves osmotically as if it has one Avogadro’s number of independent dissolved entities per liter.
Osmolarity
The osmotic pressure of a solution expressed in osmolar units. See osmolar.
Osmole
An Avogadro’s number of osmotically effective dissolved entities; a set of 6.022 × 1023 osmotically effective dissolved entities.
Osmolyte
Any solute that exerts a sufficiently large effect on the osmotic pressure of a body fluid to be of consequence for water–salt physiology.
Osmoregulation
See osmotic regulation.
Osmoregulator
An animal that maintains an approximately constant blood osmotic pressure even as the osmotic pressure of its environment varies.
Osmosis
The passive transport of water across a membrane.
Osmotic conformity
A state in which the osmotic pressure of the body fluids matches, and varies with, the osmotic pressure in the external environment.
Osmotic effector
An osmolyte that an individual animal or individual cell increases or decreases in amount to achieve the regulation of the osmotic pressure of a body fluid or the regulation of cell volume. Also called an osmoticum.
Osmotic permeability
Permeability to water moving by osmosis. See permeability.
Osmotic pressure
The property of a solution that allows one to predict whether the solution will gain or lose water by osmosis when it undergoes exchange with another solution; osmosis is always from lower osmotic pressure to higher osmotic pressure. An alternative, measurement-oriented definition is that the osmotic pressure of a solution is the difference in hydrostatic pressure that must be created between the solution and pure water to prevent any net osmotic movement of water when the solution and the pure water are separated by a semipermeable membrane. A colligative property.
Osmotic regulation
The maintenance of a constant or nearly constant osmotic pressure in body fluids regardless of the osmotic pressure in the external environment.
Osmotically free water
Water that is excreted in the urine above and beyond whatever water is absolutely required for solute excretion.
Osmoticum
See osmotic effector.
Ossicles
The ossicles of the middle ear (maleus, incus, stapes) are three bones, attached to each other in a series, that convey sound pressure waves from the tympanic membrane (eardrum) to the liquid-filled cochlea of the inner ear.
Ostia
In the heart of an arthropod, valved passages through the heart muscle through which blood enters the heart chamber.
Otolith organs
An otolith is a hard structure in the ear of a vertebrate (oto = ear, lith = stone) that stimulates hair cells to produce the sense of gravity and linear acceleration. The saccule and utricule of the inner ear are the otolith organs that mediate these senses.
Outer hair cells
In the organ of Corti of the cochlea, the three rows of hair cells that amplify the sound-produced local movements that stimulate the inner hair cells to activate cochlear sensory neurons.
Outer segment
The part of a retinal rod or cone that contains stacked membranes of photopigment and transduces light into an electrical signal.
Overexpression
A genetic manipulation whereby cells or tissues produce unusually large quantities of one or more messenger RNAs of interest.
Ovulation
The process of releasing an egg (ovum) from the ovary.
Ovum
A haploid gamete (egg) produced in the ovary.
Oxidation
Removal of electrons or hydrogen atoms from a molecule.
Oxidation water
Synonym for metabolic water.
Oxidative phosphorylation
The formation of ATP using energy released by the transport of electrons through the electron-transport chain.
Oxidative stress
See oxygen stress.
Oxy-
A prefix referring to a molecule that is capable of reversible combination with O2 that is in a state of being so combined (e.g., oxyhemoglobin).
Oxygen affinity
The readiness with which a respiratory pigment such as hemoglobin combines with O2. When oxygen affinity is high, a low O2 partial pressure is sufficient to cause extensive oxygenation.
Oxygen cascade
The sequential drop in the partial pressure of O2 from one step to the next in the series of steps by which O2 is transported from the environment of an animal to the animal’s mitochondria.
Oxygen conformity
A response in which an animal exposed to a decreasing O2 concentration in its environment allows its rate of O2 consumption to decrease in parallel.
Oxygen debt
An older term for excess postexercise oxygen consumption.
Oxygen deficit
A difference between actual O2 uptake by breathing and the theoretical O2 requirement of exercise during the first minutes at the start of exercise. During the oxygen deficit phase, aerobic catabolism based on O2 uptake by breathing is unable to meet fully the ATP requirement of exercise, and other ATP-producing mechanisms (such as phosphagen use) must contribute ATP.
Oxygen dissociation curve
Oxygen equilibrium curve
Referring to the O2-carrying properties of blood, a graph of the amount of O2 per unit of blood volume as a function of the O2 partial pressure of the blood. Also called the oxygen dissociation curve.
Oxygen regulation
A response in which an animal exposed to a decreasing O2 concentration in its environment maintains a stable rate of O2 consumption.
Oxygen stress
Collectively speaking, the destructive effects of reactive oxygen species on functional or structural properties of cells or tissues. See reactive oxygen species.
Oxygen utilization coefficient
(1) In breathing, the fraction (or percentage) of the total O2 in respired air or water that is removed by the breathing process. (2) In circulation, the fraction of the total O2 carried by blood that is removed from the blood as it passes around the body.
Oxygen-carrying capacity
In a body fluid (e.g., blood) that contains a respiratory pigment, the amount of O2 per unit of fluid volume when the respiratory pigment is saturated with O2. More generally, the maximal amount of O2 per unit of volume in a fluid under ordinary physiological conditions.
Oxygen-transport pigment
See respiratory pigment.
Oxygenation
(1) Referring to water, the dissolution of O2 in that water. (2) Referring to respiratory pigments, the combination of those pigments with O2; oxygenation of this sort is reversible and not equivalent to oxidation.
Oxyhemoglobin
Hemoglobin combined with O2.
Oxytocin
A vertebrate neurohormone synthesized by neurosecretory cells that have their cell bodies in the hypothalamus and their axon terminals in the pars nervosa of the posterior pituitary. Its major functions are to stimulate contraction of myoepithelial cells of the mammary glands to cause milk ejection and contraction of the myometrium during parturition.
P-type ATPases
One of the major families of ATPases (transporter proteins that directly cleave ATP to obtain energy for active transport).
P/O ratio
During electron transport and oxidative phosphorylation, the number of ATP molecules produced per oxygen atom reduced to H2O. A measure of the degree of coupling that exists between electron transport and oxidative phosphorylation.
P450 enzymes
A set of inducible enzymes that play roles in in the detoxification of foreign compounds. See cytochrome P450 enzymes.
P50
The partial pressure of O2 that causes a blood or respiratory pigment to become 50% saturated with O2. A measure of O2 affinity; a high P50 signifies a low O2 affinity.
Pacemaker
The cell or set of cells that spontaneously initiates a rhythm. In a heart, for example, the pacemaker is the cell or set of cells that spontaneously initiates the rhythmic contractions of the heart muscle.
Pacemaker potential
An intrinsic depolarization of a neuron or other excitable cell, leading to an action potential and thus making the cell spontaneously active.
Panting
An increase in the rate of breathing; often serves to increase the rate of evaporative cooling by increasing air flow over moist surfaces of the airways of the breathing system.
Papilla
A nipple-shaped structure. Mammalian taste buds are collected in lingual papillae on the tongue surface.
Parabronchi
The smallest-diameter tubes in the lungs of a bird. They are numerous and collectively constitute most of the lung tissue. Air capillaries—the sites of O2 and CO2 exchange—connect to the lumens of the parabronchi.
Paracellular
Referring to transport across an epithelium, transport that occurs between cells (rather than through cells).
Paracrine
A locally acting chemical signal that binds to receptors and exerts a regulatory effect on cells in the neighborhood of the cell that released it.
Parallel
In reference to the arrangement of parts in an electrical circuit, vascular system, or other analogous system in which substances flow from place to place, the parts of the system are in parallel if one path of flow branches to give rise to two or more paths in which the parts reside, so that only a fraction of the total flow passes through any one of the parts. Contrast with series.
Parasympathetic division
A division of the vertebrate autonomic nervous system that is connected to the CNS via cranial and sacral nerves; the parasympathetic and sympathetic divisions tend to exert opposing controls on autonomic effectors.
Paraventricular nucleus
A cluster of neurons in the hypothalamus; contains magnocellular neuroendocrine cell bodies that extend their axon terminals to the pars nervosa, where they secrete oxytocin and vasopressin into the general circulation.
Paravertebral ganglia
Ganglia of the sympathetic nervous system that are next to vertebrae.
Parthenogenesis
A form of asexual reproduction in which eggs produced by females develop into genetically identical clones of their female parent.
Partial compensation
See compensation.
Partial pressure
(1) The pressure exerted by a particular gas within a mixture of gases. In terms of the pressure it exerts (its partial pressure), each gas in a mixture behaves as if it alone occupies the entire volume occupied by the mixture; thus each gas’s partial pressure can be calculated from the universal gas law. (2) The concept of partial pressure is also applied to gases dissolved in aqueous solution: The partial pressure of a gas in aqueous solution is equal to the partial pressure of the same gas in a gas phase with which the solution is at equilibrium. Gas tension is synonymous with partial pressure for a gas in solution. See also universal gas law.
Parvocellular neurons
Neurons characterized by small cell bodies. In the hypothalamus, they synthesize and secrete releasing and inhibiting hormones that are carried in the hypothalamo–hypophysial portal vessels to the anterior pituitary gland (adenohypophysis).
Passive change in lung volume
In the study of ventilation, a change in lung volume that is a consequence of elastic rebound, not driven by muscle action. Contrast with active change in lung volume.
Passive electrical properties
The electrical properties of a cell that do not involve a change in membrane ion permeability, and thus involve no change in electrical resistance.
Passive exhalation
Exhalation that occurs without muscular contraction, usually driven by elastic rebound of structures stretched during inhalation.
Passive spread
See decremental spread.
Passive transport
The transport of a material by a mechanism that is capable of carrying the material only in the direction of equilibrium.
Patch-clamp recording
A method of measuring single-channel currents by sealing a glass capillary microelectrode to a patch of cell membrane. Other conformations of patch clamping can measure whole-cell current or voltage. See also single-channel current recording.
Path integration
An animal’s summation of the distances and directions of its past movements, so that the animal knows where it is (relative to home) without using landmarks.
Pay-as-you-go phase
A phase during exercise when aerobic catabolism using O2 taken up by breathing is meeting the full ATP requirement of the exercise.
Pejus temperatures
In the study of poikilotherms, a range of body temperatures in which animal performance deteriorates as body temperature is gradually raised (upper pejus range) or lowered (lower pejus range). Pejus means turning worse. Contrast with critical temperature.
Peptide hormones
Also called protein hormones; made of assemblages of amino acids and soluble in water (polar).
Perfusion
The forced flow of blood through blood vessels.
Pericardial sinus
A fluid-filled space or cavity surrounding the heart.
Perikaryon
See cell body.
Period
The length (in time or space) of a single cycle of a rhythmic activity.
Periodic breathing
Peripheral activation
Conversion of a hormone after secretion to a more physiologically potent form.
Peripheral control
Control of motor output to produce rhythmic movements, for which peripheral sensory receptors are needed to provide timing information for each stage of the sequence. Contrast with central control.
Peripheral ganglion
A collection of neuronal cell bodies outside the CNS.
Peripheral membrane protein
A protein that is associated with a cell membrane, typically on just one side or the other, but that can be removed without destroying the membrane.
Peripheral nervous system (PNS)
The portion of a nervous system outside of the central nervous system, consisting of afferent and efferent nerves that connect the central nervous system to various parts of the body.
Peristalsis
One of the gut’s principal modes of muscular activity, in which constriction of the gut at one point along its length initiates constriction at a neighboring point farther along the gut, producing a “wave” of constriction that moves progressively along the gut, propelling food material before it.
Permeability
In reference to a cell membrane or epithelium, the ease with which a particular solute can move through it by diffusion, or the ease with which water can move through it by osmosis.
Permeating
In reference to a solute, able to pass through a cell membrane or epithelium.
Permissiveness
A type of relationship between hormones in which one hormone must be present to allow another hormone to exert its effect.
PGC-1α1
An isomer of the transcriptional coactivator PGC-1α that drives the transcription of genes involved in producing the oxidative phenotype of muscle fibers, including angiogenesis, mitochondrial biogenesis, and the expression of myosin heavy chains found in oxidative muscle fibers.
PGC-1α4
An isomer of the transcriptional coactivator PGC-1α that drives the transcription of genes involved in downregulating myostatin and upregulating IGF-1, a combination that leads to hypertrophy and contractile strength.
pH
The negative of the common logarithm of the concentration of H+. The pH is inversely related to the concentration of H+ (acidity). A low pH signifies acid conditions, whereas a high pH signifies alkaline conditions. See also neutral pH.
Phasic receptor
See rapidly adapting receptor.
Phasic smooth muscles
Smooth muscles that produce rapid, intermittent contractions; the cells produce spontaneous action potentials and are electrically connected by gap junctions.
Phasing factor
An environmental cue that can entrain a biological clock, synchronizing it to environmental changes (such as the daily light-dark cycle). Also called a zeitgeber.
Phenotypic plasticity
The ability of an individual animal to express two or more genetically controlled phenotypes.
Pheromone
A chemical signal that conveys information between two or more individuals that are members of the same species. It typically signals the sexual readiness or social status of the releasing animal and triggers stereotyped behaviors or physiological changes in the detecting animal.
Phosphagens
Compounds that can donate high-energy phosphate bonds to ADP to make ATP; thus, compounds that act as stores of high-energy phosphate bonds. The two most common are creatine phosphate and arginine phosphate.
Phospholipid
A lipid compound in which a phosphate group or groups occur. A typical membrane phospholipid consists of two hydrocarbon tails linked by a phosphate group to a compound such as choline.
Phosphorylation
Covalent attachment of an orthophosphate group to an organic compound.
Photoautotroph
An organism that can obtain the energy it needs to stay alive from photons; an organism capable of photosynthesis.
Photochemical reaction
A chemical reaction triggered by light absorption.
Photoperiod
Day length; the number of hours of daylight in a 24-hour day.
Photopigment
An unstable pigment molecule that undergoes a chemical change when it absorbs light, e.g., rhodopsin.
Photoprotein
A type of chemical complex instrumental in bioluminescence in some marine animals. The complex consists of luciferin, O2, and an inactive form of catalyzing protein. Initiation of light production is often dependent on exposure of the photoprotein to Ca2+ or Mg2+.
Photoreception
Response of a sensory cell to light stimulation. Photoreceptor cells contain a photopigment that absorbs light and triggers a response.
Photoreceptor
See Photoreception.
Photosynthetic autotroph
Synonym for photoautotroph.
Phototransduction
Generation of an electrical response in a photoreceptor cell in response to a light stimulus.
Phyletic
See phylogenetic.
Phylogenetic
Having to do with the evolutionary relationships of organisms; the patterns of relationship that organisms exhibit by virtue of common evolutionary descent. Also called phyletic.
Phylogenetic reconstruction
A reconstruction of the family tree (the ancestry) of groups of related species, often using molecular genetic data.
Phylogenetically independent contrasts
See Appendix G.
Physiological work
Any process carried out by an animal that increases order and requires energy. For example, an animal does physiological work when it synthesizes proteins, generates electrical or chemical gradients by actively transporting solutes across cell membranes, or contracts muscles to move materials inside or outside its body.
Physiology
The study of function; the study of “how organisms work.”
Pilomotor
Related to the erection or compression of the hairs in the fur (pelage) of a mammal under the control of muscles attached to the bases of the hairs.
Piloting
The act of navigation by visual landmarks.
Pineal gland
A small endocrine gland in the vertebrate brain that produces melatonin.
Place cell
A neuron in the hippocampus that generates action potentials when an animal is in a particular part of its familiar environment. Place cells are so named because their activity encodes the spatial position of the animal.
Place theory
The theory that the pitch or frequency of a sound is encoded by the place of maximal vibration and maximal hair cell stimulation along the length of the basilar membrane of the cochlea of the ear.
Placenta
An organ formed jointly by the embryo and the mother in marsupial and eutherian mammals. It allows intimate juxtaposition (but not mixing) of the maternal bloodstream and the embryonic or fetal bloodstream for the exchange of materials.
Plasma clearance
The volume of blood plasma that would have to be completely cleared of a solute to obtain the amount of that solute excreted in urine over a specified period of time.
Plasma membrane
See cell membrane.
Plasma
See blood plasma.
Pleiotropy
The control of two or more distinct and seemingly unrelated traits by an allele of a single gene.
Podocytes
Distinctive cells with numerous processes that occur in structures where ultrafiltration occurs, such as the vertebrate renal glomerulus. Intricate geometric arrays of the cell processes of multiple podocytes are believed to form a critical part of the filter that determines which solutes can and cannot pass through during ultrafiltration.
Poikilosmotic animal
See osmoconformer.
Poikilotherm
An animal in which the body temperature is determined by equilibration of the body with the thermal conditions in the environment. Also called an ectotherm.
Poiseuille equation
An equation that describes the quantitative relation between the rate of flow of fluid through a horizontal tube and factors such as pressure, luminal radius, and length. The equation states that flow rate depends directly on the fourth power of the luminal radius. See Equation 25.2. Also called the Hagen-Poiseuille equation.
Polar molecule
A molecule in which electrons are unevenly distributed, so that some regions of the molecule are relatively negative while others are relatively positive.
Polarized light
In a light beam, polarization state refers to the electric field, which in general vibrates orthogonally (at right angles) to the direction in which a light wave moves. In polarized light, the vibration direction is restricted (e.g., to a plane in plane polarized light); whereas, in unpolarized light, the vibration direction is unrestricted (i.e., vibrates in all directions orthogonal to the beam). See Figure 18.6.
Polycythemia
A state of having an unusually high concentration of red blood cells in the blood; seen, for example, in humans and some other lowland mammals when they acclimatize to high altitudes.
Polyneuronal innervation
A pattern of innervation in which a single muscle fiber receives synaptic contacts from more than one motor neuron.
Polyphenic development
A developmental phenomenon in which one individual—or a set of genetically identical individuals—can assume two or more discrete, highly distinct body forms, induced by differences in the developmental environment. Particularly common in insects.
Polyphenism
See polyphenic development and seasonal polyphenism.
Polysaccharide
A carbohydrate molecule that consists of many simple-sugar (monosaccharide) molecules polymerized together. Starch, cellulose, and chitin are important examples.
Polyunsaturated fatty acid
Positive feedback
A process by which deviations of a property from a specific set-point level are reinforced, thereby tending to cause escalating changes in the property.
Postabsorptive
Referring to an animal that has not eaten for a sufficiently long time that the specific dynamic action of its last meal is over. See specific dynamic action.
Posterior pituitary
The neuroendocrine portion of the vertebrate pituitary gland in which neurohormones are released from axon terminals. The posterior pituitary is an extension of the brain. Also called the neurohypophysis.
Postganglionic neuron
In the autonomic nervous system, a neuron that extends from an autonomic ganglion to an effector.
Postgenomic
Referring to the study of an animal species or other organism after its full genome is known.
Postpartum estrus
In a female mammal, estrus that occurs soon after she has given birth.
Postsynaptic cell
A neuron or effector cell that receives a signal (chemical or electrical) from a presynaptic cell at a synapse.
Postsynaptic density
An accumulation of dense material beneath the membrane of a postsynaptic cell, visible in electron micrographs. It corresponds to the accumulation of proteins associated with the concentration of neurotransmitter receptors.
Postsynaptic potential
See synaptic potential.
Posttetanic potentiation
The augmentation of a postsynaptic potential following a period of rapidly repeated (tetanic) stimulation.
Power curve
In a muscle, the relationship between the velocity of shortening and the force exerted against a load.
Power equation
See allometric equation.
Pre-Bötzinger complex
A bilaterally arrayed pair of neuron clusters within the medulla of the brainstem of a mammal, believed to be the source of the breathing rhythm.
Preferred body temperature
The body temperature that is maintained in a poikilotherm by behavioral thermoregulation.
Preflight warm-up
In an insect, warming of the flight muscles prior to flight. Preflight warm-up is often essential for flight because the flight muscles must be warm to generate enough power for flight. It can occur by behavioral means (e.g., sun basking) or by physiological means (shivering).
Preformed water
Water that enters the body in the form of H2O. Contrast with metabolic water.
Preganglionic neuron
In the autonomic nervous system, a neuron that extends from the central nervous system to an autonomic ganglion.
Pressure
The force a fluid (liquid or gas) exerts in a perpendicular direction on solid surfaces with which it is in contact. Expressed per unit of surface area.
Presynaptic cell
A neuron or other cell that transmits a signal to a postsynaptic cell at a synapse.
Presynaptic inhibition (PSI)
Inhibition of a neuron by decreasing the amount of neurotransmitter released by an excitatory presynaptic neuron.
Presynaptic terminal
The ending of a neuronal axon at the presynaptic side of a synapse.
Primary active transport
Active transport driven by a mechanism that draws energy directly from ATP. Contrast with secondary active transport.
Primary follicle
In the vertebrate ovary, a primary oocyte surrounded by a single layer of somatic cells—called granulosa cells—that are of different histological appearance than the somatic cells of a primordial follicle. A primary follicle has started its final maturation and, unless interrupted, will gradually develop more than a single layer of somatic cells and undergo other changes, becoming a secondary follicle and then progressing further.
Primary motor cortex
A region of the cerebral cortex (anterior to the central gyrus) that exerts descending control on spinal motor neurons to control movements.
Primary oocyte
A cell in the vertebrate ovary that has initiated meiosis during the early development of a female and is destined to produce an ovum but that remains in arrest until the female becomes reproductively mature.
Primary production
The production of organic matter from inorganic chemical precursors. The most common type of primary production is photosynthesis.
Primary protein structure
The sequence of amino acids in a protein molecule.
Primary urine
The fluid initially introduced into the tubules of a kidney. It is processed as it flows through the kidney tubules, ultimately becoming the definitive urine that is excreted from the body.
Primary visual cortex
Posterior region of the mammalian cerebral cortex that receives input from the lateral geniculate nucleus; serves as the first stage of visual information processing in the cerebral cortex.
Primordial follicle
n a vertebrate ovary, a primary oocyte surrounded by a single layer of flattened somatic cells. This is the earliest stage of follicle development. See also primary follicle.
Proenzyme
An inactive form of a digestive enzyme, activated after it arrives at its site of action.
Profiling
A research strategy in which investigators look as comprehensively as possible at a class of compounds of interest, such as messenger RNAs (transcription profiling) or proteins (protein profiling, proteomics). Also called screening.
Progesterone
A sex steroid hormone secreted by the corpus luteum of most vertebrates and the placenta of eutherian mammals.
Prolactin
A hormone secreted by cells in the anterior pituitary that stimulates the production of milk in mammals and performs a variety of other regulatory functions in vertebrates related to reproduction, water and mineral balance, and caring for the young.
Propagation
Spatial transmission of a signal such as an action potential without any decrease in amplitude with distance.
Proprioceptor
A sensory receptor that provides an animal with information about the relative position or movement of parts of its body.
Protandrous
In a sequential hermaphrodite, starting life as a male and later switching to female. See hermaphrodite.
Proteasome
See ubiquitin-proteasome system.
Protein hormones
Also called peptide hormones; made of assemblages of amino acids and soluble in water (polar).
Protein kinase
A regulatory enzyme that covalently bonds a phosphate group to a protein using ATP as the phosphate donor.
Proteomics
The simultaneous detection and measurement of large suites of proteins being synthesized by cells or tissues. Also called protein profiling.
Prothoracic glands
The paired glands located in the thorax of insects that synthesize and secrete ecdysone in response to prothoracotropic hormone.
Prothoracotropic hormone
A tropic protein neurohormone of insects, secreted by neuroendocrine cells with cell bodies in the brain and axon terminals in the neurohemal region of the corpora allata; stimulates the prothoracic glands to secrete ecdysone.
Protogynous
In a sequential hermaphrodite, starting life as a female and later switching to male. See hermaphrodite.
Proton pump
In the stomach, the cellular mechanism that secretes acid into the stomach lumen during digestion.
Ptilomotor
Related to the erection or compression of the feathers in the plumage of a bird under the control of muscles attached to the bases of the feathers.
Pulmonary
Related to the lungs.
Pulmonary arteries
In a vertebrate, blood vessels that carry blood from the heart to the lungs. This blood is partially deoxygenated.
Pulmonary circuit
In the circulatory system of an air-breathing animal, the blood vessels that take blood to and from the lungs.
Pump
In the study of cellular-molecular biology, a mechanism of active transport.
Pupa
The stage of development in holometabolous insects in which the larval tissues are destroyed and replaced by adult tissues. The pupa metamorphoses into the adult.
Purinotelic
Incorporating most nitrogen from the catabolism of nitrogenous compounds into purines, such as uric acid, guanine, and xanthine.
Purkinje fibers
Modified muscle cells that form part of the conducting system in which depolarization spreads throughout the vertebrate heart. See conducting system.
Q10
QRS complex
A part of the electrocardiogram that occurs simultaneously with ventricular contraction.
Quanta
See Quantal release.
Quantal release
The release of neurotransmitter molecules in multimolecular packets (quanta) corresponding to exocytosis of synaptic vesicles.
Quantitative trait locus (QTL) analysis
A statistical method that links two types of information—phenotypic data (observable traits) and genotypic data (usually chromosomal markers)—in an attempt to explain the genetic basis of variation in complex traits. QTL analysis allows researchers to link complex phenotypes to specific regions of chromosomes.
Quaternary protein structure
In a protein molecule that is composed of two or more separate proteins, the three-dimensional arrangement of the protein subunits relative to one another.
Radiant heat transfer
Transfer of heat by electromagnetic radiation between two surfaces that are not in contact. Also called thermal radiation heat transfer.
Radiant temperature
The surface temperature of an object as judged by its rate of emission of electromagnetic energy. The radiant temperature is the surface temperature that would have to prevail for the object to emit energy at the rate it actually does if its emissivity were exactly 1 (i.e., if it were a true black body). Radiant temperature is calculated by measuring the intensity of radiation and entering it in the Stefan-Boltzmann equation with the emissivity set equal to 1. Also called the black-body temperature.
Radiotelemetry
The use of a radio transmitter placed in or on an animal to transmit data on physiological or behavioral variables.
Radular apparatus
A feeding apparatus found in snails, slugs, chitons, squids, and some other molluscs. The radula itself is a band of connective tissue, studded with teeth, that is pulled back and forth to create grinding or scraping action.
Ram ventilation
A type of gill ventilation observed in certain types of fish (e.g., tunas) in which the fish holds its mouth open as it swims forward, thereby using its swimming motions to drive water over its gills.
Rapidly adapting receptor
A sensory receptor cell that exhibits a rapidly decreasing response to a maintained stimulus. Also called a phasic receptor.
Rate-limiting reaction
In a sequence of chemical reactions, the one that proceeds slowest, thereby setting the rate of the entire sequence.
Reaction velocity
In an enzyme-catalyzed reaction, the rate at which product molecules are made from substrate molecules.
Reactive oxygen species (ROS)
A term that refers collectively to metabolically produced molecules that have a great potential to act as oxidizing agents. Included are free radicals (e.g., superoxide)—which are molecules that have an odd number of electrons—and other oxidizing agents (e.g., hydrogen peroxide and ozone). Although many reactive oxygen species play roles in normal metabolism, they can also be destructive.
Receptive field
In sensory systems, the region of a sensory surface within which stimulation changes the activity of a particular neuron.
Receptor
A protein that binds noncovalently with specific molecules and, as a consequence of this binding, initiates a change in membrane permeability or cell metabolism. Receptors mediate the response of a cell to chemical messages (signals) arriving from outside the cell. Although most receptors are in the cell membrane, some are intracellular. See also sensory receptor.
Receptor adaptation
See adaptation (meaning 2).
Receptor molecule
See sensory receptor molecule.
Receptor potential
The graded change in membrane potential that occurs in a sensory receptor cell when it is stimulated.
Receptor, sensory
See sensory receptor.
Reciprocity
A principle of motor control in which signals that activate motor neurons and muscles also inhibit activation of antagonist motor neurons and muscles.
Recruitment of motor units
A mechanism by which the force of contraction of a whole muscle can be varied depending on the number of motor units stimulated to contract. See motor unit.
Rectal pads
Pad-shaped structures, composed of thickened epithelial cells, in the rectum of some insects.
Rectal salt gland
A salt-secreting gland found in the rectum of elasmobranch fish (e.g., sharks).
Red blood cell
A hemoglobin-containing cell in the blood of an animal. Also called an erythrocyte.
Red muscle
In general, a loose term referring to a vertebrate muscle that is rich in myoglobin and thus reddish in color. In fish, a large mass of muscle that consists almost entirely of myoglobin-rich muscle fibers that make ATP mostly by aerobic catabolism; routine cruising by fish is powered by red muscle.
Redox balance (reduction–oxidation balance)
A state in which a cell has the capability to remove electrons from a compound that undergoes reversible reduction and oxidation as fast as electrons are added to the compound.
Reduction
Addition of electrons or hydrogen atoms to a molecule.
Reflex
A simple, relatively stereotyped, but graded behavioral response to a specific stimulus.
Regional heterothermy
See heterothermy.
Regulation
The maintenance of internal conditions at an approximately constant level while external conditions vary.
Relative humidity
In air of a particular temperature, the existing water vapor pressure divided by the saturation water vapor pressure characteristic of that temperature.
Relative refractory period
In a neuron or other excitable cell, the brief period following an action potential during the generation of another action potential is relatively difficult because voltage conditions are further than usual from threshold.
Relaxation volume
The volume that a structure (e.g., a lung or the thorax) assumes when there are no muscular forces tending to expand or contract it.
Release-inhibiting hormone
A hormone secreted by neuroendocrine cells in the hypothalamus of a vertebrate that travels to the anterior pituitary through the hypothalamo-hypophysial portal system and inhibits the secretion of a hormone by a specific population of anterior pituitary endocrine cells. All release-inhibiting hormones are peptides except the catecholamine dopamine, which is known to inhibit the secretion of prolactin.
Releasing hormone
A peptide hormone secreted by neuroendocrine cells in the hypothalamus of a vertebrate that travels to the anterior pituitary through the hypothalamo–hypophysial portal system and stimulates the secretion of a hormone by a specific population of anterior pituitary endocrine cells; may also exert a tropic action to maintain and support those anterior pituitary cells.
Renal
Related to kidneys.
Renal corpuscle
In the kidney of a vertebrate, a glomerulus and its associated Bowman’s capsule. See also glomerulus.
Renal pelvis
In a vertebrate kidney, an expanded tubular structure that connects to the ureter. Urine from the nephrons enters the renal pelvis and then flows into the ureter.
Renin
A substance secreted into the blood by juxtaglomerular cells of the vertebrate nephron in response to low blood pressure; converts angiotensinogen to angiotensin I.
Renin–angiotensin–aldosterone system
A hormonal complex that ensures adequate arterial blood pressure. Set in motion by renin, it produces angiotensin II, which stimulates secretion of aldosterone (important in renal conservation of sodium) and vasopressin, promotes thirst, and stimulates vasoconstriction.
Residue
In biochemistry, the modified form assumed by a small molecule when it is incorporated by covalent bonding into a larger molecule. For example, when an amino acid is incorporated into a protein, it is technically an amino acid residue because its structure is no longer the full structure of the free amino acid.
Resistance
(1) In electrical circuits, the property that hinders the flow of electric current (charge movement) through a material, measured in ohms (Ω). It is the inverse of electrical conductance, the ease of current flow, measured in siemens. See also membrane resistance. (2) In heat transfer, the property that hinders dry heat transfer either through a material or between an animal and its environment. Speaking of the latter case, resistance to heat transfer is defined to be the difference between body temperature and ambient temperature divided by the rate of dry heat transfer between the animal and the environment. Contrast with thermal conductance. (3) See vascular resistance.
Resistance exercise
Exercise that consists of relatively short periods of high-intensity muscular actions against a large load, often repeated with intervening interruptions. The periods of high-intensity activity are usually characterized by significant dependence on anaerobic mechanisms of ATP production. Also called resistance training. Contrast with endurance exercise.
Resonant frequency
In any elastic system, the frequency at which the system oscillates when left alone following activation by a pulse of energy; the natural frequency. The energy cost of energy-driven oscillation tends to be lowest if the oscillation is at the resonant frequency of the system involved.
Respiratory acidosis or alkalosis
Acidosis or alkalosis that is caused by an abnormally rapid or slow rate of removal of CO2 from the body fluids by breathing. Contrast with metabolic acidosis or alkalosis.
Respiratory airways
In the lung of a vertebrate, the airways in which O2 and CO2 are exchanged between the lung air and blood (i.e., excluding airways that are too poorly vascularized or too thick-walled for exchange to occur).
Respiratory chain
Synonym for electron-transport chain.
Respiratory gases
Oxygen (O2) and carbon dioxide (CO2).
Respiratory pigments
Any of the metalloprotein pigments that undergo reversible combination with O2 and thus are able to pick up O2 in certain places in an animal’s body (e.g., the breathing organs) and release it in other places (e.g., systemic tissues). Respiratory pigments include hemoglobin, hemocyanin, hemerythrin, and chlorocruorin. Also called O2-transport pigments.
Respiratory quotient (RQ)
The moles of CO2 produced by a cell expressed as a ratio of the moles of O2 simultaneously consumed. Because the RQ changes with the type of foodstuff being oxidized, it can be used to assess what foodstuffs a cell is using in aerobic catabolism.
Resting membrane potential (Vm)
The normal electrical potential across the cell membrane of a cell at rest.
Rete
Shorthand for rete mirabile. See rete mirabile.
Rete mirabile
A Latin expression, meaning literally “wonderful net.” A morphological term referring to any intricately complex vascular system composed of closely juxtaposed, small-diameter arterial and venous blood vessels. The term rete is a shorthand synonym.
Reticular theory
Theory in the nineteenth century, now discarded, that cells in the CNS were cytoplasmically connected to each other in a syncytial reticulum. Contrast neuron doctrine.
Retina
The layer of photoreceptor cells and other neurons that line the inside of a vertebrate eye.
Retinal
The aldehyde of vitamin A, one of two components of the photopigment rhodopsin.
Retinular cell
Photoreceptor cell in the ommatidium of a compound eye.
Retrieval
The movement of channel or transporter protein molecules out of the membrane where they are active to intracellular locations where they are inactive.
Retrograde messenger
In a synapse, a chemical signal thought to be released by a postsynaptic cell that alters the synaptic properties (such as neurotransmitter release) of the presynaptic cell.
Reversal potential
The membrane potential at which the amplitude of a voltage response (such as a postsynaptic potential or receptor potential) is zero because there is no net driving force for ion flow. For example, EEPSP is the reversal potential of an EPSP.
Rhabdomere
Collection of microvilli from the retinular cells of an ommatidium of a compound eye.
Rhabdomeric photoreceptor
A photoreceptor cell in which the photopigment rhodopsin is collected in microvillar membranes. Contrast with ciliary photoreceptor.
Rhodopsin
The light-absorbing pigment of photoreceptors that initiates the visual response to light; composed of retinal and the protein opsin.
Rhythmic behavior
Stereotyped, repetitive sequences of movement such as walking, swimming, flying, and breathing.
Rhythmogenesis
Generation of a rhythm. Usually refers to rhythm generation by neurons or sets of neurons, such as the sets of neurons that rhythmically originate nerve impulses that stimulate the breathing muscles to contract.
Right-to-left shunting
In the study of blood flow through the central circulation of an air-breathing fish, amphibian, or nonavian reptile, diversion of systemic venous blood directly back into the systemic circuit, bypassing the lungs.
RNA interference (RNAi)
A cellular process that destroys specific mRNA molecules (produced naturally in cells) when specific double-stranded RNA molecules are introduced into cells. The pathway effectively silences the genes that produced the targeted mRNAs.
Rod
A type of photoreceptor in the vertebrate retina. Rods are larger than cones, respond at lower light levels, and are used for nocturnal vision.
Root effect
A decrease in the amount of O2 a respiratory pigment can bind at saturation—and thus a decrease in the oxygen-carrying capacity of blood—caused by a decrease in pH or an increase in CO2 partial pressure. Unusual; observed only in certain fish and molluscs.
ROS
Rumen
An expanded nonacidic stomach chamber in a ruminant mammal, housing a mixed microbial community of symbiotic microbes that perform functions important for the mammal’s nutrition.
Ryanodine receptor
The calcium channel of the sarcoplasmic reticulum in striated muscles.
Sacculus
Otolithic organ in the vertebrate inner ear containing hair cells that are stimulated by vertical movements and forces (including gravity).
Salinity
The sum total concentration of inorganic dissolved matter in water, usually expressed as grams of dissolved matter per kilogram of water.
Salt
A synonym for inorganic ion, or a compound formed by inorganic ions.
Salt glands
Organs other than kidneys that excrete concentrated solutions of inorganic ions. Examples include the cranial salt glands of marine birds and the rectal salt glands of marine elasmobranch fish.
Saltatory conduction
Propagation of action potentials in a spatially discontinuous manner along a myelinated axon by jumping from one node of Ranvier to another.
Sarcolemma
The cell membrane of a muscle fiber.
Sarcomere
The contractile unit of striated muscle that consists of contractile, regulatory, and cytoskeletal proteins. Many sarcomeres in series, delineated by Z discs, constitute a myofibril.
Sarcopenia
The loss of skeletal muscle mass as a result of aging. It involves both the loss of contractile proteins from individual muscle fibers and the loss of complete fibers by cell death.
Sarcoplasmic reticulum (SR)
A system of internal compartments in a muscle cell that envelops myofibrils and stores Ca2+ ions. The sarcoplasmic reticulum has calcium ATPase pumps that transport Ca2+ ions into its lumen and calcium channels that open in response to excitation along the associated transverse tubules.
Satellite cells
Muscle stem cells that lie immediately outside the sarcolemma.
Saturated air
Air in which a temperature-specific maximum partial pressure of water vapor prevails. Fully humidified air.
Saturated enzyme
An enzyme that is catalyzing its reaction at a maximum rate because substrate molecules are abundant enough that they bind with the enzyme at as great a number per unit of time as is possible.
Saturated fatty acid
A fatty acid in which all the bonds between carbon atoms in the carbon-chain backbone of the molecule are single bonds. Also called a SFA or a saturate.
Saturated respiratory pigment
A respiratory pigment that has combined with as much O2 as it can possibly hold.
Saturated transporter
A transporter protein that is catalyzing transport at a maximum rate because the molecules being transported are abundant enough that they bind with the transporter at as great a number per unit of time as is possible.
Saturation deficit
In air of a particular temperature, the difference between the saturation water vapor pressure characteristic of that temperature and the existing water vapor pressure.
Saturation kinetics
The kinetics characteristic of a chemical reaction or other chemical process that is limited to a maximum velocity by a limited supply of some type of molecule with which other molecules must reversibly combine for the reaction or process to take place.
Saturation water vapor pressure
The maximum possible water vapor pressure that can stably exist in a gas of a particular temperature. If the water vapor pressure rises above the saturation water vapor pressure, condensation occurs. The saturation water vapor pressure varies strongly with temperature.
Scaffolding (or scaffold) proteins
Proteins that function to bring other proteins together in a relatively stable complex. Presynaptic terminals have scaffolding protein complexes involved in neurotransmitter release, and postsynaptic cells have scaffolding complexes mediating neurotransmitter receptor trafficking and action.
Scaling
The study of the relations between physiological (or morphological) features and body size within sets of phylogenetically related species, e.g., the study of metabolism-weight relations.
Schwann cell
A type of ensheathing non-neuronal glial cell found in the vertebrate peripheral nervous system. Schwann cells form, for example, the myelin sheath of myelinated axons.
Screening
See profiling.
Scrotum
In a male mammal, an external sac that houses the testes (testicles).
Seasonal polyphenism
A phenomenon seen in some insects that go through two or more generations per year, in which genetically identical individuals can assume two or more discrete, highly distinct body forms depending on the season during which they develop.
Second law of thermodynamics
A law, believed to apply without exception in the biosphere, stating that if an isolated system undergoes internal change, the net effect of the change in the system as a whole is always to increase disorder (entropy).
Second messenger
An intracellular signaling molecule that is produced inside a cell in response to the binding of a chemically different extracellular signaling molecule to specific cell-membrane receptors.
Secondary active transport
Active transport driven by a mechanism that does not draw energy directly from ATP, but instead obtains it from the potential energy inherent in an electrochemical gradient. During secondary active transport, an electrochemical gradient of a solute (e.g., Na+) is created using ATP-bond energy, and the active-transport mechanism obtains its energy from that gradient. Contrast with primary active transport.
Secondary compounds
Compounds of diverse chemical types (e.g., alkaloids, tannins, or terpenes) that are found in tissues and that have as their major function the deterrence of tissue consumption by a predator.
Secondary lamellae
In fish gills, the microscopically fine folds of tissue on the surfaces of the gill filaments that serve as the primary sites of exchange of respiratory gases between the ambient water and blood.
Secondary oocyte
The cell formed at the time of ovulation by the primary oocyte when it concludes its first meiotic division and extrudes the first polar body.
Secondary protein structure
The arrangement of the amino acids within subregions of a protein molecule into highly regular geometric shapes. The two most common types of such highly ordered arrays of amino acids are the α-helix and the β-sheet (pleated sheet).
Segmentation
One of the gut’s principal modes of muscular activity, in which circular muscles contract and relax in patterns that push the gut contents back and forth.
Selective permeability
The state of having a high permeability to some solutes but a low permeability to others. See also permeability.
Semelparity
A type of reproductive life history in which individuals are physiologically capable of only one bout of reproduction during their lives. In semelparous species, individuals are often programmed to die after reproducing once. Contrast with iteroparity.
Semelparous
Characterized by semelparity.
Semen
The sperm-containing fluid emitted by a male mammal during ejaculation. Most of the fluid volume is composed of secretions of accessory sexual glands, (notably the prostate gland and seminal vesicles in humans).
Semicircular canal
A component of the vestibular organ of the vertebrate inner ear containing receptors that respond to head rotation.
Seminiferous tubules
Small tubules in the testes (testicles) of a vertebrate, responsible for sperm production.
Semipermeable membrane
A membrane that is permeable only to water. All true semipermeable membranes are human-made, as there are no natural biological membranes that are strictly semipermeable.
Sense organ
A complex multicellular structure specialized to detect a particular type of sensory stimulus.
Sensillum
A sensory hair of arthropods; not related to vertebrate hair, but rather a hollow chitinous projection of the exoskeleton that is associated with sensory receptor neurons.
Sensitivity
The ability of a sensory cell to distinguish stimuli of different intensity. Receptor sensitivity may also refer to the adequate stimulus, the kind of stimulus to which the receptor is responsive.
Sensitization
Enhancement of a learned behavioral response to a harmless stimulus after exposure to a strong or harmful stimulus.
Sensory adaptation
See adaptation (meaning 2).
Sensory modality
The class of stimulus that evokes a sensory response. The classical sensory modalities are vision (light is the stimulus that evokes a response), hearing (sound), touch, smell, and taste.
Sensory neuron
A sensory receptor that is a neuron, or a peripheral neuron that is excited by a non-neuronal sensory receptor cell.
Sensory receptor cell
A sensory cell that is specialized to respond to a particular kind of environmental stimulus.
Sensory receptor molecule
A molecule in a sensory receptor cell that is particularly sensitive to a kind of sensory stimulus, and that participates in transducing a stimulus into a cellular response.
Sensory system
The sense organs (or other sensory receptors) for a particular sensory modality and all of the central processing areas and pathways associated with those organs (or other receptors).
Sensory transduction
The process by which the energy of a physical stimulus is converted into an electrical signal in a sensory receptor cell.
Septate junction
A type of junction between epithelial cells that differs in fine structure from a tight junction but otherwise has similar properties. Found in invertebrates.
Sequential hermaphroditism
See hermaphroditism.
Series
In relation to the arrangement of parts in an electrical circuit, vascular system, or other analogous system in which substances flow from place to place, the parts of the system are in series if they occur sequentially along a single path of flow, so that all flow must occur sequentially through all parts. Contrast with parallel.
Serosal surface
See basal surface.
Sertoli cells
Somatic cells in the seminiferous tubules of the testes of vertebrates that function to support spermatogenesis.
Sexual reproduction
The formation of a new, genetically unique individual from the union of male and female gametes.
Shivering
(1) In a mammal or bird, the unsynchronized contraction and relaxation of motor units in skeletal muscles in high-frequency rhythms, producing heat rather than organized motion as the primary product. (2) In an insect, contraction of the flight muscles in a nonflying mode to generate heat rather than flight.
Short-chain fatty acids (SCFA)
Fatty acids that consist of relatively few carbon atoms, including acetic acid (2 carbons), propionic acid (3), and butyric acid (4). They are produced, for example, by symbiotic fermenting microbes and are readily absorbed and metabolized by animals. Also called volatile fatty acids.
Sigmoid kinetics
In an enzyme-catalyzed reaction, an s-shaped relation between reaction rate and substrate concentration. In the study of hemoglobin or other respiratory pigments, an s-shaped relation between the percentage of oxygenated O2-binding sites and the O2 partial pressure.
Signal transduction
In the study of cell signaling, the translation of a signal from one chemical form (e.g., a hormone) into another chemical form (e.g., a second messenger inside a cell).
Simple cell
A neuron found in the mammalian primary visual cortex that has an elongated orientation-selective receptive field, so that it responds most to a bar or edge at a particular angle of orientation. The receptive fields of simple cells have distinct excitatory and inhibitory subregions.
Simple diffusion
Transport of solutes, water, gases, or other materials that arises from the molecular agitation that exists in all systems above absolute zero and from the simple statistical tendency for such agitation to carry more molecules out of regions of relatively high concentration than into such regions.
Simple epithelium
An epithelium consisting of a single cell layer.
Single effect
The difference produced by use of metabolic energy between adjacent parts of the two oppositely flowing fluid streams in a countercurrent multiplier system. See also countercurrent multiplication.
Single-channel current recording
The ability to record the ionic currents flowing though a single membrane channel by attaching a microelectrode to the membrane surrounding the channel. See also patch-clamp recording.
Single-unit smooth muscle
A muscle made up of unstriated muscle cells electrically coupled to neighboring cells by gap junctions; when stimulated, all of the cells are depolarized and contract together. Contrast with multiunit smooth muscle.
Sinoatrial (S-A) node (sinus node)
In the heart of a mammal or bird, a cluster of modified muscle cells in the right atrial wall where the pacemaker cells for the heart are located, responsible for initiating heart beats.
Sinuses
In an open circulatory system, large spaces, between nonvascular structures, that serve as passageways for blood flow.
Skeletal muscle
Muscle that produces locomotory movements or other external movements of the body. In vertebrates, skeletal muscle is attached to the endoskeleton. In most types of invertebrates, it is attached to the exoskeleton, shell, or other external covering. Vertebrate skeletal muscle consists of large, cylindrical, multinucleate striated cells (muscle fibers). Invertebrate skeletal muscle may consist of striated or smooth muscle cells, depending on the phylogenetic group under consideration.
Sliding-filament theory
The well-documented theory that muscle contraction results from active interaction between thick and thin myofilaments, which causes them to slide past each other.
Slow oxidative (SO) muscle fibers
Muscle fibers that are poised to make ATP principally by aerobic catabolism, develop contractile tension slowly, have relatively low peak power outputs, and are relatively resistant to fatigue.
Slowly adapting receptor
A sensory receptor cell that responds to a maintained stimulus in a way that decreases slowly and incompletely. Also called a tonic receptor.
Smooth muscle
Muscle that consists of small, spindle-shaped, uninucleate cells without striations. Thin actin filaments and thick myosin filaments are present, but not organized into sarcomeres. In vertebrates, smooth muscle is found in hollow and tubular internal organs, such as certain blood vessels and the gut.
Sociobiology
The study of the social relations and behaviors of animals from an evolutionary perspective.
Solute
A substance that is in solution (i.e., dissolved in solvent).
Solvent drag
Movement of solutes (dissolved entities) through a membrane driven by simultaneous osmotic movement of water (solvent).
Soma (plural somata)
See cell body.
Somatic effectors
Skeletal (striated) muscle, as opposed to internal effectors.
Somatic nervous system
The part of the vertebrate peripheral nervous system that controls skeletal muscles and provides afferent information from sensory receptors not associated with internal organs.
Somatotopic map
The topographic representation or mapping of the body surface onto a region of the brain—for example, in the vertebrate somatosensory cortex.
Spatial summation
The summation of postsynaptic potentials that result from presynaptic action potentials at different synapses onto a single postsynaptic cell.
Specific dynamic action (SDA)
A rise in the metabolic rate of an animal caused by the processing of ingested food.
Specificity
Sensory specificity is the ability to distinguish among different stimulus types (modalities or qualities).
Spermatogenesis
The formation of haploid sperm through the process of meiosis in the testis.
Spermatozoa
Haploid gametes, produced by spermatogenesis in the testis. Also called sperm cells or sperm.
Sphincter
In a vertebrate, a circular muscle, located between two chambers, that can contract tightly and steadily (tonically) for long periods of time, thus preventing exchange between the chambers.
Spinal nerves
In a vertebrate, segmental nerves of the peripheral nervous system that attach to the spinal cord.
Spinal reflex
A reflex mediated by neural circuits of the vertebrate spinal cord.
Spiracle
In a terrestrial arthropod (e.g., insect), a porelike aperture on the surface of the body that opens into the breathing system (e.g., tracheal system).
Spongy myocardium
Myocardium in which the muscle cells are sufficiently widely spaced that the tissue is perfused by the blood flowing through the lumens of the heart chambers. The myocardium of many fish is spongy.
Spontaneous ovulation
Ovulation (release of an egg from the ovaries of a female) that results from processes endogenous to the female, more or less independent of the actual act of mating. Contrast with induced ovulation.
Standard amino acid
One of the 20–22 amino acids that are employed by organisms to synthesize proteins.
Standard metabolic rate (SMR)
The metabolic rate of a poikilothermic (ectothermic) animal when it is resting and fasting. The SMR is specific to the body temperature prevailing during measurement.
Standard temperature and pressure (STP)
A temperature of 0°C and a pressure of 1 atm (101 kPa; 760 mm Hg). Gas volumes are often expressed under these conditions as a way of standardizing the effects of temperature and pressure on volume.
Star compass
A mechanism by which an animal can use the positions of stars and constellations (and an internal clock) to determine compass direction in nocturnal navigation.
Starling–Landis hypothesis
The concept that the ventricle of the heart tends to contract with greater force when it is stretched to a great extent at the start of contraction rather than stretched to a small extent.
Statocyst
A sense organ that can detect acceleration and the direction of gravitational force.
Statolith
A stony mineral concretion in a statocyst that is denser than the medium in which it sits, so that it is pulled downward by gravity and stimulates mechanoreceptors.
Stefan-Boltzmann equation
An equation that relates the temperature of a surface and the rate at which the surface emits electromagnetic energy.
Stenohaline
Referring to aquatic animals, able to live only within a narrow range of environmental salinities. Contrast with euryhaline.
Stenothermal
Referring to poikilotherms, able to live only within a narrow range of body temperatures. Contrast with eurythermal.
Steroid hormones
Nonpolar hormones synthesized on demand from cholesterol, secreted by diffusion through the cell membrane and circulated in the blood or hemolymph bound to carrier molecules; typically bind to intracellular receptors of the target cells and exert actions through genomic means. Some bind to receptors on the target cell membrane and exert nongenomic actions.
Stimulus
(1) At a cellular level, a form of energy (sometimes called stimulus energy) that excites sensory receptor cells; the form of energy is specific for each type of receptor cell. (2) At a whole-animal level, a change in the external environment or in internal conditions that an animal can detect and respond to. (2) A change in the external environment or in internal conditions that can be detected by an animal.
Stoichiometry
The existence of fixed ratios in chemical reactions. For instance, the fact that hydrogen and oxygen atoms react in a 2-to-1 ratio to form water is an example of stoichiometry.
Stomatogastric ganglion
A small ganglion containing 30 or more neurons that controls rhythmic activity of stomach muscles in crustaceans. The stomatogastric ganglion is a model system for study of central pattern generators.
Straight-through pathways
In the retina, neural pathways that extend through the retina from external photoreceptors to internal ganglion cells. Contrast with lateral pathways.
Stress response
The response of an animal to a threatening situation. In vertebrates, it typically involves functions of the autonomic nervous system and the hypothalamus–pituitary–adrenal cortex axis.
Stretch reflex
A vertebrate spinal reflex in which muscle stretching activates a muscle spindle stretch receptor, generating nerve impulses in 1a afferent axons that excite motor neurons innervating the same muscle to oppose the stretch.
Stretch-gated channel
A channel protein that opens and closes in response to stretching or pulling forces that alter the physical tension on a membrane.
Striated muscle cells
Also called striated muscle fibers; in vertebrates, they make up skeletal and cardiac muscles.
Striated muscle
Muscle that consists of cells in which the thick myosin and thin actin filaments are arranged in sarcomeres. The sarcomeres in a cell are aligned in register to form stripes or striations at right angles to the long axis of the cell. In vertebrates, skeletal and cardiac muscles are striated.
Stroke volume
In reference to a heart or other organ that pumps fluid by rhythmic cycles of contraction, the volume of fluid pumped per cycle.
Subcellular membrane
Submaximal exercise
Exercise that requires less than an individual’s maximal rate of O2 consumption.
Substrate
One of the initial reactants of an enzyme-catalyzed reaction.
Substrate-binding site
See active site.
Substrate-level phosphorylations
ATP-producing reactions other than those associated with the electron-transport chain.
Sulfur-oxidizing bacteria
Bacteria that employ oxidation of reduced sulfur compounds (e.g., H2S) as a source of energy so they can function as chemoautotrophs. See chemoautotroph.
Summation
In excitable cells, the addition of graded subthreshold potentials (electrical events). In muscle cells, the addition of twitches (mechanical events) produced by high frequencies of action potentials.
Sun compass
A mechanism by which an animal can use the sun’s position and an internal clock to determine compass direction in navigation.
Supercooling
Cooling of a solution to below its freezing point without freezing.
Supercooling point
The highest temperature at which freezing is almost certain to occur promptly in a supercooled solution. (Freezing of a supercooled solution is probabilistic. At some temperatures freezing is unlikely. The temperature needs to be lowered to the supercooling point for prompt freezing to be likely.)
Suprachiasmatic nucleus
The part of the hypothalamus of the brain that acts as the master circadian clock in mammals.
Supramaximal exercise
Exercise that requires ATP at a greater rate than it can be made aerobically even when an individual’s rate of O2 consumption is maximized.
Supraoptic nucleus
A cluster of neurons in the hypothalamus; contains magnocellular neuroendocrine cell bodies that extend their axon terminals to the pars nervosa where they secrete oxytocin and vasopressin into the general circulation.
Surface-to-volume ratio
The ratio of the total area of the outer surface of a three-dimensional object over the volume of that object.
Suspension feeding
Feeding on objects suspended in water that are very small by comparison to the feeding animal. See also filter feeding.
Sweating
The secretion onto the skin surface of a low-salinity aqueous solution (sweat) by specialized sweat glands to increase the rate of evaporative cooling. Occurs only in some groups of mammals.
Symmorphosis
A hypothesis about the evolution of multiple organ systems in a species, which posits that the performance limits of all systems remain roughly matched because it would make no sense for any one system to have evolved capabilities that could never be used because of more-restrictive limits in other systems.
Sympathetic chain
The chain of segmental sympathetic ganglia in a vertebrate nervous system.
Sympathetic division
A division of the vertebrate autonomic nervous system that is connected to the CNS via thoracic and lumbar spinal nerves; the sympathetic and parasympathetic divisions tend to exert opposing controls over autonomic effectors.
Symporter
See cotransporter.
Synapse
A specialized site of communication between two neurons, between a neuron and an effector, or between a non-neuronal sensory cell and a neuron.
Synaptic antifacilitation
A decrease in the amplitude of postsynaptic responses to repeated presynaptic action potentials. Also called synaptic depression.
Synaptic cleft
The extracellular gap between presynaptic and postsynaptic cells at a synapse, typically 20–40 nm wide.
Synaptic current
The current that flows through the postsynaptic membrane in synaptic transmission and produces a postsynaptic potential.
Synaptic facilitation
An increase in the amplitude of postsynaptic responses that occurs after repeated presynaptic action potentials.
Synaptic homeostasis
A form of synaptic plasticity that provides a means for neurons and neuron circuits to maintain stable function in the face of perturbations (such as developmental or activity-dependent changes in synapse number or strength). Sleep may play roles in synaptic homeostasis.
Synaptic plasticity
Change in properties of synapses or strength of synaptic interactions with time or circumstance. Changes of nervous system function during development or learning are thought to reflect synaptic plasticity.
Synaptic potential
A graded change in a postsynaptic cell’s membrane potential produced by synaptic input. Also called a postsynaptic potential.
Synaptic transmission
The process whereby one neuron influences the excitability of another neuron or effector. Synaptic transmission can be either chemical or electrical.
Synaptic vesicles
Membrane-bound vesicles in a presynaptic terminal, into which neurotransmitter molecules are concentrated.
Synergism
Interactions between two or more agents (e.g., hormones) whereby they have a greater effect acting together than the simple sum of their individual effects.
Systemic circuit
In a circulatory system, the blood vessels that take blood to and from the systemic tissues.
Systemic tissues
All tissues other than those of the breathing organs.
Systole
The period of contraction during each beating cycle of a heart.
Systolic pressure
During the beating cycle of the heart, the blood pressure prevailing during periods of heart contraction.
Tachycardia
A heart rate that is unusually high.
Tagged
See epigenetic mark.
Target cell
A cell that responds to a chemical signaling molecule such as a paracrine or hormone because it expresses specific receptors for that molecule. It may express receptors for more than one signaling molecule. Its sensitivity depends on the number of receptors present, and can be changed by upregulation or downregulation of the receptors.
Taste
Chemoreception of stimuli that are dissolved or suspended in liquids, typically requiring higher stimulus concentrations than olfaction. Taste chemoreceptors are often, but not always, localized around the mouth.
Taste bud
A collection of epithelial taste receptor cells and support cells on the tongue or, in fish, on the skin surface.
Telemetry
See radiotelemetry.
Telencephalon
The anterior part of the vertebrate forebrain.
Teleost fish
The principal group of fish having bony skeletons.
Temperature
A measure of the speed or intensity of the ceaseless random motions that all the atoms and molecules of any substance undergo on an atomic-molecular scale. More exactly, the temperature of a substance is proportional to the product of the mean square speed of random molecular motions and the molecular mass.
Temperature coefficient
The ratio of the rate of a process at one body temperature over the rate of the same process at a body temperature 10°C lower. Symbolized Q10.
Temperature-dependent sex determination
The phenomenon, seen in some nonavian reptiles, whereby the sex of offspring is determined by their body temperature during early development.
Temperature gradient
Technically, the difference in temperature between two places divided by the distance separating those two places. Often used more loosely to refer simply to a difference in temperature. Also called a thermal gradient.
Temporal heterothermy
See heterothermy.
Temporal organization
The tendency for a physiological or behavioral system to be organized in time.
Temporal summation
The summation of synaptic potentials in response to repeated presynaptic action potentials at the same synapse.
Tendons
Connective tissue attachments of skeletal muscles to bones.
Tension (gas)
A synonym of partial pressure. See partial pressure.
Tension (muscular)
The force produced by cross-bridge action in a contracting muscle.
Tertiary protein structure
The natural arrangement of an entire protein molecule in three dimensions, including its secondary structure and the other patterns of folding that give the molecule its particular conformation. Tertiary structure is flexible because it is stabilized by noncovalent bonds.
Testicular recrudescence
The regrowth of the testes—and their return to producing sperm—after a period when they have been reduced in size and nonfunctional.
Testosterone
A sex steroid hormone, produced by the Leydig cells of the testes, that is essential for male secondary sexual characteristics and reproduction. An androgen.
Tetanus
Summed twitches of skeletal muscles produced by trains of motor action potentials. Fused tetanus is a smooth rise in tension produced by a high-frequency train of action potentials. Unfused tetanus is produced by a lower-frequency train of action potentials and shows some relaxation of each twitch between action potentials.
Theca cells
Somatic connective tissue cells that form the outer layer of a developing ovarian follicle.
Thermal conductance
A measure of the ease of dry heat transfer between an animal and its environment. Contrast with resistance (meaning 2).
Thermal gradient
See temperature gradient.
Thermal hysteresis
The phenomenon of the freezing point of a solution being different from its melting point.
Thermal hysteresis protein (THP)
A protein or glycoprotein that acts as an antifreeze by chemically interfering with the formation or growth of ice crystals. Thermal hysteresis proteins lower the freezing point more than the melting point, thus their name.
Thermal radiation heat transfer
Thermogenesis
In the context of thermal relations, production of heat for the specific function of warming tissues.
Thermogenic tissue or process
In the context of thermal relations, a tissue or process that specifically increases production of heat when activated.
Thermogenin
An older term referring to uncoupling protein 1. See UCP1.
Thermoneutral zone (TNZ)
In a homeotherm, the range of ambient temperatures over which the metabolic rate is constant regardless of ambient temperature.
Thermoregulation
The maintenance of a relatively constant body temperature.
Thick filaments
Polymers of myosin molecules in muscle cells. See myosin.
Thin filaments
Polymers of G-actin monomers in muscle cells. See actin.
Thyroid-stimulating hormone (TSH)
A peptide tropic hormone secreted by the anterior pituitary gland; travels in the general circulation, binds to its receptors on cells of the thyroid gland and stimulates synthesis and secretion of thyroid hormones.
Tidal flow
Flow that occurs alternately in and out through a single set of passageways.
Tidal volume
In an animal that exhibits tidal breathing, the amount of air inhaled and exhaled per breath.
Tight junction
A place where the cell membranes of adjacent cells in an epithelium are tightly joined so that there is no extracellular space between the cells.
Time constant (τ)
The time required for an exponential process to reach 63% of completion. In neurophysiology, it is a measure of the time needed to change membrane potential and is proportional to the product of resistance and capacitance.
Time difference
In sensory physiology, a time difference between responses of different receptors can localize a stimulus; for example, sound that reaches the left ear first comes from the left.
Time-energy budget
A method used to estimate an animal’s average daily metabolic rate (ADMR). In this method, the time per day spent in each type of activity is measured and multiplied by an estimate of the energy cost of the activity per unit of time to get the total daily energy cost of the activity. The total costs of all activities are then added to get the ADMR. See also average daily metabolic rate.
Tissue
A group of similar cells organized into a functional unit.
Tissue fluids
See interstitial fluids.
Titin
A giant elastic protein molecule that in a striated muscle spans an entire half-sarcomere from Z disc to M line.
Tonic muscle fibers
Smooth muscle cells that constitute tonic smooth muscles; they maintain contractile force (tone) for long periods but do not generate spontaneous contractions or action potentials.
Tonic receptor
Tonic smooth muscles
A category of smooth muscles that maintain contractile force (tone) for long periods of time, e.g., sphincter muscles.
Total carbon dioxide concentration
The amount of CO2 a solution or body fluid takes up per unit of volume to reach a particular CO2 partial pressure, regardless of the chemical form the CO2 assumes when in the solution or body fluid.
Total fluid energy
An important concept in understanding blood flow, defined in Figure 25.8.
Totipotent
Capable of all things. For animals, chemical energy is the only form of totipotent energy.
Trabeculae
Strands of muscle tissue that run through the open central cavity of a heart chamber, crisscrossing from one part of the chamber wall to another.
Trachea
A principal tube in the breathing system of a terrestrial animal, typically with reinforced walls. (1) In vertebrates, the trachea is the initial, large airway that carries air from the buccal cavity to the lungs. (2) In insects, a trachea is any of multitudinous airways that ramify throughout the body to form the tracheal respiratory system. See also tracheole.
Tracheal gills
In an aquatic insect, evaginated breathing structures characterized by a high density of small tracheae, into which O2 diffuses from the water.
Tracheole
A very fine, thin-walled end-tubule at the innermost reaches of the tracheal breathing system of an insect or other tracheate arthropod. The tracheoles are believed to be the principal sites of O2 and CO2 exchange with the tissues.
Tract
A bundle of axons within a vertebrate central nervous system.
Trafficking
The movement of protein molecules between active and inactive locations in a cell, thereby controlling the functional activity of the protein molecules. A common scenario for trafficking is for transport-protein molecules to be active in transport when in the cell membrane but inactive when in an intracellular membrane. See insertion and retrieval.
Trail following
A trail is a marked path or course; trail following is navigation by simply following an interconnected series of local sensory cues. These cues can be visual, olfactory, or any other modality.
Transcellular
In the study of transport through an epithelium, transport that occurs through the cells rather than by passageways between cells.
Transcription factors
Factors that control transcription of DNA.
Transcription profiling
A research strategy in which investigators detect and measure large sets of messenger RNA molecules simultaneously, as a way of assessing patterns of gene transcription in cells or tissues. Also called transcriptomics.
Transcriptomics
Transducin
A G protein that is activated by rhodopsin in photoreceptors, leading to a receptor potential.
Transduction
(1) In the study of energy, the transformation of one form of energy into another. (2) In the study of cell signaling, the translation of a signal from one chemical form to another chemical form. (3) In neurophysiology, the conversion of stimulus energy into an electrical signal in sensory receptor cells; the electrical signal is usually a receptor potential.
Transient receptor potential (TRP) channel
A kind of ion channel found in membranes of many sensory cells; it opens in response to stimuli and produces a receptor potential.
Transpirational water loss
Water loss across the integument of an animal that occurs without sweating or any other active mechanism of transporting water across the integument; passive water loss across the integument.
Transport
An entirely general term referring to any and all movements of solutes, water, gases, or other materials from place to place, regardless of the mechanisms of movement.
Transporter
A membrane protein that mediates the transport of solute molecules across a membrane and must undergo reversible, noncovalent bonding with the solute molecules in order to do so. Transporters participate in facilitated diffusion and active transport. Also sometimes called a carrier.
Transverse tubules (t-tubules)
Fingerlike indentations of the cell membrane at regular intervals over the entire surface of a muscle cell. Transverse tubules conduct electrical excitation into the interior of the cell and are intimately associated with the sarcoplasmic reticulum.
Triacylglycerol
A fat or oil; a lipid composed of glycerol esterified with three fatty acids. Also called a triglyceride.
Triglyceride
See triacylglycerol.
Trophoblast
In a mammal, the outer layer of cells of a blastocyst that will form the fetal portion of the placenta.
Tropic action
An action performed by a hormone acting on a target endocrine gland to stimulate secretion of hormone by the target gland and also to maintain the structure and function of the target gland.
Tropic hormones
Chemical messengers that bind to receptors of endocrine target cells to both stimulate secretion of hormone and also maintain the structure and functional capabilities of the target cells. Contrast with direct acting hormones.
Tropomyosin (TM)
In muscles, a coiled protein molecule that spans seven actin monomers on a thin filament and is associated with one troponin molecule.
Troponin (TN)
In muscles, a protein consisting of three subunits that is associated with actin and tropomyosin on the thin filaments.
Twitch
A single contraction and relaxation of a skeletal muscle fiber produced by an action potential that triggers release of Ca2+ ions from the sarcoplasmic reticulum. A single twitch is produced by many repeated cross-bridge power strokes that draw the thin filaments toward the center of each sarcomere.
Twitch fibers
In vertebrates, skeletal muscle fibers that generate an action potential when stimulated by a motor neuron action potential and produce a contractile event called a twitch. See twitch.
Tympanal organ
An organ of hearing in which sound vibrates a tympanal membrane (“eardrum”) to activate auditory receptor cells. The term is usually used for insect hearing organs, although the vertebrate ear is also a tympanal organ.
U/P ratio
The concentration of urine expressed as a ratio of the concentration of blood plasma. The osmotic U/P ratio is the ratio of urine osmotic pressure over plasma osmotic pressure. The U/P ratio for a particular ion is the ratio of the urine concentration of that ion over the plasma concentration of that ion.
Ubiquitination
Tagging of proteins for destruction. Target proteins are linked with one or more molecules of ubiquitin, a small tagging protein. Proteins thus tagged are later broken up by proteasomes.
Ubiquitin–proteasome system
A cellular mechanism that destroys proteins by breaking them up into their amino acids.
UCP1
An uncoupling protein that is expressed in the mitochondria of brown adipose tissue in mammals. UCP1 permits protons from the space between the inner and outer mitochondrial membranes to move to the matrix (core) of a mitochondrion without driving the production of ATP; thus it promotes uncoupling of oxidative phosphorylation and production of heat from the energy released by electron transport.
Ultrafiltrate
An aqueous solution produced by ultrafiltration. Also called filtrate.
Ultrafiltration
Pressure-driven bulk flow (oozing, streaming) of fluid out of the blood plasma across the walls of blood capillaries (or sometimes through other barriers), considered a form of filtration because solutes of high molecular weight are left behind while ones of low molecular weight travel with the fluid. In ordinary tissues, fluid that leaves the blood plasma by ultrafiltration enters the tissue interstitial fluids. In kidneys that form primary urine by ultrafiltration, the fluid that leaves the blood plasma enters the kidney tubules (e.g., nephrons).
Umami
One of five taste qualities in mammals, stimulated by glutamate. It is the savory taste of proteinaceous foods.
Uncoupling of oxidative phosphoryl-ation
The state of making little or no ATP from the energy that is released by the transport of electrons through the electron-transport chain.
Uncoupling protein (UCP)
Unless otherwise stated, refers to UCP1. See UCP1.
Universal gas law
In the study of gases, an equation that relates pressure (P), volume (V), molar quantity (n), and absolute temperature (T): PV = nRT, where R is the universal gas constant (8.314 J/mol•K, where K is a Kelvin and equivalent to one degree Celsius). Also called the ideal gas law.
Unloading
Deoxygenation of a respiratory pigment.
Unsaturated
In lipid chemistry, characterized by one or more double bonds between carbon atoms in a carbon chain.
Unsaturated fatty acid
A fatty acid in which one or more of the bonds between carbon atoms in the carbon-chain backbone of the molecule are double bonds; also called a UFA or an unsaturate. Monounsaturated fatty acids (MUFAs) have a single double bond in their structure. Polyunsaturated fatty acids (PUFAs) have two or more double bonds in their structure.
Unstriated muscle cells
Contractile cells in which the actin and myosin myofilaments are not arranged in sarcomeres. See smooth muscle.
Uphill transport
See active transport.
Upper-critical temperature
In a homeotherm, the ambient temperature that represents the upper limit of the thermoneutral zone. See also thermoneutral zone.
Upregulation
An upward shift in the catalytic activity of an enzyme, the rate of functioning of a biochemical pathway, or the rate of some other similar process brought about in a controlled manner by a regulatory system.
Ureotelic
Incorporating most nitrogen from the catabolism of nitrogenous compounds into urea. “Most” is defined differently by different authorities; a common approach is to categorize an animal as ureotelic if 50% or more of the nitrogen released by catabolism is incorporated into urea.
Ureter
The tube that carries urine out of a kidney, often discharging it into the bladder.
Uricotelic
Incorporating most nitrogen from the catabolism of nitrogenous compounds into uric acid or closely similar compounds such as urate salts. “Most” is defined differently by different authorities; a common approach is to categorize an animal as uricotelic if 50% or more of the nitrogen released by catabolism is incorporated into uric acid.
Urine
The fluid excreted by a kidney.
Utriculus
Otolithic organ in the vertebrate inner ear containing hair cells that are stimulated by horizontal movements and accelerations.
V̇O2max
An animal’s maximal rate of O2 consumption. It is usually measured by inducing an animal to exercise at its peak sustained exercise intensity. Also called aerobic capacity.
Vaporization
A change in the physical state of a material from a liquid to a gas.
Vasa recta
Blood vessels of minute diameter that are arranged in long hairpin shapes and that constitute the principal vasculature of the medulla of the mammalian kidney. They function as countercurrent diffusion exchangers because blood flow is in opposite directions in the two limbs of each hairpin.
Vascular endothelial growth factor (VEGF)
A locally acting cytokine that stimulates angiogenesis.
Vascular endothelium
See endothelium.
Vascular resistance
The resistance to blood flow through a blood vessel or system of blood vessels, calculated as the pressure drop divided by the flow rate.
Vasoconstriction
Reduction in the inside (luminal) diameter of a blood vessel by contraction of smooth muscles in the vessel wall.
Vasodilation
An increase in the inside (luminal) diameter of a blood vessel permitted by relaxation of smooth muscles in the vessel wall.
Vasomotor
Related to changes in the inside (luminal) diameters of blood vessels mediated by contraction and relaxation of smooth muscles in the blood vessel walls.
Vasopressin
A neurohormone of mammals, consisting of nine amino acids, that stimulates the reabsorption of water by the collecting ducts in the kidneys. Also called antidiuretic hormone. Its molecular structure is similar to that of the neurohormone vasotocin that controls water conservation in many nonmammalian vertebrates. Both are synthesized by neurosecretory cells that have their cell bodies in the hypothalamus and their axon terminals in the pars nervosa of the posterior pituitary.
Vein
A macroscopic blood vessel that carries blood toward the heart.
Venae cavae
Large veins that collect venous blood from many parts of the body for return to the heart.
Venous reserve
In the study of O2 transport, the amount of O2 remaining in venous blood after the blood has passed through the systemic tissues.
Ventilation
Forced flow (convection) of air or water into and out of structures used for external respiration or over body surfaces used for external respiration.
Ventilation-perfusion matching
In a breathing organ, matching of the rate of blood flow and the rate of ventilation so that efficiency is promoted in the transfer of O2 from air or water to the blood.
Ventral aorta
In fish, the vessel that carries blood from the heart to the gills.
Ventricle
A relatively muscular heart chamber.
Vesicle
A small, membrane-bound, spherical organelle in the cytoplasm of a cell.
Vestibular organ
A vertebrate sense organ consisting of statocysts (maculae) and semicircular canals, which together detect gravity and acceleration.
Viscosity
Internal friction in a moving fluid; a lack of intrinsic slipperiness between fluid layers that are moving at different linear velocities. Fluids that are particularly high in internal friction—low in internal slipperiness—have high viscosities and exhibit syruplike properties. Viscosity can also be thought of as resistance to shear forces within a moving fluid.
Vital capacity
In the study of lung function, the difference between lung volume after maximal inhalation and lung volume after maximal exhalation.
Vitamin
An organic compound that an animal must obtain from food, symbiotic microbes, or another source other than the animal’s biosynthesis because the animal cannot synthesize it, yet requires it in small amounts.
Volatile fatty acids
Voltage
A measure of the potential energy present because of charge separation. Separated electrical charges exert electrostatic force on each other. These forces can cause charges to move, and when charges move, work is done. Voltage, also called electrical potential or potential difference, provides a measure of the rate of charge movement that can be achieved.
Voltage clamp
An experimental method to measure ionic current flow by imposing a selected membrane potential on a cell and monitoring the exogenously applied current necessary to maintain that potential by bucking the ionic current.
Voltage threshold
The critical value of membrane depolarization that is just enough to trigger an action potential.
Voltage-gated channel
An ion channel that opens in response to membrane depolarization.
Voltage-gated channel superfamily
The evolutionarily related set of known voltage-gated cation channels.
Volume regulation
The maintenance of a constant or nearly constant volume (amount) of body fluid. The term can be applied to cells, in which case it refers to a constant volume of intracellular fluid. It can also be applied to whole animals, in which case it refers to a constant volume of all body fluids.
Volumes percent (vol %)
Milliliters (at STP) of gas dissolved or chemically combined within a fluid per 100 milliliters of the fluid.
Vomeronasal organ
An accessory olfactory organ of vertebrates that mediates many (but not all) sensory responses to pheromones.
Wasting
See atrophy.
Water channel
A membrane protein that aids passive water transport—osmosis—through a cell membrane.
Water vapor
Water in the gaseous state.
Water vapor pressure
(1) In reference to a gas phase, the prevailing partial pressure of gaseous water. (2) In reference to an aqueous solution, the partial pressure of gaseous water that the solution will create by evaporation in a gas phase with which it is in contact if permitted to come to equilibrium with the gas phase. A colligative property.
Water vapor pressure of the aqueous solution
See water vapor pressure, definition (2).
Water-vapor-pressure depression
The difference between the water vapor pressure of a solution and that of pure water under the same conditions. A colligative property.
Watt (W)
A unit of power (rate of energy use) equal to one joule per second.
Weak bond
See noncovalent bond.
Weight
In biology, commonly used as a synonym for mass (as stated in the U.S. National Institute of Standards and Technology Guide to the SI, in common parlance in the United States, weight is used as a synonym for mass). When weight is used in this way, it is expressed in kilograms. In physics, the force exerted on a mass by Earth’s gravitational acceleration, expressed in Newtons. For physics calculations, weight and mass are dramatically different in meaning; see any physics text.
Weight-specific metabolic rate
An animal’s metabolic rate divided by its weight: metabolic rate per unit of weight.
White adipose tissue
The ordinary fat-storage tissue of vertebrates, exemplified by the “fat” we speak of in poultry or beef prepared for food. Also called white fat. Contrast with brown adipose tissue.
White matter
A histological region of the vertebrate central nervous system that consists largely of tracts of neuronal axons. The abundance of myelin imparts a glistening white sheen to the tissue.
White muscle
In general, a loose term referring to a vertebrate muscle that is poor in myoglobin and thus whitish (rather than reddish) in color. In fish, a large mass of muscle that consists almost entirely of myoglobin-poor muscle fibers that make ATP mostly by anaerobic catabolism; white muscle powers burst exercise in fish.
Work
See external work, internal work, physiological work.
Xeric
Able to live steadily in the open air and thus face the full drying power of the terrestrial environment.
Z disc
In striated muscle, a web of accessory proteins at each end of a sarcomere that anchors the proteins titin and nebulin and the actin thin filaments. Also called Z line, Z band.
Z line
See Z disc.
Zeitgeber
See phasing factor.
Zona pellucida
An extracellular layer of glycoproteins secreted by a primary oocyte.
Zooxanthellae
Symbiotic algae living in the tissues of animals, such as reef-building corals.
Zygote
A genetically unique, diploid cell produced by the union of an egg (ovum) and a sperm in sexual reproduction.
Zymogen
An inactive form of a digestive enzyme, activated after it arrives at its site of action.