Neuroscience 6e Glossary

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A

abducens nerve (VI): Cranial nerve VI, an efferent nerve that controls the lateral rectus muscle of the eye.
accessory olfactory bulb (AOB): The target of axons from the VNO, adjacent to the main olfactory bulb that relays vomeronasal information to the hypothalamus and other basal forebrain regions.
accommodation: 1. Dynamic changes in the lens of the eye that enable the viewer to focus. When viewing distant objects, the lens is made relatively thin and flat; for near vision, the lens becomes thicker and rounder and has more refractive power. 2. Term used by Piaget (along with assimilation) to describe how children might react to a new person, event, or object by modifying their scheme of thought.
acetylcholine (Ach): Neurotransmitter at motor neuron synapses, in autonomic ganglia, and in a variety of central synapses. Binds to two types of acetylcholine receptors (AChRs), either ligand-gated ion channels (nicotinic receptors) and G-protein-coupled receptors (muscarinic receptors).
acetylcholinesterase (AChE): Enzyme in the synaptic cleft that clears the cleft of acetylcholine released by the presynaptic cell. AChE hydrolyzes ACh into acetate and choline; the choline is then transported back into nerve terminals, where it is used to resynthesize ACh.
actin: A cytoskeletal protein involved in maintaining cell shape and organelle movement.
actin cytoskeleton: The meshwork of polymers of the fibrillary protein actin that forms a flexible, but strong scaffold to allow for localization of key proteins and organelles in the cytoplasm, and maintains the integrity of the cell membrane so that the cell retains its volume. The actin cytoskeleton is also essential for force generation in motile cells or motile cell extensions like axonal or dendritic growth cones.
action potential: The electrical signal generated and conducted along axons (or muscle fibers) by which information is conveyed from one place to another in the nervous system (or within muscle fibers).
action tremors: See intention tremors.
activation: The time-dependent opening of ion channels in response to a stimulus, typically membrane depolarization.
active transporters: Transmembrane proteins that actively move ions into or out of cells against their concentration gradients. Their source of energy may be ATP or the electrochemical gradients of various ions. See also co-transporters; ion exchangers.
active zone: The location within the presynaptic terminal where synaptic vesicles fuse with the presynaptic plasma membrane to discharge their neurotransmitters into the synaptic cleft.
adaptation: In the context of evolution, moving animal phenotypes closer to the demands of their environments. Roughly synonymous with evolving “fitness.”
adenylyl cyclase III (ACIII): Membrane-bound enzyme that can be activated by G-proteins to catalyze the synthesis of cyclic AMP from ATP.
adrenaline: See epinephrine.
affective–motivational: The fear, anxiety, and autonomic nervous activation that accompany exposure to a noxious stimulus.
afferent neurons: Neurons or axons that conduct action potentials from the periphery toward the central nervous system.
agnosias: The inability to name objects; literally means “not knowing.”
alar plate: Embryonic gray matter structure in the brainstem and spinal cord that gives rise to sensory nuclei of the brainstem and the dorsal horn of the spinal cord.
allelic silencing: A genomic mechanism that renders one of the two copies of each gene (paternal and maternal copies on each chromosome) unable to be expressed. Allelic silencing is thought to be due to a combination of direct binding of transcription regulators to regulatory sequences of one but not the other allele, as well as local histone modifications. Allelic silencing is a more general example of specific forms of transcriptional selection of one copy versus another of a particular gene. Other examples include parental imprinting when either the paternal or maternal allele of a gene is selectively expressed and the other suppressed, and X-inactivation in females when one of two copies of identical genes on each X chromosome is silenced to maintain appropriate gene dosage.
allodynia: The induction of pain by a normally innocuous stimulus.
allometry: Measurement of one or more body parts in relation to the body as a whole.
α-fetoprotein: A protein that actively sequesters circulating estrogens, preventing maternal estrogen from affecting the sexual differentiation of the fetus.
alpha (α) motor neurons: Neurons in the ventral horn of the spinal cord that innervate force-generating, extrafusal fibers of skeletal muscle.
amacrine cells: Retinal neurons that mediate lateral interactions between bipolar cell terminals and the dendrites of ganglion cells.
amblyopia: Diminished visual acuity as a result of the failure to establish appropriate visual cortical connections in early life.
amnesia: The pathological inability to remember or establish memories; retrograde amnesia is the inability to recall existing memories, whereas anterograde amnesia is the inability to lay down new memories.
AMPA receptors: See ionotropic glutamate receptors.
ampullae: The juglike swellings at the base of the semicircular canals that contain the hair cells and cupulae. See also cupula.
amygdala: A nuclear and corticoid complex in the anterior-medial temporal lobe that forms part of the limbic forebrain; its major functions concern implicit processing with respect to autonomic, emotional, and sexual behavior.
androgen insensitivity syndrome (AIS): A condition in which, due to a defect in the gene that codes for the androgen receptor, testosterone cannot act on its target tissues. Also called testicular feminization.
anesthesia: Procedures that reduce sensation during surgical procedures, most often to alleviate pain or to create a state of unconsciousness.
anesthetics: Drugs that produce anesthesia.
anomalous trichromats: Individuals with one or more atypical cone opsins.
anopsias: A large deficit in the visual field resulting from pathological changes in some component of the primary visual pathway.
anosmias: Loss of the sense of smell; can be total or restricted to a single odorant.
anosmin: The protein encoded by the gene mutated in Kallman’s syndrome, also known as KAL1. Anosmin is a transmembrane Ca²⁺ independent cell adhesion molecule. In the absence of anosmin protein, olfactory receptor neurons do not extend their axons to the olfactory bulb nor do GNRH neurons migrate to the hypothalamus. Thus, individual lacking anosmin because of a specific gene mutation are both unable to smell things (anosmia) and also sterile due to a lack of gonadotrophin releasing hormone secretion.
anterior cerebral arteries: Major vessels derived from the internal carotid arteries that supply the anterior and medial aspects of the frontal and parietal lobes, including associated deep structures.
anterior chamber: The part of the eye that is just behind the cornea and in front of the lens.
anterior choroidal artery: Branch of the proximal middle cerebral artery that supplies blood to the medial temporal lobe and deep white matter and gray matter, including parts of the basal ganglia and the internal capsule.
anterior cingulate cortex (ACC): The portion of the midline frontal lobe comprising the anterior extent of the cingulate gyrus and adjacent cortex; its dorsal regions are associated with executive functions.
anterior circulation: Vasculature derived from the internal carotid arteries that supplies blood to the forebrain.
anterior commissure: A small commissural fiber tract that lies anterior to the third ventricle and inferior to the genu of the corpus callosum; like the callosum, it serves to connect the two hemispheres, but its origins and terminations are mainly in the ventral frontal lobe, olfactory bulb, and anterior temporal lobe.
anterior communicating arteries: Small vessels that cross the midsagittal plane joining the two anterior cerebral arteries, forming the anterior aspect of the circle of Willis.
anterior inferior cerebellar artery (AICA): Long circumferential branch of the basilar artery that supplies dorsolateral aspects of the caudal pons and anterior and inferior aspects of the cerebellum
anterior insula: A functional division of the cerebral cortex located within the lateral sulcus between the temporal lobe and the frontal and parietal lobes. It is associated with emotion and homeostatic regulation.
anterior spinal artery: Principal artery on the anterior aspect of the spinal cord supplied by the vertebral arteries and the medullary arteries; gives rise to some 200 unilateral sulcal branches that alternate left and right along the length of the spinal cord supplying blood to the anterior two-thirds of the cord.
anterograde: Signals or impulses that travel “forward,” e.g., from the cell body to the axon terminal, from the presynaptic terminal to the postsynaptic cell, or from the CNS to the periphery.
anterograde amnesia: The inability to lay down new memories.
anterolateral columns: See ventral columns.
anterolateral system: Ascending sensory pathway in the spinal cord and brainstem that carries information about pain and temperature to the thalamus.
anteroventral paraventricular nucleus (AVPV): A sexually dimorphic hypothalamic nucleus that regulates cyclical ovulation in female mammals.
antiporters: Active transporters that use the energy from ionic gradients to carry multiple ions across the membrane in opposite directions.
apoptosis: Cell death resulting from a programmed pattern of gene expression; also known as programmed cell death.
aprosodias: The inability to infuse language with its normal emotional content. See also prosody.
aqueduct of Sylvius: See cerebral aqueduct.
aqueous humor: A clear, watery liquid that supplies nutrients to the cornea and lens of the eye.
arachnoid mater: One of the three coverings of the brain that make up the meninges; lies between the dura mater and the pia mater and forms a spiderweb-like network of trabeculae (arachnoid mater means “spiderweb-like mother) that allows for the flow of cerebrospinal fluid and the distribution of superficial blood vessels in the subarachoid space.
arachnoid villi: Protrusions of arachnoid mater into the superior sagittal sinus that allow for the passage of cerebrospinal fluid from the subarachnoid space into the venous drainage.
archicortex: Phylogenetically, the simplest and most primitive division of the cerebral cortex, which occurs in the hippocampus.
aromatase: The enzyme that converts testosterone, and some additional steroids, to the active form of estrogen, 17β estradiol.
arousal: 1. A global state of the brain (or the body) reflecting an overall level of responsiveness. 2. The degree of intensity of an emotion.
arrestin: A protein that binds to rhodopsin.
assimilation: Term used by Piaget (along with accommodation) to describe how children adapt to their environment during development.
associational systems: Neural cell circuits that are not part of the relatively defined sensory (input) and motor (output) systems; they mediate the most complex and least well-defined brain functions that require the integration or association of signals from multiple sensory and/or motor systems.
associativity: A mechanism that serves to link together two or more independent processes. For example, associative learning results from the pairing of unconditioned and conditioned stimuli presented to an experimental subject. In the hippocampus, associativity allows a weakly activated group of synapses to undergo long-term potentiation when a nearby group of synapses is strongly activated.
astrocytes: One of the three major classes of glial cells found in the central nervous system; important in maintaining and regulating, in a variety of ways, an appropriate chemical environment for neuronal signaling; also involved in the formation of the blood-brain barrier, the secretion of substances that influence the construction of new synaptic connections, and the proliferation of new cells in the adult brain that retain characteristics of stem cells.
ATPase pumps: Membrane pumps that use the hydrolysis of ATP to translocate ions against their electrochemical gradients.
auditory area 1 (A1): The cortical target of the neurons in the medial geniculate nucleus; the terminus of the primary auditory pathway.
auditory meatus: Opening of the external ear canal.
Auerbach’s plexus: See myenteric plexus.
augmentation: An activity-dependent form of short-term synaptic plasticity that enhances synaptic transmission over a time course of a few seconds. Augmentation is caused by an increase in the amount of neurotransmitter released in response to presynaptic action potentials and results from persistent calcium signaling within presynaptic terminals, perhaps due to actions on the SNARE-regulatory protein, munc13.
Australopithecines: The first ancestors in the human lineage about 3–4 million years ago. The subsequent lineage includes Homo habilis, H. erectus, Neanderthals, and H. sapiens.
autonomic ganglia: Collections of autonomic motor neurons outside the central nervous system that innervate visceral smooth muscles, cardiac muscle, and glands.
autonomic motor division: See visceral motor division.
autonomic nervous system: The components of the nervous system (peripheral and central) concerned with the regulation of smooth muscle, cardiac muscle, and glands. Also known as the visceral motor system; sometimes called the “involuntary” nervous system. Consists of sympathetic and parasympathetic divisions, and a semi-autonomous division in the gut, the enteric nervous system.
autophagy: A cellular state in which proteins and other macromolecules within a cell are isolated and eventually trafficked to lysosomes for degradation. Autophagy is regulated by a number of genes, many of which respond to cellular damage, infection, or oxidative stress.
autosomes: Any chromosome other than the X and Y sex chromosomes.
auxilin: An accessory protein that promotes the actions of HSC70 during vesicle uncoating after endocytosis.
axial sections: See horizontal sections.
axon: The neuronal process (typically, much longer than any dendrite) that conveys the action potential from the nerve cell body to its terminals.
axonal transport: The process that allows cellular components, such as proteins and organelles, to move within the axons of neurons.
Aδ group: Myelinated, faster conducting pain fibers.

B

bacteriorhodopsin: A protein that, in response to light of the proper wavelength, acts as a proton pump transporting protons from inside the cell to outside; in its native host, the resulting proton gradient is subsequently converted into chemical energy; when engineered into a neuron for optogenetics, it hyperpolarizes the neuron when exposed to light.
Balint’s syndrome: A neurological syndrome, caused by bilateral damage to the posterior parietal and lateral occipital cortex, that has three hallmark symptoms: (1) simultanagnosia, the inability to attend to and/or perceive more than one visual object at a time; (2) optic ataxia, the impaired ability to reach for or point to an object in space under visual guidance; and (3) oculomotor apraxia, difficulty voluntarily directing the eye gaze toward objects in the visual field with a saccade. Simultanagnosia is the sign most closely associated with the syndrome, and the one most studied from a cognitive neuroscience standpoint.
basal cells: Basal cells are found in the region of the olfactory epithelium adjacent to the lamina propria, where blood vessels and connective tissue that support the olfactory epithelium are found. They retain neural stem cell identity and can generated new olfactory receptor neurons throughout life.
basal forebrain nuclei: Cerebral nuclei anterior to the hypothalamus and ventral to the basal ganglia; give rise to widespread modulatory projections to diverse targets in the cerebral hemispheres.
basal ganglia: Cerebral nuclei lying deep in the subcortical white matter of the cerebral hemispheres lateral and central to the lateral ventricle. The caudate, putamen, and globus pallidus are the major components of the basal ganglia; together with the subthalamic nucleus and substantia nigra, these structures modulate the initiation and suppression of behavior.
basal lamina: A thin layer of extracellular matrix material (primarily collagen, laminin, and fibronectin) that surrounds muscle cells and Schwann cells. Also underlies all epithelial sheets. Also called the basement membrane.
basal plate: Embryonic gray matter structure in the brainstem and spinal cord that gives rise to motor nuclei of the brainstem and the ventral horn of the spinal cord.
basilar artery: Major vessel formed by the fusion of the two vertebral arteries that lies along the ventral midline of the pons; gives rise to the anterior inferior and superior cerebellar arteries before bifurcating and giving rise to the paired posterior cerebral arteries at the midbrain.
basilar membrane: The membrane that forms the floor of the cochlear duct, on which the cochlear hair cells are located.
basket cells: Inhibitory interneurons in the cerebellar cortex whose cell bodies are located within the Purkinje cell layer and whose axons make basket-like terminal arbors around Purkinje cell bodies, providing lateral inhibition that focuses the spatial distribution of Purkinje cell activity.
belt and parabelt regions: Regions of the auditory cortex that surround the core region.
bHLH (basic helix-loop-helix): Neurogenic transcription factors (named for a shared basic helix-loop-helix amino acid motif that defines their DNA-binding domain) that have emerged as central to the differentiation of distinct neural and glial fates.
binocular field: The two symmetrical, overlapping visual hemifields. The left hemifield includes the nasal visual field of the right eye and the temporal visual field of the left eye; the right hemifield includes the temporal field of the right eye and the nasal field of the left eye.
biogenic amines: Category of small-molecule neurotransmitters; includes the catecholamines (epinephrine, norepinephrine, dopamine), serotonin, and histamine.
bipolar cells: Retinal neurons that provide a direct link between photoreceptor terminals and ganglion cell dendrites.
bitter: One of the five basic tastes; the taste quality, generally considered unpleasant, produced by substances like quinine or caffeine. Compare salty, sour, and sweet. Bitter is transduced by taste cells via the T2R G-protein-coupled taste receptors.
blindsight: A pathological phenomenon in which patients with damage to the primary visual cortex are blind in the affected area of the contralateral visual field.
blood oxygenation level-dependent (BOLD): Endogenous signals reflecting the oxygenation of hemoglobin in blood that are modulated by changes in the local level of neural activity; for example, when neural activity in a local brain region increases, more oxygen is consumed and within seconds the local microvasculature responds by increasing the flow of oxygen-rich blood to the active region, thus constituting a BOLD signal that may be detected by fMRI.
blood–brain barrier: A diffusion barrier between the cerebral vasculature and the substance of the brain formed by tight junctions between capillary endothelial cells and the surrounding astrocytic endfeet.
bone morphogenetic proteins (BMPs): Peptide hormones that play important roles in neural induction and differentiation.
Bowman’s glands: Mucous producing specializations composed of secretory cells surrounding a lumen that is continuous with the surface of the olfactory epithelium.
brachium conjunctivum: See cerebellar peduncles.
brachium pontis: See cerebellar peduncles.
brain: The rostral (supraspinal) portion of the central nervous system comprised of the cerebral hemispheres, diencephalon, cerebellum, and brainstem.
brain-derived neurotrophic factor (BDNF): One member of a family of neutrophic factors, the best-known constituent of which is nerve growth factor (NGF).
brainstem: The portion of the brain that lies between the diencephalon and the spinal cord; comprises the midbrain, pons, and medulla.
branchial motor nuclei: Brainstem nuclei (derived from the basal plate) that give rise to efferent fibers innervating striated muscle fibers derived from embryonic branchial (pharyngeal) arches; located in an intermediate position in the tegmentum after embryonic migration from a more dorsal location.
Broca’s aphasia: Difficulty producing speech as a result of damage to Broca’s area in the left frontal lobe. Also called motor, expressive, or production aphasia.
Broca’s area: An area in the left inferior frontal lobe specialized for the production of speech and the expression of language in non-vocal forms.
Brodmann’s area 17: Another name for the primary visual cortex (V1) in the occipital lobe; major cortical target of the retinal sensory cells. Also called striate cortex because the prominence of layer 4 in myelin-stained sections gives this region a striped (striated) appearance.

C

C fiber group: Unmyelinated, slower conducting pain fibers.
c-fos: A transcription factor, originally isolated from cellular feline osteosarcoma cells, that binds as a heterodimer, thus activating gene transcription.
Ca²⁺/calmodulin-dependent protein kinase, type II: A protein kinase that is activated by the second messenger, calcium ions, binding to the calcium-binding protein calmodulin. Once activated by calcium and calmodulin, this protein kinase can phosphorylate numerous substrate proteins to alter their signaling properties.
CACNA genes: Ca²⁺ channel genes.
cadherins: A family of calcium-dependent cell adhesion molecules found on the surfaces of growth cones and the cells over which they grow.
calbindin: A protein that slows transient changes in intracellular calcium concentration by reversibly binding calcium ions.
calcarine sulcus: Major sulcus on the medial aspect of the occipital lobe that divides the cuneus and lingual gyri; the primary visual (striate) cortex lies largely within this sulcus.
calcium imaging: Method of monitoring by optical means the levels of calcium within cells using calcium-sensitive fluorescent dyes; calcium dynamics within the cytoplasm of neurons reflect the integration of synaptic inputs and the generation of postsynaptic electrical activity.
calcium pump: ATPases that remove calcium ions from the cytoplasm of cells. Calcium pumps are found both on the plasma membrane and intracellular membranes, such as the endoplasmic reticulum.
calmodulin: A calcium-binding protein that serves as a sensor for many calcium-regulated intracellular signaling processes.
cAMP response element binding protein (CREB): A protein activated by cyclic AMP that binds to specific regions of DNA, thereby increasing the transcription rates of nearby genes.
CAMs: The general abbreviation for all cell adhesion molecules.
cataracts: Opacities in the lens of the eye that cause a loss of transparency and, ultimately, degrading vision.
catecholamines: A term referring to molecules containing a catechol ring and an amino group; examples are the neurotransmitters epinephrine, norepinephrine, and dopamine.
cauda equina: The collection of segmental ventral and dorsal roots that attach to the lumbosacral enlargement through the caudal segments of the spinal cord and pass out of the spinal canal through caudal intervertebral foramina and the sacrum.
caudate: One of the three major components of the striatum (the other two are the putamen and nucleus accumbens)
cavernous sinus: Dural venous sinus in the anterior middle cranial fossa; drains venous blood from ventral cerebral hemisphere and the face. Cranial nerves III, IV, two divisions of V, and VI, and the internal carotid artery all pass through the cavernous sinus.
cell-associated signaling molecules: Chemical signals that are attached to, or embedded in, the plasma membrane.
cell-impermeant molecules: Chemical signals that are incapable of permeating the plasma membrane, either because they are too hydrophobic or are attached to the membranes of other nearby cells. Such molecules activate intracellular signaling by activating receptors on the plasma membrane of their target cells.
cell-permeant molecules: Hydrophobic chemical signals that are capable of permeating the plasma membranes of their target cells. Such molecules activate intracellular signaling by activating receptors in the cytoplasm or nucleus of their target cells.
central autonomic network: Collection of nuclei and cortical regions in the brain that integrate visceral sensory signals, distribute those signals to more widespread brain regions, and give rise to signals that govern visceral motor activity.
central nervous system (CNS): The brain and spinal cord of vertebrates (by analogy, the central nerve cord and ganglia of invertebrates).
central pattern generators: Oscillatory spinal cord or brainstem circuits responsible for programmed, rhythmic movements such as locomotion.
central sensitization: Increased excitability of neurons in the dorsal horn following high levels of activity in nociceptive afferents and that can result in hyperalgesia and allodynia.
central sulcus: A major sulcus on the dorsolateral aspect of the cerebral hemispheres that forms the boundary between the frontal and parietal lobes. The anterior bank of the sulcus contains the primary motor cortex; the posterior bank contains the primary sensory cortex.
cephalic flexure: Sharp bend in the neural tube which during early neurulation balloons out to form the prosencephalon, which in turn will give rise to the forebrain and later to the cerebral hemispheres.
cerebellar ataxia: A pathological inability to make coordinated movements, associated with lesions or congenital malformation of the cerebellum.
cerebellar cortex: Laminated, superficial gray matter of the cerebellum.
cerebellar peduncles: Three bilateral pairs of tracts that convey axons to and from the cerebellum. The superior cerebellar peduncle, or brachium conjunctivum, is primarily an efferent motor pathway; the middle cerebellar peduncle, or brachium pontis, is an afferent pathway arising from the pontine nuclei. The smallest but most complex is the inferior cerebellar peduncle, or restiform body, which encompasses multiple afferent and efferent pathways.
cerebellum: Prominent hindbrain structure concerned with motor coordination, posture, and balance; derived from the embryonic metencephalon. Composed of a three-layered cortex and deep nuclei; attached to the brainstem by the cerebellar peduncles.
cerebral achromatopsia: Loss of color vision as a result of damage to extrastriate visual cortex.
cerebral akinetopsia: A rare disorder in which one is unable to appreciate the motion of objects.
cerebral angiography: An x-ray based means for imaging blood vessels in the brain involving injection of a contrast agent into the systemic circulation.
cerebral aqueduct: Narrow channel derived from the lumen of the neural tube in the dorsal mesencephalon that connects the third and fourth ventricles. Also called the aqueduct of Sylvius.
cerebral cortex: The superficial gray matter of the cerebral hemispheres derived from the outer aspect of the telencephalic vesicles.
cerebral hemispheres: The two symmetrical halves of the cerebrum derived from the telencephalic vesicles.
cerebral nuclei: Masses of gray matter located in the deep or basal regions of the cerebral hemispheres, including the basal ganglia, basal forebrain nuclei, septal nuclei, and nuclear components of the amygdala.
cerebral peduncles: Paired “stalks” (peduncle means stalk) of white matter that define the ventral aspect of the midbrain; contain major axon tracts that originate in the cerebral cortex and terminate in the brainstem and spinal cord, including the corticopontine, corticobulbar and corticospinal tracts.
cerebrocerebellum: Lateral part of the cerebellar hemisphere, greatly expanded in humans, that receives input from the cerebral cortex via axons from pontine relay nuclei and sends output to the premotor and prefrontal cortex via the thalamus; concerned with the planning and execution of complex spatial and temporal sequences of skilled movement.
cerebrospinal fluid (CSF): Clear and cell-free fluid that fills the ventricular system of the central nervous system produced by choroid plexus in the ventricles.
cervical: Rostral region of the spinal cord related to the upper trunk and upper extremities.
cervical enlargement: The spinal cord expansion that relates to the upper extremities; includes spinal segments C3–T1.
channel-linked receptors: Receptors that are ligand-gated ion channels; binding of ligands leads to channel opening.
channelrhodopsin: Typically, a protein that, in response to light of the proper wavelength, opens a channel that is permeable to cations; when engineered into a neuron for optogenetics, it depolarizes the neuron when exposed to light; anion-conducting channelrhodopsins have also been discovered, which would have inhibitory effects when activated in mature neurons.
characteristic frequency: The lowest threshold of a tuning curve.
chemical synapses: Synapses that transmit information via the secretion of chemical signals (neurotransmitters).
chemoaffinity hypothesis: The idea that nerve cells bear chemical labels that determine their connectivity.
cholinergic nuclei: Nuclei in which synaptic transmission is mediated by acetylcholine.
Chordin: An endogenous antagonist of Bmps that acts in combination with Noggin.
choroid plexus: Specialized, highly vascularized epithelium in the ventricular system that produces cerebrospinal fluid.
chromosomal sex: The sex of an individual organism based upon the sex chromosomes in its genome.
chronic traumatic encephalopathy (CTE): A neurological syndrome that is caused by repeated concussive force delivered to the head. This syndrome is common in athletes who sustain constant forceful blows to the head, including boxers, hockey players and football players. In addition, soldiers exposed to repeated explosive blasts at close range are at risk. While alive, individuals with CTE suffer a gradual dementia like decline in cognition and social behaviors. At autopsy, their brains have several signs of neurodegenerative and neuroinflammatory damage, and resemble those of individuals, often far older in age, with Alzheimer’s disease or other age-related neurodegenerative disorders.
ciliary body: Two-part ring of tissue encircling the lens of the eye. The muscular component is important for adjusting the refractive power of the lens. The vascular component produces the fluid that fills the front of the eye.
ciliary muscle: Muscle that controls the shape of the lens.
cingulate gyrus: Prominent gyrus on the medial aspect of each cerebral hemisphere, lying just superior to the corpus callosum; a major component of the limbic forebrain.
cingulate sulcus: Prominent sulcus on the medial aspect of each cerebral hemisphere.
circadian: Refers to variations in physiological functions that occur on a daily basis.
circle of Willis: Arterial anastomosis on the ventral aspect of the midbrain; connects the posterior and anterior cerebral circulation.
circumvallate papillae: Circular structures that form an inverted V on the rear of the tongue (three to five on each side, with the largest in the center). Circumvallate papillae are moundlike structures surrounded by a trench (like a moat). These papillae are much larger than fungiform papillae. Compare foliate papillae and fungiform papillae.
cisterns: Large, cerebrospinal fluid-filled spaces that lie within the subarachnoid space.
Clarke’s nucleus: A group of relay neurons (also called the dorsal nucleus of Clarke) located in the medial aspect of the intermediate gray matter of the spinal cord (lamina VII) in spinal levels T1 through L2–3; conveys proprioceptive signals originating in the lower body to the ipsilateral cerebellum and the dorsal column nuclei via the dorsal spinocerebellar tract.
classical conditioning: Also called conditioned reflex. The modification of an innate reflex by associating its normal triggering stimulus with an unrelated stimulus. The unrelated stimulus comes to trigger the original response by virtue of this repeated association. Compare operant conditioning.
clathrin: The most important protein for endocytotic budding of vesicles from the plasma membrane; its three-pronged “triskelia” attach to the vesicular membrane to be retrieved.
CLCN genes: Cl^– channel genes.
climbing fibers: Axons that originate in the inferior olivary nuclei, ascend through the inferior cerebellar peduncle, and make terminal arborizations that invest the proximal dendritic trees of Purkinje cells; induce complex spikes and long-term depression in cerebellar Purkinje neurons.
co-transmitters: Two or more types of neurotransmitters within a single synapse; may be packaged into separate populations of synaptic vesicles or co-localized within the same synaptic vesicles.
co-transporters: Active transporters that use the energy from ionic gradients to carry multiple ions across the membrane in the same direction.
coccygeal: Most caudal region of the spinal cord.
cochlea: The coiled structure in the inner ear where vibrations caused by sound are transduced into neural impulses.
cochlear microphonics: The generation of sound by the cochlea, thought to be driven by the cochlear amplifier.
cognitive neuroscience: The field of neuroscience devoted to studying and understanding cognitive functions.
coincidence detector: A device that detects the simultaneous presence of two or more signals. In the context of long-term synaptic plasticity, a mechanism for detecting the coioncidence of two or more synaptic signals; for example, NMDA receptors detect the simultaneous occurrence of presynaptic glutamate release and postsynaptic depolarization during long-term synaptic potentiation.
coincidence detectors: A neuron that detects simultaneous events, as in sound localization.
collagens: Fibrillary extracellular matrix proteins with multiple binding domains for a variety of cell surface receptors.
columns: Term used to describe an elongated gray matter structure (e.g., the motor neuronal pool in the ventral horn of the spinal cord that innervates a muscle) or a subdivision of white matter (e.g., a region of white matter in the spinal cord containing long axon tracts).
coma: A pathological state of profound and persistent unconsciousness.
commissures: Axon tracts that cross the midline of the brain or spinal cord.
common currency theory: A hypothesis about the function of the reward system. In this theory, the brain uses a single scale to compare the values of all goods; the scale is consistent for different items regardless of their type.
competitive interaction: The struggle among nerve cells, or nerve cell processes, for limited resources essential to survival or growth.
computational map: An assembly of neural circuits in a specific brain region that represent inputs that do not have a direct correspondence to a topographic map, such as those in the somatosensory or visual systems. Some cognitive capacities, including language and declarative memory, are thought to depend on computational maps.
computerized tomography (CT): Radiographic procedure in which a three-dimensional image of a body structure is constructed by computer from a series of cross-sectional X-ray images.
concha: A component of the external ear.
conditional mutations: A genetic engineering approach, typically reliant upon the Cre/lox system, whereby an exogenous recombinase enzyme recognizes unique DNA excision sequences (loxP sequences) introduced at the 5’ and 3’ ends of an endogenous gene and eliminates the intervening sequence.
conditioned learning: The generation of a novel response that is gradually elicited by repeatedly pairing a novel stimulus with a stimulus that normally elicits the response being studied.
conduction aphasia: Difficulty producing speech as a result of damage to the connection between Wernicke’s and Broca’s language areas.
conduction velocity: The speed at which an action potential is propagated along an axon.
conductive hearing loss: Diminished sense of hearing due to the reduced ability of sounds to be mechanically transmitted to the inner ear. Common causes include occlusion of the ear canal, perforation of the tympanic membrane, and arthritic degeneration of the middle ear ossicles. Contrast with sensorineural hearing loss.
cones: Photoreceptor cells specialized for high visual acuity and the perception of color.
conflict: A psychological process that arises when multiple competing demands compete for control of behavior or attention. It is usually associated with increased error rates and/or reaction times.
confluence of sinuses: Dural venous sinus formed by the junction of the superior sagittal sinus with the transverse sinuses.
congenital adrenal hyperplasia (CAH): Genetic deficiency that leads to overproduction of androgens and a resultant masculinization of external genitalia in genotypic females.
conjugate eye movements: The paired movements of the two eyes in the same direction, as occurs in saccades, smooth pursuit eye movements, optokinetic movements, and the vestibulo-ocular reflex.
connexins: Transmembrane proteins that serve as the subunits of connexons, the transcellular channels that permit electrical and metabolic coupling between cells at electrical synapses. See gap junctions.
connexons: Precisely aligned, paired transmembrane channels that form gap junctions between cells. They are formed from connexins, members of a specialized family of channel proteins.
contralateral neglect syndrome: Neurological condition in which the patient does not acknowledge or attend to the left visual hemifield or the left half of the body. The syndrome typically results from lesions of the right parietal cortex.
convergence: Innervation of a target cell by axons from more than one neuron. In vision refers specifically to the convergence of both rod and cone photoreceptor cells onto retinal ganglion cells.
cornea: The transparent surface of the eyeball in front of the lens; the major refractive element in the optical pathway.
coronal: Standard anatomical planes of section; any vertical plane passing parallel to the medial-to-lateral axis through the head (in humans, parallel to the face), dividing the head into anterior (front) and posterior (rear) segments. Also known as frontal.
corpus callosum: The large medial fiber bundle that connects the cortices of the two cerebral hemispheres.
cortex (pl. cortices): The superficial mantle of gray matter (a sheet-like array of nerve cells) covering the cerebral hemispheres and cerebellum, where most of the neurons in the brain are located.
corticobulbar tract: Pathway carrying motor information from the motor cortex to brainstem nuclei.
corticocortical connections: Connections made between cortical areas in the same hemisphere, or between corresponding areas in the two hemispheres via the cerebral commissures.
corticospinal tract: Pathway carrying motor information from the motor cortex to the spinal cord. Essential for the performance of discrete voluntary movements, especially of the hands and feet.
corticostriatal pathway: Excitatory (glutamateric) projections from deep layers of the cerebral cortex to the striatum. Projections are organized topographically with distinct cortical areas projecting to distinct divisions of the striatum.
covert attention: The focusing of visual attention toward a location or item in the visual field without shifting the direction of gaze. Can apply to other sensory modalities or to attentional paradigms. Compare overt attention.
cranial nerve ganglia: The sensory ganglia associated with the cranial nerves; these correspond to the dorsal root ganglia of the segmental nerves of the spinal cord.
cranial nerves: The 12 pairs of sensory, motor, and mixed sensorimotor nerves that innervate targets in the head and neck.
cranial placodes: The local thickening of the non-neural surface ectoderm of the mid-gestation embryo that undergo a form of neural induction so that they can generate cranial peripheral sensory neurons (mechanoreceptor cells in the cranial sensory ganglia, olfactory and vomeronasal receptor neurons in the nose, and hair cells in the inner ear) as well as the lens in the eye.
Cre recombinase: A viral DNA cutting enzyme used to excise a floxed exons. See Cre/lox.
Cre/lox: A genetic engineering system for achieving conditional mutations of endogenous mammalian genes using introduced loxP sequences, which are not found in mammalian genomes but occur in bacterial genomes and are targeted by certain viruses, and a viral DNA cutting enzyme, Cre recombinase. With expression of the Cre DNA introduced into host genome, the resulting Cre recombinase engages the loxP binding sites, and the intervening endogenous exon targeted for elimination (the so-called floxed sequence) is excised.
credit assignment: When multiple events occur and are associated with different values, the decision-maker must determine which event produces which value. This process, known as credit assignment, is trivially easy in many cases, but in other cases, can be quite difficult.
cribiform plate: A bony structure of the facial portion of the skull comprising many small fensetra (tiny holes), at the level of the eyebrows, that separates the olfactory epithelium from the brain. The axons from the olfactory sensory neurons pass through the tiny holes of the cribriform plate to enter the brain.
crista: The hair cell-containing sensory epithelium of the semicircular canals.
critical periods: Restricted developmental periods during which the nervous system is particularly sensitive to the effects of experience.
cuneate nucleus: Somatosensory relay nucleus in the lower medulla containing second-order sensory neurons that relay mechanosensory information originating in peripheral receptors in the upper body (excluding the face) to the contralateral thalamus.
cuneate tract: Lateral division of the dorsal column in the upper half of the spinal cord containing the central processes of first-order afferents and the postsynaptic dorsal column projection; conveys mechanosensory signals derived from the upper body excluding the face.
cuneus gyrus: Gyral structure on the superior aspect of the medial occipital lobe forming the upper bank of the calcarine sulcus; portion of the primary visual cortex that represents the inferior quadrant of the contralateral visual hemifields.
cupula: Gelatinous structure in the semicircular canals in which the hair cell bundles are embedded.
cyclic nucleotide-gated channels: A class of ion channels that are activated and inactivated by second messenger cascades. These second messenger cascades usually involve the activation of a G-protein that is coupled to a G-protein-coupled receptor leading to increased phosphorylation capacity of adenylyl or guanyl cyclases: enzymes that can phosphorylate the channels and modify their permeability to ions.
cytoarchitectonic areas: Distinct regions of the neocortical mantle identified by differences in cell size, packing density, and laminar arrangement (layering). Most prominent in humans is the 6-layered neocortex. The evolutionary older archicortex (or hippocampal cortex) has 3–4 layers, and the ancient paleocortex has 3 layers.

D

decerebrate rigidity: Excessive tone in extensor muscles as a result of damage to descending motor pathways at the level of the brainstem.
declarative memory: Memories available to consciousness that can be expressed by language.
decussation of the superior cerebellar peduncle: Midline crossing of fibers in the superior cerebellar peduncles as they converge in the tegmentum of the rostral pons.
deep cerebellar nuclei: Subcortical nuclei at the base of the cerebellum that give rise to output from the cerebellum to the thalamus and brainstem; integrate afferent signals to the cerebellum and cortical processing conveyed by Purkinje neurons.
delayed response genes: Genes whose protein products are not produced rapidly after a triggering stimulus. The delay in gene expression is caused by the requirement for transcriptional regulators that must first be synthesized in response to the stimulus.
delayed response task: A behavioral paradigm used to test cognition and memory.
delta ligands: Transmembrane proteins whose ectodomain (the region of the protein that extends beyond the cell’s outer membrane) binds to receptors (notch proteins) on the surfaces of adjacent cells to initiate a signaling cascade that promotes local cellular differentiation.
delta waves: Slow (<4 Hz) electroencephalographic waves that characterize stage IV (slow-wave) sleep.
dendrites: Neuronal processes (typically, much shorter than the axon) arising from the nerve cell body that receive synaptic input.
dendritic polarization: The process by which the dendrite of a nerve cell, which is the primary site for synaptic input at post-synaptic specializations becomes distinguished from the axon, which is the primary site for synaptic output at presynaptic terminals.
dentate nucleus: Largest and most lateral of the deep cerebellar nuclei; source of output from the cerebrocerebellum to the premotor and prefrontal cortex via the thalamus and to the parvocellular red nucleus.
depolarization: Displacement of a cell’s membrane potential toward a less negative value.
dermatomes: The area of skin supplied by the sensory axons of a single dorsal root ganglion.
dichromatic: The color vision that arises when animals have only two cone types.
diencephalon: Portion of the brain derived from the posterior part of the embryonic forebrain vesicle that lies just rostral to the midbrain; comprises the thalamus and hypothalamus.
diffusion tensor imaging (DTI): A type of magnetic resonance imaging used in live humans that allows for the selective visualization of large axon tracts in the brain based upon the alignment of the water molecules in myelinated axons bundled together and extending in the same direction.
dimorphic: Having two different forms depending on genotypic or phenotypic sex.
diplopia: Double vision.
disconjugate (disjunctive) eye movements: Movements of the two eyes in opposite directions, such as in vergence eye movements.
dissociated sensory loss: Loss of mechanosensation on one side of the body accompanied by pain and temperature deficits on the other side of the body, often caused by lateral hemisection of the spinal cord.
divergence: The branching of a single axon to innervate multiple target cells.
dopamine: A catecholamine neurotransmitter that is involved many brain functions, including motivation, reward and motor control.
dorsal anterior cingulate cortex (dACC): A functional division of the prefrontal cortex that surrounds the corpus callosum and includes the cingulate gyrus; important in emotional and visceral motor behavior.
dorsal columns: Major ascending tracts in the spinal cord that carry mechanosensory information from first-order sensory neurons in dorsal root ganglia and second-order, postsynaptic dorsal column projection neurons to the dorsal column nuclei; also called the posterior funiculi.
dorsal horns: The dorsal portions of the spinal cord gray matter derived from the alar plate; populated by neurons that process somatosensory information.
dorsal motor nucleus of the vagus nerve: Visceral motor nucleus of the rostral medulla containing parasympathetic preganglionic neurons that innervate thoracic and upper abdominal visceral, mediating a range of autonomic functions.
dorsal nucleus of Clarke: Column of relay neurons in the medial aspect of the intermediate gray matter of the spinal cord from T1–L3; receives first-order sensory signals from proprioceptors that supply the lower body and gives rise to ipsilateral dorsal spinocerebellar tract. Also called Clarke’s nucleus.
dorsal root ganglia: The segmental sensory ganglia of the spinal cord; they contain the cell bodies of the first-order neurons of all somatic sensory and visceral sensory pathways arising in the spinal cord.
dorsal roots: The bundle of axons that runs from the dorsal root ganglia to the dorsal horn of the spinal cord, carrying somatosensory information from the periphery.
dorsal spinocerebellar tract: Axonal projection arising from Clarke’s nucleus to the ipsilateral cerebellum and the dorsal column nuclei; conveys proprioceptive signals originating in the lower body.
dorsolateral geniculate nucleus: The portion of the thalamus that sends (and receives) axons to the cerebral cortex via the internal capsule.
dorsolateral prefrontal cortex (DLPFC): A functional division of the prefrontal cortex roughly corresponding to the middle and superior frontal gyri, as located anterior to motor cortex and the frontal eye fields. Compare ventrolateral prefrontal cortex.
dorsolateral tract of Lissauer: A small bundle of mostly unmyelinated axons that is situated on the posterior margin of the dorsal horn and that conveys pain information to second order neurons in Rexed’s laminae 1, 2, and 5.
dreaming: A unique state of awareness that entails some features of memory and hallucinations in the sense that the experience of dreams is not related to corresponding sensory stimuli arising from the present environment.
DSCAM: A class of transmembrane cell adhesion molecules encoded by the DSCAM genes. In Drosophila, the DSCAM gene structure allows for a total of 37,000 possible splice variants based upon its complex structure of exons for alternative splicing. The DSCAMs can either bind homophilically (binding between the same isoforms) or heterophilically (binding between different isoforms) to initiate avoidance of dendrites or axon branches from the same neuron, or recognition and apposition of dendrites or axons from different cells.
dura mater: The thick external covering (dura mater means “tough mother”) of the brain and spinal cord; one of the three components of the meninges, the other two being the pia mater and arachnoid mater
dynamin: A GTP-hydrolyzing enzyme involved in the fission of membranes during endocytosis.
dynorphins: A class of endogenous opioid.
dysdiadochokinesia: Difficulty performing rapid alternating movements.
dysmetria: Inaccurate movements due to faulty judgment of distance, especially over- or underreaching; characteristic of cerebellar pathology.

E

ectoderm: The most superficial of the three embryonic germ layers; gives rise to the nervous system and epidermis.
Edinger–Westphal nucleus: Midbrain visceral motor nucleus containing the parasympathetic preganglionic neurons that constitute the efferent limb of the pupillary light reflex.
efferent neurons: Neurons or axons that conduct information away from the central nervous system toward the periphery.
electrical synapse: Synapses that transmit information via the direct flow of electrical current at gap junctions.
electrochemical equilibrium: The condition in which no net ionic flux occurs across a membrane because ion concentration gradients and opposing transmembrane potentials are in exact balance.
electroencephalography (EEG): The study of electrical potentials generated in the brain recorded from electrodes placed on the scalp.
electrophysiological recording: Measure of the electrical activity across the membrane of a nerve cell by use of electrodes.
embryonic stem cells (ES cells): Cells derived from pre-gastrula embryos that have the potential for infinite self-renewal and can give rise to all tissue and cell types of the organism. See also glial stem cells; neural stem cells.
emmetropic: Having normal vision.
end plate current (EPC): A macroscopic postsynaptic current resulting from the summed opening of many ion channels; produced by neurotransmitter release and binding at the motor end plate.
end plate potential (EPP): Depolarization of the membrane potential of skeletal muscle fiber, caused by the action of the transmitter acetylcholine at the neuromuscular synapse.
end plates: The complex postsynaptic specializations at the site of nerve contact on skeletal muscle fibers.
endocannabinoids: A family of endogenous signals that participate in several forms of synaptic transmission, interacting cannabinoid receptors. These receptors are the molecular targets of the psychoactive component of the marijuana plant, Cannabis.
endocrine: Referring to the release of signaling molecules whose effects are made widespread by distribution in the general circulation.
endoderm: The innermost of the three embryonic germ layers. Gives rise to the digestive and respiratory tracts and the structures associated with them.
endogenous antagonists: The endogenous antagonists are secreted proteins from the notochord and other sources that bind to Bmp ligands and inactivate them. The specification of the neuroectoderm relys upon the activity of the endogenous antagonists to prevent the undifferentiated ectoderm from becoming epidermal ectoderm.
endogenous attention: A form of attention in which processing resources are directed voluntarily to specific aspects of the environment; typically prompted by experimental instructions or, more normally, by an individual’s goals, expectations, and/or knowledge. Compare exogenous attention.
endogenous opioids: Peptide neuritransmitters in the central nervous system that have the same pharmacological effects as morphine and other derivatives of opium, being agonists at opioid receptors, virtually all of which contain the sequence Tyr-Gly-Gly-Phe. There are three classes: dynorphins, endorphins, and enkephalins.
endolymph: The potassium-rich fluid filling both the cochlear duct and the membranous labyrinth; bathes the apical end of the hair cells.
endorphins: A family of neuropeptides first identified as endogenous mimics of morphine. These neuropeptides, as well as morphine, act by activating opioid receptors.
engram: A term used to describe the physical basis of a stored memory.
enkephalins: A class of endogenous opioid.
enteric nervous system (ENS): A subsystem of the visceral motor system, made up of small ganglia and individual neurons scattered throughout the wall of the gut; influences gastric motility and secretion. Also called the enteric system.
enzyme-linked receptors: Receptors that activate intracellular signaling processes via their enzymatic activity. Most of these receptors are protein kinases that phosphorylate intracellular target proteins.
ephrin: A large family of transmembrane cell surface adhesion molecules, also referred to as Eph ligands that can bind to and activate the protein kinase activity of ephrin receptors. Ephrins also can initiate retrograde signaling in the cell whose membrane they are embedded in.
ephrin receptors: Ephrin or Eph receptors are a large family of transmembrane receptor tyrosine kinases (RTKs). Upon binding of an eprhin (also known as an eph ligand) embedded in the membrane of a neighboring cell or process, the tyrosine kinase domain of the receptor is activated leading to phosphorylation of multiple protein targets and initiation of signaling that changes the cytoskeleton, cell motility and gene expression.
epibrachial placodes: The cranial placodes found in the branchial arches that contribute neural precursors to cranial sensory ganglia to generate mechanoreceptor sensory neurons.
epinephrine: Catecholamine hormone and neuro-transmitter that binds to adrenergic G-protein-coupled receptors.
epithelial-to-mesenchymal transition: The dissolution of tight junctions and other molecular specializations that maintain cells in a sheet like (epithelial) arrangement so that the liberated cells can acquire motile capacity and migrate to distal locations. The delamination and migration of the neural crest is the best-known example of an epithelial-to-mesenchymal transition in the embryo. These changes, resulting in migratory cells, can also be pathological in mature tissues, leading to metastatic tumor formation when transformed cells escape epithelial constraints.
equilibrium potential: The membrane potential at which a given ion is in electrochemical equilibrium.
error-related negativity (ERN): An electrophysiological marker that occurs when participants make errors in cognitive tasks.
esotropia: A type of ocular misalignment in which one or both eyes of an individual turn inward (toward the nose).
estradiol: The principal estrogen, secreted by ovarian follicles.
estrogen: A steroid hormone (including estradiol) that affects sexual differentiation during development and reproductive function and behavior in mature adults.
evaluation: Assigning a specific value to an option or possible action. Usually based on learned associations with past experiences with similar options or actions.
event related potential (ERP): Averaged EEG recordings measuring time-locked brain responses to repeated presentations of a stimulus or repeated execution of a motor task.
excitatory: Postsynaptic potentials that increase the probability of firing a postsynaptic action potential (EPSPs).
executive control: The cognitive functions that allow flexible and goal-directed control of thought and behavior.
exocytosis: A form of cell secretion resulting from the fusion of the membrane of a storage organelle, such as a synaptic vesicle, with the plasma membrane.
exogenous attention: Also called reflexive attention. A form of attention in which processing resources are directed to specific aspects of the environment in response to a sudden stimulus change, such as a loud noise or sudden movement, that attracts attention automatically. Compare endogenous attention.
exotropia: A type of ocular misalignment in which one or both eyes of an individual turn outward (toward the temporal bones, or the ears).
express saccades: A reflexive type of saccade in response to the sudden appearance of a sensory stimulus; mediated by a direct pathway from the retina (and auditory and somatosensory centers in the brainstem) to the superior colliculus.
expressive aphasia: See Broca’s aphasia.
external cuneate nucleus: A group of relay neurons just lateral to the cuneate nucleus in the caudal medulla; conveys proprioceptive signals originating in the upper body, excluding the face, to the ipsilateral cerebellum via the cuneocerebellar tract.
external segment: A lateral subdivision of the globus pallidus.
exteroception: The modality of touch and pressure.
extracellular matrix cell adhesion molecules: Fibrillar proteins usually secreted by epithelial cells that form a macromolecular meshwork to provide a substrate for binding and stability of epithelial cells (via the basal lamina, which is composed of extracellular matrix adhesion molecules) or signals for cell motility and force generation. Fibronectin, laminins, and collagens are the main subtypes of extracellular matrix (ECM) cell adhesion molecules.
extracellular recording: Recording the electrical potentials in the extracellular space near active neurons. Compare intracellular recording.
extrafusal fibers: Fibers of skeletal muscles that generate primary biomechanical force during muscle contraction; a term that distinguishes ordinary muscle fibers from the specialized intrafusal fibers associated with muscle spindles.

F

facial nerve (VII): Cranial nerve VII, a mixed sensorimotor nerve that conveys visceral afferents (taste from anterior two-thirds of tongue) and somatosensory afferents (mechanosensation from skin on or near pinna) to the brainstem, and branchial motor efferents from the brainstem to the muscles of mastication and visceral motor efferents to lacrimal and salivary glands. Middle of three cranial nerves that attaches to the brainstem at the junction of the pons and medulla.
far cells: Visual cortical neurons that change their rate of firing in response to retinal disparities that arise from points beyond the plane of fixation.
fast fatigable (FF) motor units: Large motor units comprising large, pale muscle fibers that generate large amounts of force; however, these fibers have sparse mitochondria and are therefore easily fatigued. FF motor units are especially important for brief exertions that require large forces, such as running or jumping.
fast fatigue-resistant (FR) motor units: Motor units of intermediate size comprising muscle fibers that are not as fast as FF motor units, but generate about twice the force of S motor units and are resistant to fatigue.
fastigial nucleus: Most medial of the deep cerebellar nuclei; source of output from the median spinocerebellum to brainstem upper motor neurons.
fibroblast growth factor (FGF): A peptide growth factor, originally defined by its mitogenic effects on fibroblasts; also acts as an inducer during early brain development.
fibronectin: A large cell adhesion molecule that binds integrins.
filopodia: Slender protoplasmic projections, arising from the growth cone of an axon or a dendrite, that explore the local environment.
first pain: A category of pain perception described as sharp.
flocculus: Lateral portion of the vestibulocerebellum that receives input from the vestibular nuclei in the brainstem and the vestibular nerve; coordinates the vestibulo-ocular reflex and movements that maintain posture and equilibrium.
floorplate: A specialized region of columnar neuroepithelial cells at the midline of the ventral neural tube, just above the notochord, that becomes a source for secreted signals, particularly Sonic hedgehog (Shh) that establish the ventral-dorsal pattern of the neural tube.
folia: The gyral formations of the cerebellar cortex.
foliate papillae: Folds of tissue containing taste buds. Foliate papillae are located on the rear of the tongue lateral to the circumvallate papillae, where the tongue attaches to the mouth. Compare circumvallate papillae and fungiform papillae.
foramina of Monro: See interventricular foramina.
forebrain: The anterior portion of the brain derived from the prosencephalon that includes the diencephalon and telencephalon.
fornix: Axon tract, best seen from the medial surface of the divided brain, that interconnects the septal nuclei and hypothalamus with the hippocampus.
fourth ventricle: The ventricular space derived from the lumen of the neural tube that lies between the pons and rostral medulla and the cerebellum.
fovea: Area of the retina specialized for high acuity in the center of the macula; contains a high density of cones and few rods.
foveation: Aligning the foveae with a visual target and maintaining fixation.
foveola: Capillary-free and rod-free zone in the center of the fovea.
free nerve endings: Afferent fibers that lack specialized receptor cells; they are especially important in the sensation of pain and tempertaure.
frontal: See coronal.
frontal eye fields: A region of the frontal lobe that lies in a rostral portion of the premotor cortex and that contains cells that respond to visual and motor stimuli.
frontal lobe: The hemispheric lobe that lies anterior to the central sulcus and superior to the lateral fissure.
frontal-parietal attention network: A postulated cortical network for controlling attention.
functional magnetic resonance imaging (fMRI): Magnetic resonance imaging that detects changes in blood flow and therefore identifies regions of the brain that are particularly active during a given task.
fundus: The inner surface of the retina.
fungiform papillae: Mushroom-shaped structures (maximum diameter 1 millimeter) that are distributed most densely on the edges of the tongue, especially the tip. Taste buds (an average of six per papilla) are buried in the surface.
fusiform gyrus: See occipitotemporal gyrus.

G

G-protein-coupled receptors: A large family of neurotransmitter or hormone receptors, characterized by seven transmembrane domains; the binding of these receptors by agonists leads to the activation of intracellular G-proteins. See also metabotropic receptors.
G-proteins: Proteins that are activated by exchanging bound GDP for bound GTP (and thus also known as GTP-binding proteins).
gamma (γ) motor neurons: Class of spinal motor neurons specifically concerned with the regulation of muscle spindle length; these neurons innervate the contractile elements of intrafusal muscle fibers in muscle spindles.
ganglia (sing. ganglion): Collection of hundreds to thousands of neurons found outside the brain and spinal cord along the course of peripheral nerves.
ganglion cells: Neurons located in a ganglion.
ganglionic eminences: The bilaterally symmetric accumulations of forebrain neural precursors at the ventromedial aspect of the telencephalon that give rise to the nuclei of the basal ganglia (caudate, putamen, globus pallidus, subthalamic nucleus) as well as the majority of GABAergic interneurons that migrate into the cerebral cortex.
gap junctions: Specialized intercellular contacts formed by channels that directly connect the cytoplasm of two cells.
gastrulation: The cell movements (invagination and spreading) that transform the embryonic blastula into the gastrula.
gate theory of pain: The idea that the flow of nociceptive information through the spinal cord is modulated by the concomitant activation of low threshold mechanoreceptors.
gaze centers: Collections of local circuit neurons in the reticular formation that organize the output of cranial nerves III, IV, and VI to control eye movements along the horizontal or vertical axis.
gene: Hereditary unit located on the chromosomes; genetic information is carried by linear sequences of nucleotides in DNA that code for corresponding sequences of amino acids.
generator potential: See receptor potentials.
genetic analysis: The analysis of the relationship between single genes and the phenotypes to which each gene contributes.
genetic engineering: A methodological means for inducing mutations in genes or otherwise editing or altering the structure and/or the function of targeted genes for experimental or therapeutic benefit. Also called reverse genetics.
genome-wide association studies (GWAS): A statistical correlation of likely associated genes drawn from analyses of large cohorts of individuals with the same phenotype or clinical diagnoses.
genomics: The comprehensive analysis of nuclear DNA sequences within or between species or individuals. Genomic analyses include “whole exome” evaluation in which all nuclear regions that code RNA transcripts are assessed or compared, and “whole genome” evaluation in which the entire DNA sequence of an individual or organism is assessed or compared.
germ layers: The three layers—ectoderm, mesoderm, and endoderm—of the developing embryo from which all adult tissues arise. Neural cells and structures arise from the ectoderm and from the mesodermally-generated notochord.
ghrelin: Peptide hormone secreted by the stomach prior to feeding thought to signal hunger.
glaucoma: Condition in which the eye’s aqueous humor is not adequately drained, resulting in increased intraocular pressure, reduced blood supply to the eye, and eventual damage to the retina.
glial cells (glia): The support cells associated with neurons (astrocytes, oligodendrocytes, and microglial cells in the central nervous system; Schwann cells in peripheral nerves; and satellite cells in ganglia).
glial scarring: Local proliferation of glial precursors and extensive growth of processes from existing glia within or around the site of a brain injury.
glial stem cells: Neural precursor cells in the adult brain that retain the capacity to proliferate and generate both additional precursor cells and differentiated glial cells (and, in some cases, differentiated neurons).
globus pallidus: Principal component of the pallidum. External division is involved with the indirect pathway from striatum to pallidum. Internal division provides major output from the basal ganglia to motor circuits in the thalamus and brainstem.
glomeruli: Characteristic collections of neuropil in the olfactory bulb; formed by dendrites of mitral cells and terminals of olfactory receptor cells, as well as processes from local interneurons.
glossopharyngeal nerve (IX): Cranial nerve IX, a mixed sensorimotor nerve that conveys visceral afferents (taste from posterior one-third of tongue and sensation from oropharynx and middle ear) and somatosensory afferents (mechanosensation from skin on or near pinna) to the brainstem, and branchial motor efferents from the brainstem to muscles of the soft palate and pharynx and visceral motor efferents to a salivary (parotid) gland. Attaches to the rostral medulla in a cleft between the inferior olive and the inferior cerebellar peduncle, just superior to the roots of the vagus nerve.
glutamate–glutamine cycle: A metabolic cycle of glutamate release and resynthesis involving both neuronal and glial cells.
glymphatic system: Vascular, glial, and lymphatic system that allows for the passage of cerebrospinal fluid (CSF) from arterial perivascular space through the substance of the brain and back into venous perivascular space; with this flow of CSF, metabolic wastes and discarded proteins are rinsed from the parenchyma and pass out of the brain, especially during sleep when extracellular spaces in the brain expand.
Goldman equation: Mathematical formula that permits membrane potential to be calculated for case where a membrane is permeable to multiple ions.
G_olf: A G-protein found uniquely in olfactory receptor neurons.
Golgi cells: Inhibitory interneurons in the granular cell layer of the cerebellar cortex that provide inhibitory feedback from parallel fibers to granule cells, regulating the temporal properties of the granule cell input to the Purkinje cells.
Golgi tendon organs: Receptors at the interface of muscle and tendon that provide mechanosensory information to the central nervous system about muscle tension.
gracile nucleus: Somatosensory nucleus in the caudal medulla containing second-order sensory neurons that relay mechanosensory information originating in peripheral receptors in the lower body to the contralateral thalamus via the medial lemniscus.
gracile tract: Medial division of the dorsal column containing the central processes of first-order afferents and the postsynaptic dorsal column projection; conveys mechanosensory signals derived from the lower body (also conveys visceral pain signals from the lower abdominal viscera).
granulations: See arachnoid villi.
gray matter: General term that describes regions of the central nervous system rich in neuronal cell bodies and neuropil; includes the cerebral and cerebellar cortices, the nuclei of the brain, and the central portion of the spinal cord.
group Ib afferents: Axons of primary sensory neurons that innervate Golgi tendon organs.
growth cone: The specialized end of a growing axon (or dendrite) that generates the motive force for elongation.
GTP-binding protein (G-protein): Proteins that are activated by exchanging bound GDP for bound GTP.
gustatory nucleus: The portion of the solitary nucleus in the brainstem that receives input from sensory neurons that are activated by taste receptors cells in taste buds in the tongue. The neurons of the gustatory nucleus then send their axons to the ventral posterior nucleus of the thalamus to relay taste information to the insular cortex.
gustducin: A specialized G-protein that is activated by binding of specific tastants to a subclass of G-protein coupled taste receptors.
gyri (sing. gyrus): Folds in the cerebral cortex. Also called convolutions.
gyrification index: Measure of cortical folding (in relation to brain weight or volume).

H

habituation: Reduced behavioral responsiveness to the repeated occurrence of a sensory stimulus.
hair cells: The sensory cells in the inner ear that transduce mechanical displacement into neural impulses.
halorhodopsin: A protein that, in response to light of the proper wavelength, opens a channel that is selectively permeable to chloride ions; when engineered into a mature neuron for optogenetics, it inhibits the neuron when exposed to light.
haptics: The exploration and perception of somatosensory stimuli using active touching and proprioception.
Hebb’s postulate: The idea that when pre- and postsynaptic neurons fire action potentials at the same time, the synaptic association between those cells strengthens. Sometimes phrased as “cells that fire together wire together,” the postulate provides one explanation for the formation of certain neural networks.
helicotrema: The opening at the apex of the cochlea that joins the perilymph-filled cavities of the scala vestibuli and scala tympani.
hemianopsia: A loss of vision confined to either the nasal or temporal visual fields of each eye. Also called heteronomous hemianopsia.
hemiballismus: A basal ganglia syndrome resulting from damage to the subthalamic nucleus and characterized by involuntary, ballistic movements of the limbs.
hemispatial (or contralateral) neglect syndrome): Neurological condition in which the patient does not acknowledge or attend to the left visual hemifield or the left half of the body. The syndrome typically results from lesions of the right parietal cortex.
heteronomous hemianopsia: A loss of vision confined to the temporal visual field of each eye. Also called bitemporal hemianopsia. Due to lesions at the optic chiasm.
heterotrimeric G-proteins: A large group of proteins consisting of three subunits (α, β, and γ) that can be activated by exchanging bound GDP for GTP, resulting in the liberation of two signaling molecules—αGTP and the βγ dimer.
hindbrain: See rhombencephalon.
hippocampus: A cortical structure in the dorsomedial margin of the parahippocampal gyrus; in humans, concerned with short-term declarative memory, among many other functions.
histamine: A biogenic amine neurotransmitter, derived from the amino acid histidine, that is involved in arousal, attention, and other central and peripheral functions.
histamine-containing neurons: Neurons in the tuberomammillary nucleus (TMN) of the hypothalamus that contribute to the neuronal basis of wakefulness.
homeobox genes: The family of genes that encode transcription factor proteins with a homeodomain DNA binding domain (the homeobox). The homeobox genes are essential for early anterior-posterior patterning in most animal embryos.
homeotic genes: Genes that determine the developmental fate of an entire segment of an animal. Mutations in these genes drastically alter the characteristics of the body segment (as when wings grow from a fly body segment that should have produced legs).
hominins: Term for ancestral humans.
homologous recombination: An endogenous cellular mechanism for DNA replication and repair involving DNA polymerases and ligases; may be used in genetic engineering to replace (“recombine”) a native sequence of nucleotides in a gene with an exogenous sequence.
homonymous hemianopsia: A loss of vision in both left and right hemifields due to lesions of the optic tract.
homonymous quadrantanopsia: A loss of vision in both left of right quadrants of the visual field due to lesions of the optic radiation.
horizontal cells: Retinal neurons that mediate lateral interactions between photoreceptor terminals and the dendrites of bipolar cells.
horizontal gaze center: See paramedian pontine reticular formation.
horizontal sections: Standard anatomical planes of section; when standing, horizontal sections are parallel to the ground. Also known as axial sections.
Hox genes: A group of conserved genes characterized by a specific DNA sequence—the homeobox—and that specify body axis (particularly the anterior–posterior axis) and regional identity in the developing vertebrate embryo.
Hsc70: An ATP-hydrolyzing enzyme involved in dissociation of clathrin-coats on vesicles following endocytosis.
Huntington’s disease: An autosomal dominant genetic disorder in which a single gene mutation results in personality changes, progressive loss of control of voluntary movement, and eventually death. Primary target early in the disease is medium spiny neurons of the striatum that participate in the indirect pathway.
hydrocephalus: Enlarged ventricles that can compress the brain and expand the entire cranium as a result of increased cerebrospinal fluid pressure (typically due to a mechanical outflow blockage).
hyperacusis: A painful sensitivity to moderate or even low-intensity sounds.
hyperalgesia: Increased perception of pain.
hyperopic: Far sighted.
hyperpolarization: The displacement of a cell’s membrane potential toward a more negative value.
hypnotic: Drug that induces sleep.
hypocretin: Another name for orexin.
hypoglossal nerve (XII): Cranial nerve XII, a somatic motor nerve that conveys efferents from the brainstem (hypoglossal nucleus) to the extrinsic muscles of the tongue. Attaches to the rostral medulla in a cleft between the medullary pyramid and the inferior olive.
hypothalamus: Heterogeneous collection of small nuclei in the base of the diencephalon that plays an important role in the coordination and expression of visceral motor activity, as well as neuroendocrine and somatomotor activities that promote homeostasis and allostasis. The diverse functions in which hypothalamic involvement is at least partially understood include: the control of blood flow, the regulation of energy metabolism, the regulation of reproductive activity, and the coordination of responses to threatening conditions.

I

immediate early gene: A gene whose protein product is produced rapidly, within 30-60 minutes, after a triggering stimulus. These often serve as transcriptional activators for delayed response genes. C-fos is one of the best-known examples of an immediate early gene.
immediate memory: Sensory impressions that last only a few seconds.
inactivation: The time-dependent closing of ion channels in response to a stimulus, typically membrane depolarization.
inferior cerebellar peduncles: See cerebellar peduncles.
inferior colliculi (sing. colliculus): Paired hillocks on the dorsal surface of the midbrain; concerned with auditory processing.
inferior division: Referring to the region of the visual field of each eye that corresponds to the bottom half of the retina.
inferior frontal gyri: Inferior of three, parallel longitudinal gyri anterior to the precentral sulcus; part of the prefrontal cortex and anterior premotor cortex, including Broca’s area in the left hemisphere.
inferior oblique muscles: Extraocular muscles that extort the eyeballs when in the primary position (eyes straight ahead) and rotate upward when in adduction.
inferior olivary nucleus: Prominent nucleus in the ventral medulla; source of climbing fiber input to the contralateral cerebellum; induces complex spikes and long-term depression in cerebellar Purkinje neurons. Also called inferior olive.
inferior olive: See inferior olivary nucleus.
inferior parietal lobules: Gyral formations of the lateral (inferior) parietal lobes that are involved in associating somatosensory, visual, auditory, and vestibular signals and generating a neural construct of the body, the position of its parts, and its movements (body image or schema).
inferior rectus muscles: Extraocular muscles that rotate the eyeballs downward.
inferior salivatory nuclei: Visceral motor nucleus of the caudal pons containing parasympathetic preganglionic neurons that mediate salivation.
inferior temporal gyri: Inferior of three, parallel longitudinal gyri that define the inferior-lateral margin of the temporal lobe; involved in associational functions pertaining to stimulus recognition.
infundibulum: The connection between the hypothalamus and the pituitary gland; also known as the pituitary stalk.
inhibitory: Postsynaptic potentials that decrease the probability that a postsynaptic cell will generate an action potential (IPSPs).
inositol trisphosphate (IP₃) receptor: A ligand-gated ion channel in the endoplasmic reticulum membrane. This receptor binds to the second messenger, IP₃, and elevates cytoplasmic calcium concentration by mediating flux of calcium out of the lumen of the endoplasmic reticulum.
insular cortex (insula): The portion of the cerebral cortex that is buried within the depths of the lateral fissure by the growth inferior frontal and parietal lobes and the superior temporal lobe. The posterior portion of the insula is concerned largely with visceral and autonomic function, including taste, while more rostral portions are involved in implicit feelings and their impact on social cognition. Also called the insula.
insular taste cortex: A neocortical region located with the gyri of the insula, a region covered by dorsal region of the temporal lobe and the ventral region of the parietal lobe. Taste information from the tongue.
integrins: A family of receptor molecules found on growth cones that bind to cell adhesion molecules such as laminin and fibronection.
intelligence quotient (IQ): A widely used but highly controversial measure of human intelligence based on standard tests.
intention tremors: Tremor that occurs while performing a voluntary motor act; characteristic of cerebellar pathology. Also called action tremor.
interhemispheric connections: The corpus callosum and anterior commissure, together. Mediate corticocortical connections between cortical regions in the opposite hemispheres.
intermediolateral cell column: Rod-shaped distribution of sympathetic preganglionic neurons in the lateral, intermediate gray matter of the spinal cord; in thoracic segments, accounts for a lateral protrusion of gray matter into the white matter known as the lateral horn.
internal arcuate fibers: Axons of dorsal column nuclei in the caudal brainstem that sweep across the midline and turn in the rostral direction forming the medial lemniscus.
internal capsule: Large, fan-shaped white matter tract that lies between the diencephalon and the basal ganglia, formed by the growth of axons supplying the cerebral cortex (mainly from the thalamus) and axons originating in the cerebral cortex and terminating in subcortical targets; features an anterior limb, a genu, and a posterior limb.
internal carotid arteries: Large arteries, one on each side of the head, that carry blood to the head. They divide into an external branch (supplying the neck and face), and an internal branch (supplying the brain and eye).
internal segment: A medial subdivision of the globus pallidus.
interneurons: Technically, a neuron in the pathway between primary sensory and primary effector neurons; more generally, a neuron whose relatively short axons branch locally to innervate other neurons. Also known as local circuit neuron.
interoception: The sense of the internal state of the organism.
interphotoreceptor retinoid binding protein (IRBP): A critical protein in the retinoid cycle.
interposed nuclei: Intermediate deep cerebellar nuclei; source of output from the paramedian spinocerebellum to the motor cortex via the thalamus and to brainstem upper motor neurons.
intersexuality: Having a biologically ambiguous or intermediate sex based either upon indeterminate chromosomal sex (XXY males), primary sex characteristics (differentiation of gonads, genitalia), or secondary sex characteristics.
interstitial nuclei of the hypothalamus (INAH): Four cell groups located slightly lateral to the third ventricle in the anterior hypothalamus of primates; thought to play a role in sexual behavior.
interventricular foramina: Narrow channels that allow for the passage of cerebrospinal fluid from the paired lateral ventricles into the single third ventricle; form in each hemisphere between the fornix and medial aspect of the anterior thalamus. Also known as the foramina of Monro.
intracellular receptors: Receptors that participate in signal transduction by binding to cell-permeant chemical signals. The activate form of these receptors typically interact with nuclear DNA and produce new mRNA and protein within target cells.
intracellular recording: Recording the potential between the inside and outside of a neuron with a microelectrode. Compare extracellular recording.
intracellular signal transduction: A process that converts binding of ligands to plasma membrane receptors to intracellular signaling processes.
intrafusal muscle fibers: Specialized muscle fibers found in muscle spindles.
intraparietal sulcus: Prominent longitudinal sulcus of the posterior parietal lobe that divides the superior and inferior parietal lobules.
ion channels: Integral membrane proteins possessing pores that allow only certain ions to diffuse across cell membranes, thereby conferring selective ionic permeability.
ion exchangers: Membrane transporters that exchange intracellular and extracellular ions against their concentration gradient by using the electrochemical gradient of other ions as an energy source. See also antiporters and co-transporters.
ion selectivity: The ability of channels to discriminate between different ions.
ionotropic receptors: Receptors in which the ligand binding site is an integral part of the receptor molecule.

J

joint receptors: Mechanoreceptors found in and around joints; especially important for monitoring finger movements during fine manual manipulations.
jugular veins: Principal means for draining venous blood from the cranium; arise from the sigmoid sinuses as they pass through the jugular foramina in the base of the skull.

K

kainate receptors: See ionotropic glutamate receptors.
kairomones: Volatile chemicals from other species indicating predator, prey, or symbiotic status that bind specifically to vomeronasal receptor proteins localized on subsets of vomeronasal sensory receptor cells.
KAL1: See anosmin.
KCN genes: K⁺ channel genes.
KIF21A: A member of the Kinesin family of molecular motors that interact with the microtubule cytoskeleton as well as with macromolecules and organelles referred to as cargo so that the cargo is transported from the cell body to the periphery of the cell. KIF21A and related kinesins are essential for moving proteins and organelles from the cell body to the axon as well as transporting cargos back to the cell body, often as endosomes for signaling to the cell body, or as membrane bound compartments for protein turnover and degradation.
kinocilium: A true ciliary structure which, along with the stereocilia, comprises the hair bundle of vestibular and fetal cochlear hair cells in mammals (it is not present in the adult mammalian cochlear hair cell).
koniocellulary (K-cell) pathway: A third poorly understood pathway from retina to cortex characterized by the anatomical location of it cells in the lateral geniculate nucleus that process of short wavelength light.

L

L1: A member of the Ca²⁺ independent family of transmembrane cell surface adhesion molecules. L1 binds homophillically (thus to other L1 molecules on adjacent cells) It signals primarily through non-receptor tyrosine kinases Fyn and Src. It is particularly essential for the fasciculation (binding together) of axons within growing nerves.
labyrinth: A set of interconnected chambers in the internal ear, comprising the cochlea, vestibular apparatus, and the bony canals in which these structures are housed.
lamellipodium: A sheetlike extension, rich in actin filaments, on the leading edge of a motile cell or growth cone.
laminae (sing. lamina): Cell layers that characterize the neocortex, hippocampus, and cerebellar cortex. The gray matter of the spinal cord is also arranged in laminae.
laminins: Large cell adhesion molecules that bind integrins. Laminin is a major component of the extracellular matrix.
large dense-core vesicles: A type of synaptic vesicle characterized by a large diameter (typically 90–250 nanometers) and the presence of an electron-dense core. These vesicles typically contain and release neuropeptides.
lateral columns: The lateral regions of spinal cord white matter that convey motor information from the brain to the ventral horn via the lateral corticospinal tract and convey proprioceptive signals from spinal cord neurons to the cerebellum via the spinocerebellar tracts.
lateral corticospinal tract: Spinal portion of the corticospinal tract in the lateral column of the spinal cord derived from the contralateral motor cortex; governs skilled movements of the extremities.
lateral fissure: The cleft on the lateral surface of the brain that separates the temporal lobe below from the frontal and parietal lobes above. Also called the Sylvian fissure.
lateral horns: Lateral protrusion of intermediate gray matter into the adjacent white matter known; characteristic feature of the thoracic segments of the spinal cord. See intermediolateral cell column.
lateral olfactory tract: The bundle of mitral and tufted cell axons that relay olfactory information to the accessory olfactory nuclei, the olfactory tubercle, the pyriform and entorhinal cortices, and portions of the amygdala.
lateral rectus muscles: Extraocular muscles that rotate the eyeballs laterally.
lateral superior olive (LSO): A structure in the auditory brainstem that aids in sound localization that aids in sound localization by computing interaural intensity differences.
lateral ventricles: Largest of the ventricles derived from the lumen of the neural tube that expanded in the formation of the paired telencephalic vesicles; components of the lateral ventricles are present in each lobe of the cerebral hemisphere: anterior (frontal) horn, body, atrium, posterior (occipital) horn, and temporal horn.
lateral vestibulospinal tract: Ipsilateral projection of the anterior white matter of the spinal cord from the lateral vestibular nuclei to the medial ventral horn; mediates reflexes that activate extensor (anti-gravity) muscles with rapid lateral roll of the head, as when jostled to one side when standing on a moving train.
lenticulostriate arteries: Numerous small branches of the middle and anterior cerebral arteries that penetrate deep into the anterior cerebral hemispheres supplying blood to deep white matter and cerebral nuclei, including the basal ganglia.
leptin: Peptide hormone secreted by adipocytes following feeding thought to signal satiety.
lesion studies: The method of observing and documenting change in function following damage (lesion) of a distinct brain region, nerve, or tract; damage may be acquired in humans or induced experimentally in non-human models; predominant method of studying the human nervous system prior to the advent of modern neurophysiological and brain imaging tools.
ligand-gated ion channels: Ion channels that respond to chemical signals rather than to the changes in membrane potential generated by ionic gradients. The term covers a large group of neurotransmitter receptors that combine receptor and ion channel functions into a single molecule.
light adaptation: Gain control of vision according to the prevailing level of illumination.
limbic forebrain: Constellation of cortical and subcortical structures in the frontal and temporal lobes that form a medial rim of cerebrum roughly encircling the corpus callosum and diencephalon (limbic means “border” or “rim”). Comprises an olfactory division that processes olfactory cues; a parahippocampal division that generates cognitive maps in spatial frameworks that facilitate the acquisition of episodic and declarative memory; and an amygdaloid/orbital cortical division that is important in the experience and expression of emotion.
limbic lobe: Cortex that lies superior to the corpus callosum on the medial aspect of the cerebral hemispheres; forms the cortical component of the limbic system.
limbic system: Term that refers to those cortical and subcortical structures concerned with the emotions; the most prominent components are the cingulate gyrus, the hippocampus, and the amygdala.
lingual gyrus: Gyral structure on the inferior aspect of the occipital lobe forming the lower bank of the calcarine sulcus; portion of the primary visual cortex that represents the superior quadrant of the contralateral visual hemifields.
lobes: The four major divisions of the cerebral hemispheres named for the overlying cranial bones (frontal, parietal, occipital, and temporal).
local circuit neurons: General term referring to a neuron whose activity mediates interactions among other neurons in the CNS; exemplified by short-axon neurons in the spinal cord that mediate transmission of signals from sensory neurons to motor neurons. Interneuron is often used as a synonym.
long circumferential arteries: Long branches of the vertebral and basilar arteries that supply dorsolateral aspects of the brainstem and cerebellum.
long-term depression (LTD): A form of long-term synaptic plasticity that produces a persistent, activity-dependent weakening of synaptic transmission.
long-term memory: Memories that last days, weeks, months, years, or a lifetime.
long-term potentiation (LTP): A form of long-term synaptic plasticity that produces a persistent, activity-dependent strengthening of synaptic transmission.
longitudinal: Standard anatomical planes of section that pass through the CNS parallel to its long axis.
loudness: The sensory quality elicited by the intensity of sound stimuli.
lumbar: Caudal region of the spinal cord between the thoracic and sacral regions related to the lower extremities.
lumbosacral enlargement: The spinal cord expansion that relates to the lower extremities; includes spinal segments L1–S2.
luminance: The physical measure of light intensities.
luminance contrast: The difference between the level of illumination that falls on the receptive field center and the level of illumination that falls on the surround.

M

macroscopic currents: Ionic currents flowing through large numbers of ion channels distributed over a substantial area of membrane.
macula: The sensory epithelium of the otolith organs, comprising hair cells and associated supporting cells.
macula lutea: The central region of the retina that contains the fovea (the term derives from the yellowish appearance of this region in ophthalmoscopic examination); also, the sensory epithelia of the otolith organs.
macular sparing: The loss of vision throughout wide areas of the visual field, with the exception of foveal vision.
magnetic resonance imaging (MRI): A noninvasive technique that uses magnetic energy and radiofrequency pulses to generate images that reveal structural and/or functional information in the living brain.
magnetic source imaging (MSI): A non-invasive means for localizing brain activity that combines magnetoencephalography with structural magnetic resonance imaging.
magnetoencephalography (MEG): A passive and noninvasive functional brain-imaging technique that measures the tiny magnetic fields produced by active neurons, in order to identify regions of the brain that are particularly active during a given task.
magnocellular layers: A component of the primary visual pathway specialized for the perception of motion; so named because of the relatively large (“magno”) cells involved.
major histocompatibility complex (MHC): A macromolecular assembly of immune proteins that mediate recognition of antigens derived from an individual organism itself (“self”) and those derived from external sources (“other”). The MHC provides a unique molecular signature for an individual, and is thought to act as a stimulus in for the vomeronasal system.
mammillary bodies: Small prominences on the ventral surface of the posterior diencephalon; anatomically and functionally part of the caudal hypothalamus that is related to the hippocampus and its role in memory formation.
mechanoreceptors: Receptors specialized to sense mechanical forces.
mechanosensitive: Ion channels that respond to mechanical distortion of the plasma membrane.
medial geniculate complex (MGC): The thalamic nucleus in the primary auditory pathway. Compare dorsolateral geniculate nucleus.
medial lemniscus: Axon tract in the brainstem that carries mechanosensory information from the dorsal column nuclei to the ipsilateral thalamus.
medial longitudinal fasciculus: Axon tract that carries excitatory projections from the abducens nucleus to the contralateral oculomotor nucleus; important in coordinating conjugate eye movements.
medial nucleus of the trapezoid body (MNTB): A structure that provides inhibitory input to the lateral superior olive.
medial rectus muscles: Extraocular muscles that rotate the eyeballs medially.
medial superior olive (MSO): A structure in the auditory brainstem that aids in sound localization by computing interaural time differences.
medial vestibulospinal tract: Bilateral projection of the anterior-medial white matter of the spinal cord from the medial vestibular nuclei to the cervical cord; mediates reflexes that extend the arms and dorsiflex the neck with rapid downward pitch of the head, as when falling forward.
medium spiny neurons: The principal projection neurons of the striatum.
medullary arteries: Segmental branches of the descending aorta that supply blood to the vertebral column and the spinal cord.
medullary pyramids: Longitudinal bulges on the ventral aspect of the medial medulla formed by the corticospinal tract and a small remnant of the corticobulbar tract.
Meissner afferents: Encapsulated cutaneous mechanosensory receptors in the tips of dermal papillae specialized for the detection of fine touch and pressure.
Meissner’s plexus: See submucous plexus.
melanopsin: A photopigment located in the retinal ganglion cells that help to set the biological clock.
melatonin: Sleep-promoting neurohormone produced in the pineal gland.
membrane conductance: The reciprocal of membrane resistance. Changes in membrane conductance result from, and are used to describe, the opening or closing of ion channels.
meninges: The external covering of the brain and spinal cord; includes the pia, arachnoid, and dura mater.
Merkel cell afferents: Peripheral axons of primary sensory neurons that make synaptic connections with Merkel cells forming Merkel cell-neurite complexes; Merkel afferents transduce the dynamic aspects of stimuli giving rise to fine touch sensations.
Merkel cell–neurite complexes: Specializations at the boundary of the epidermis and dermis comprising Merkel cells in the tips of the primary epidermal ridges (coinciding with fingerprint ridges on the skin surface) and terminals of primary sensory afferents in the dermis; transduce light touch and pressure stimuli with high spatial resolution.
Merkel cells: Specialized cells in the basal epidermis that contact Merkel afferents forming Merkel cell-neurite complexes; Merkel cells signal the static aspect of a touch stimulus, such as light pressure, and release peptide neurotransmitters onto the terminals of Merkel afferents.
mesencephalic locomotor region: Collection of neurons in the reticular formation of the midbrain tegmentum that can trigger locomotion and change the speed and pattern of the movement by changing the level of activity delivered to the spinal cord through reticulospinal projections originating in the pons and medulla.
mesencephalic trigeminal nucleus: Array of pseudounipolar proprioceptive neurons in the rostral pons and midbrain (mesencephalon) on the ventral-lateral margin of the periaqueductal gray; mediates sensory limb of myotatic reflexes for jaw muscles and other striated muscles of the anterior cranium.
mesencephalon: See midbrain.
mesoderm: The middle of the three embryonic germ layers; gives rise to muscle, connective tissue, skeleton, and other structures.
mesopic vision: Vision in light levels at which both the rods and cones are active.
metabotropic receptors: Receptors that are indirectly activated by the action of neurotransmitters or other extracellular signals, typically through the aegis of G-protein activation. Also called G-protein-coupled receptors.
metencephalon: The part of the embryonic hindbrain (the entire rhombencephalon at earliest stages) that generates the pons and the cerebellum, and the trigeminal (V), abducens (VI), facial (VII) and vestibulocochlear (VIII) cranial nerves and surrounds the fourth ventricle in the mature brainstem.
Meyer’s loop: That part of the optic radiation that runs in the caudal portion of the temporal lobe.
microglial cells: One of the three major classes of glial cells found in the central nervous system derived primarily from hematopoietic precursor cells; function as scavenger cells that remove cellular debris from sites of injury or normal cell turnover, and secrete signaling molecules that modulate local inflammatory responses.
microscopic currents: Ionic currents flowing through single ion channels.
microtubule cytoskeleton: The polymers of a variety of tubulin proteins that form a framework of parallel tubes that provide stability and resilience to long cellular processes, especially dendrites and axons. Polymerization of tubulins occurs as an axon or dendrite extends from the cell body of a differentiating neuron. Once the process is stable, this arrangement of parallel microtubules maintains the volume of the process and also acts as a set of tracks upon which molecules or organelles are transported from the cell body to distal domains of the processes using the molecular “motor” proteins kinesin or dynein.
midbrain: The most rostral portion of the brainstem; identified by the superior and inferior colliculi on its dorsal surface, and the cerebral peduncles on its ventral aspect. Also known as the mesencephalon.
middle cerebellar peduncles: See cerebellar peduncles.
middle cerebral arteries: Major vessels derived from the internal carotid arteries that supply the lateral aspects of the frontal, parietal and temporal lobes, including associated deep structures.
middle frontal gyri: Middle of three, parallel longitudinal gyri anterior to the precentral sulcus; part of the prefrontal cortex and anterior premotor cortex.
middle temporal area (MT): Region of the extrastriate cortex in which neurons respond mainly to movement without regard to color.
middle temporal gyri: Middle of three, parallel longitudinal gyri that define the lateral aspect of the temporal lobe; part of the lateral temporal network that encodes language content and, at its posterior margin, contains visual areas involved in motion discrimination.
midsagittal: Standard anatomical plane of section; the vertical plane passing from anterior to posterior through the midline dividing the body (and brain) into right and left sections.
miniature end plate potentials (MEPPs): Small, spontaneous depolarization of the membrane potential of skeletal muscle cells, caused by the release of a single quantum of acetylcholine.
mirror motor neurons: Neurons in the posterior frontal and inferior parietal lobes that respond during the execution of goal-oriented action and the observation of the same actions, even when such actions are not executed.
mitral cells: The major output neurons of the olfactory bulb.
monitoring: The process that evaluates the appropriateness of a given behavior for the current context; examples include evaluating the accuracy of answers generated during a memory test or the adequacy of a response rule in an executive function paradigm.
monomeric G-proteins: GTP-binding proteins, also called small G-proteins, that relay signals from activate cell surface receptors to intracellular targets. In contrast to heterotrimeric G-proteins, monomeric G-proteins consist of a single protein. Also called small G-proteins.
mosaic brain evolution: Brain evolution considered in terms of proposed functional modules.
mossy fibers: Afferent axons to the cerebellum from all sources except for the inferior olivary nuclei; the vast majority enter the cerebellum via the inferior and middle cerebellar peduncles.
motor aphasia: See Broca’s aphasia.
motor cortex: Region of the cerebral cortex in the posterior frontal lobe that gives rise to corticobulbar and corticospinal projections and is concerned with motor behavior. Includes the primary motor cortex in the anterior bank of the central sulcus that is essential for the voluntary control of movement, and the premotor cortex (anterior to the primary motor cortex) that is involved in planning and programming voluntary movements.
motor neurons: By common usage, nerve cells that innervate and send efferent signals to skeletal muscle.
motor systems: A broad term used to describe all the central and peripheral structures that support motor behavior.
motor unit: A motor neuron and the skeletal muscle fibers it innervates.
muscarinic ACh receptors (mAChRs): Metabootropic ACh receptors that can be pharmacologically identified by their selective activation by muscarine.
muscle spindles: Highly specialized mechanosensory organs found in most skeletal muscles; provide proprioceptive information about muscle length.
muscle tone: The normal, ongoing tension in a muscle; measured by resistance of a muscle to passive stretching.
myelencephalon: The part of the hindbrain that gives rise to the medulla and the glossopharyngeal (IX), vagal (X), Spinal accessory (XI), and hypoglossal (XII) cranial nerves (motor neurons and neural crest that will form related cranial ganglia).
myelin: The multilaminated wrapping around many axons formed by oligodendrocytes or Schwann cells.
myelination: Process by which glial cells (oligodendrocytes or Schwann cells) wrap around axons to form multiple layers of glial cell membrane, thus insulating the axonal membrane and increasing conduction velocity.
myenteric plexus: Network of neurons in the enteric division of the visceral motor system concerned with regulating the musculature of the gut. Also called Auerbach’s plexus.
myopic: Near sighted.
myotatic reflex: A fundamental spinal reflex that is generated by the motor response to afferent sensory information arising from muscle spindles; also called a “stretch” or “deep tendon” reflex. The knee jerk reaction is a common example.

N

N-acetyl aspartate (NAA): An abundant metabolite in the neurons synthesized in mitochondria from the amino acid aspartic acid and acetyl-coenzyme A.
Na⁺/Ca²⁺ exchanger: An active transport protein that removes calcium from the cytoplasm of cells by exchanging intracellular calcium ions for extracellular sodium ions.
nasal division: Referring to the region of the visual field of each eye in the direction of the nose. See also binocular field.
nasal mucosa: The nasal mucosa is the general term for the entire epithelial lining of the nasal cavities. It includes both the non-neural respiratory epithelium and the neural olfactory sensory epithelium and their constituent cells. It is named based upon the layer of mucous that coats the entire outer surface of the respiratory and olfactory epithelia in the nose.
Neanderthals: A group in the human lineage that evolved from Homo erectus about 50,000 years ago in what is now Europe and the Middle East.
near cells: Visual cortical neurons that respond to retinal disparities that arise from points in front of the plane of fixation.
near reflex triad: Reflexive response induced by changing binocular fixation to a closer target; comprises convergence, accommodation, and pupillary constriction.
neocortex: The six-layered cortex that forms the surface of most of the cerebral hemispheres (all cerebral cortex lateral and dorsal to the rhinal sulcus).
Nernst equation: A mathematical formula that predicts the electrical potential generated ionically across a membrane at electrochemical equilibrium.
nerve cells: See neurons.
nerve growth factor (NGF): A neurotrophic protein factor required for survival and differentiation of sympathetic ganglion cells and certain sensory neurons. Preeminent member of the neurotrophin family of growth factors.
nerves: A collection of peripheral axons that are bundled together and travel a common route.
netrins: A family of diffusible molecules that act as attractive or repulsive cues to guide growing axons.
neural circuits: A collection of interconnected neurons mediating a specific function.
neural crest cells: Cells that migrate to become a variety of cells types and structures, including peripheral sensory neurons, enteric neurons, and glial cells.
neural crest: A transient region where the edges of the folded neural plate come together, at the dorsalmost limit of the neural tube. Gives rise to neural crest cells that migrate to become a variety of cells types and structures, including peripheral sensory neurons, enteric neurons, and glial cells as well as facial bones, teeth, and melanocytes in the skin.
neural induction: The mechanism by which ectodermal cells, in response to local signals available in the embryo, acquire neural stem cell identity.
neural plate: The thickened region of the dorsal ectoderm of a neurula that gives rise to the neural tube.
neural precursor cells: Undifferentiated stem cells in the embryonic neural tube.
neural stem cells: The neuroectodermal cells, established immediately after gastrulation via signals from the notochord, that have the capacity to give rise to all neuronal and glial cell types of the CNS and PNS, plus the non-neural derivatives of the neural crest.
neural tube: The primordium of the brain and spinal cord; derived from the neural ectoderm.
neuregulin1 (Nrg1): Neuregulin 1 is a member of the broader class of secreted neuregulin ligands. Via binding to the Erb family of neuregulin receptors, neuregulin1 and related neuregulins can mediate cell motility, local receptor clustering or a variety of growth responses.
neurexins: Adhesion molecules of the presynaptic membrane in developing synapses. Neurexins bind to neuroligin in the postsynaptic membrane, promoting adhesion, and help localize synaptic vesicles, docking proteins, and fusion molecules.
neuroblasts: Dividing cells, the progeny of which develop into neurons; immature nerve cells.
neuroectoderm: The portion of the outermost embryonic germ layer (the ectoderm) that based upon its proximity to the mesodermally derived notochord differentiates into a field of multipotent neural stem cells that give rise to the entire nervous system (CNS and PNS).
neuroectodermal precursor cells: The cells in the neuroectoderm that can give rise via proliferation to a variety of neural and glial cell classes.
neuroethology: The field of study devoted to using evolutionary and comparative approaches for observing complex behaviors of animals in their native environments (e.g., social communication in birds and non-human primates) and inferring underlying mechanisms for nervous system regulation.
neuroglia: See glial cells.
neuroligins: Postsynaptic binding partners of the presynaptic adhesion molecule neurexin. Promote the clustering of receptors and channels of the postsynaptic density as the synapse matures.
neuromeres: The repeating units of the neural tube.
neurons: Also called nerve cells. Cells specialized for the generation, conduction, and transmission of electrical signals in the nervous system.
neuropathic pain: A chronic, intensely painful experience that is difficult to treat with conventional analgesic medications.
neuropeptides: A general term describing a large number of peptides that are synthesized by neurons and function as neurotransmitters or neurohormones.
neuropil: The dense tangle of axonal and dendritic branches, the synapses between them, and associated glia cell processes that lies between neuronal cell bodies in the gray matter of the brain and spinal cord.
neurotransmitters: Substances released by synaptic terminals for the purpose of transmitting information from one cell (the presynaptic cell) to another (the postsynaptic cell).
neurotrophic factors: Chemical substances, secreted by cells in a target tissue, that promote the growth and survival of neurons.
neurotrophin 4 or 5 (NT-4/5): NT-4 and NT-5 are names used for the same member of the neurotrophin family of secreted growth and survival signaling molecules. NT-4/5 signals through the TrkB receptor as the LNGFR/P75.
neurotrophin-3 (NT-3): A member of the neurotrophin family of secreted growth and survival signaling molecules. NT-3 binds with high affinity to both the TrkB and TrkC neurotrophin receptor-kinases, as well as with low affinity to the low affinity neurotrophin receptor (LNGFR, also known as P75). NT-3 has trophic effects on a variety of peripheral and central neurons.
neurotrophins: A family of trophic factor molecules that promote the growth and survival of several different classes of neurons.
neurulation: The process by which the neuroectoderm rounds into the neural tube. This process establishes the midline of the neural plate as the ventral midline of the CNS and the lateral edges of the neural plate as the dorsal, or alar regions that give rise to the neural crest at the margins, or the dorsal CNS.
niche: The tissue environment necessary to support the retention, quiescence, and when elicited, proliferation of tissue specific stem cells in an adult organism.
nicotinic ACh receptor (nAChR): Ionotropic ACh receptors that can be pharmacologically identified by their selective activation by nicotine.
NMDA receptors: See ionotropic glutamate receptors.
nociceptors: Cutaneous and subcutaneous receptors (especially free nerve endings) specialized for the detection of harmful (noxious) stimuli.
nodes of Ranvier: Periodic gaps in the myelination of axons where action potentials are generated.
nodulus: Medial portion of the vestibulocerebellum that receives input from the vestibular nuclei in the brainstem and the vestibular nerve; coordinates the vestibulo-ocular reflex and movements that maintain posture and equilibrium.
Noggin: An endogenous antagonist of the Bmp family of Tgfβ ligands that binds secreted Bmps and inactivates them, preventing the acquisition of epidermal fate for ectodermal cells that go on to become neural plate neural stem cells. Noggin is secreted by the notochord, in combination with positive signals like Shh that drive neuronal differentiation.
non-rapid eye movement (non-REM) sleep: Collectively, those phases of sleep (stages I–IV) characterized by the absence of rapid eye movements.
nondeclarative memory: Unconscious memories such as motor skills and associations. Also called procedural memory.
noradrenaline: See norepinephrine.
noradrenergic neurons: Neurons that contribute to the neuronal basis of wakefulness.
norepinephrine: Catecholamine hormone and neurotransmitter that binds to α- and β-adrenergic receptors, both of which are G-protein-coupled receptors. Also known as noradrenaline.
Notch cell surface receptors: These transmembrane proteins transduce signals upon binding a delta ligand on the surface of a neighboring cell. Delta binding to Notch activates a local cascade at the inner surface of the cell membrane that recruits a protease to cleave the intracellular domain of the Notch protein. This Notch intracellular domain (NICD) then complexes with other cytoplasmic proteins and is translocated to the nucleus to regulate gene expression.
notochord: A transient, cylindrical structure of mesodermal cells underlying the neural plate (and later the neural tube) in vertebrate embryos. Source of important inductive signals for neural development.
NSF: NEM-sensitive fusion protein. An enzyme responsible for dissociating complexes of SNARE proteins.
nuclei of the lateral lemniscus: A brainstem nucleus in the primary auditory pathway.
nucleus (pl. nuclei): Collection of nerve cells in the brain and spinal cord that are anatomically discrete, and which typically serve a particular function.
nucleus ambiguus: Branchial motor nucleus of the rostral medulla containing somatic motor neurons that innervate striated muscles of the larynx and pharynx; also contains a visceral motor division with parasympathetic preganglionic neurons that mediate slowing of the heart rate.
nucleus of the solitary tract: Nucleus of the caudal pons and rostral medulla that contains a rostral gustatory division and a caudal visceral sensory division; integrates inputs relayed from the rostral division and several primary and secondary visceral sensory afferents that are relevant to the autonomic control of the gut, the cardiovascular system, and other target organs; receives visceral and taste information via several cranial nerves and relays this information to the thalamus.
nutrient: A chemical that is needed for growth, maintenance, and repair of the body but is not used as a source of energy.
nystagmus: Repetitive rotational movements of the eyes normally elicited by large-scale motion of the visual field (optokinetic nystagmus), with each cycle involving a slower phase driven by central circuits in the brainstem and higher brain centers and a faster, reflexive phase resetting the position of the eye in the orbit; in the absence of physiological visual or vestibular stimuli, nystagmus may indicate brainstem or cerebellar pathology.

O

occipital lobe: The posterior lobe of the cerebral hemisphere; primarily devoted to vision.
occipitotemporal gyrus: Longitudinal gyrus of the inferior temporal lobe between the parahippocampal gyrus and the inferior temporal gyrus; part of the ventral “what” visual processing stream concerned with object recognition. Posterior portion also known as the fusiform gyrus.
ocular dominance columns: The segregated termination patterns of thalamic inputs representing the two eyes in the primary visual cortex of some mammalian species.
oculomotor nerve (III): Cranial nerve III, a mixed efferent nerve with somatic motor components that controls the superior rectus, the inferior rectus, the medial rectus and the inferior oblique eye muscles, as well as the levator palpebrae superioris—a muscle that retracts the upper eyelid; also contains a parasympathetic component that constricts the pupil.
odorant receptor gene family: The largest set of genes in most animals’ genomes devoted to a singular function: the binding and transduction of volatile chemicals in the air that are sensed as odors. All odorant receptors are members of the 7 transmembrane G-protein coupled receptor class of cell surface receptors. There numbers in vertebrates are between 600 and 2000 individual genes.
odorants: Molecules capable of eliciting responses from receptors in the olfactory mucosa.
OFF-center neuron: A visual neuron whose receptive field center is inhibited by light.
olfactory bulbs: Telencephalic structures that lie on the orbital surface of the frontal lobe and receive axons from cranial nerve I; contain local circuit neurons and project neurons that transmit olfactory signals to the olfactory cortex via the olfactory tracts.
olfactory cilia: Actin based protrusions from the apical domain of an olfactory sensory receptor neuron. The olfactory cilia are the site of concentration of odorant receptor molecules and the cytoplasmic signaling intermediates necessary for odor transduction and the initiation of odor processing in the olfactory pathway.
olfactory ensheathing cells: The glial cells that surround the unmyelinated axons of the peripheral portion of the olfactory nerve. These cells have several properties of Schwann cells that fulfill a similar function for peripheral somatosensory, motor and autonomic axons in peripheral nerves throughout the body.
olfactory epithelium: Pseudostratified epithelium that contains olfactory receptor cells, supporting cells, and mucus-secreting glands.
olfactory nerve (I): The set of bundles of unmyelinated axons originating from the olfactory receptor neurons in the olfactory epithelium of the nose that project through the cribiform plate and terminate in the olfactory bulb.
olfactory receptor neurons (ORNs): Bipolar neurons in olfactory epithelium that contain receptors for odorants.
olfactory tracts: The projections from the olfactory bulb to the various divisions of the olfactory cortex in the ventromedial forebrain. See lateral olfactory tract.
oligodendrocytes: One of the three major classes of glial cells found in the central nervous system; their major function is to lay down myelin, which facilitates the efficient generation and rapid conduction of action potentials; also produce signaling molecules that modulate growth cone activity in regenerating axons.
ON-center neuron: A visual neuron whose receptive field center is excited by light.
Onuf’s nucleus: Sexually dimorphic nucleus in the human spinal cord that innervates striated perineal muscles mediating contraction of the bladder in males, and vaginal constriction in females.
operant conditioning: A form of conditioning shaped by reward rather that pairing a reflex response with an arbitrary signal.
opsins: Proteins in photoreceptors that absorb light (in humans, rhodopsin and the three specialized cone opsins).
optic chiasm: The junction of the two optic nerves on the ventral aspect of the diencephalon, where axons from the nasal divisions of each retina cross the midline.
optic disk: The region of the retina where the axons of retinal ganglion cells exit to form the optic nerve and where the ophthalmic artery and vein enter the eye. Also called the optic papilla.
optic nerve (II): The nerve (cranial nerve II) containing the axons of retinal ganglion cells; extends from the eye to the optic chiasm.
optic papilla: The region of the retina where the axons of retinal ganglion cells exit to form the optic nerve and where the ophthalmic artery and vein enter the eye. Also called the optic disk.
optic radiation: Portion of the internal capsule that comprises the axons of lateral geniculate neurons that carry visual information to the striate cortex.
optic tectum: The first central station in the visual pathway of many non-mammalian vertebrates (analogous to the superior colliculus in mammals).
optic tract: The axons of retinal ganglion cells after they have passed through the region of the optic chiasm en route to the lateral geniculate nucleus of the thalamus.
optic vesicles: The evagination of the forebrain vesicles that generates the retina and induces lens formation in the overlying ectoderm.
optogenetics: The use of genetic tools to induce neurons to become sensitive to light, such that experimenters can excite or inhibit a cell by exposing it to light.
optokinetic eye movements: Movements of the eyes that compensate for head movements; the stimulus for optokinetic movements is large-scale motion of the visual field.
optokinetic nystagmus: Repeated reflexive responses of the eyes to ongoing large-scale movements of the visual scene.
orbital gyri: Gyral formations on the ventral aspect of the frontal lobes that lie superior to the orbits in the anterior cranial fossae; part of the prefrontal cortex that is involved in implicit processing, including emotion, bodily feeling, and related aspects of cognition.
orbitofrontal cortex (OFC): The division of the prefrontal cortex that lies above the orbits in the most rostral and ventral extension of the sagittal fissure; important in emotional processing and decision making.
orexin: A peptide secreted by the hypothalamus, which promotes waking. Also called hypocretin.
orthologous genes: Genes expressed in model organisms that are identical or similar to target genes (typically expressed in humans and associated with disease) based on sequence and chromosomal location.
orthostatic hypotension: Fall in blood pressure upon standing up as a result of blood pooling in the lower extremities.
oscillations: Rhythmic patterns of either sub-threshold or spike related electrical activity in the brain that continues over extended periods of time, and can influence the capacity for neurons to engage in plastic changes in synaptic connections.
oscillopsia: Inability, as a result of vestibular damage, to fixate visual targets while the head is moving.
ossicles: The bones of the middle ear.
otoconia: The calcium carbonate crystals that rest on the otolithic membrane overlying the hair cells of the sacculus and utricle.
otolith organs: The two organs in the labyrynth of the inner ear—the utricle and saccule—that respond to linear accelerations of the head and static head position relative to the gravitational axis.
otolithic membrane: The gelatinous and fibrous membrane on which the otoconia lie and in which the tips of the hair bundles are embedded.
oval window: Site where the middle ear ossicles transfer vibrational energy to the cochlea.
overshoot phase: The peak, positive-going phase of an action potential, caused by high membrane permeability to a cation such as Na⁺ or Ca²⁺.
overt attention: The focusing of attention (typically visual) by voluntarily shifting gaze. Compare covert attention.

P

P2X receptors: A family of ionotropic purinergic neurotransmitter receptors.
p75: The alternate name for the low affinity neurotrophin receptor (LNGFR). P75/LNGFR interacts with the AKT kinase pathway, and is particularly important for neurotrophin signaling that influences neuronal survival or death.
Pacinian afferents: Encapsulated cutaneous mechanosensory receptors in the deep dermis (also found in other tissues) specialized for the detection of high-frequency vibrations.
pain matrix: A broad array of brain areas, including the somatosensory cortex, insular cortex, amygdala, and anterior cingulate cortex, whose activity is associated with the experience of pain.
paleocortex: Phylogenetically primitive cortex with few cell layers on the inferior and medial aspect of the temporal lobe within the parahippocampal gyrus and the junction of the temporal and frontal lobes.
pallidum: Division of the basal ganglia that receives input from the striatum and provides inhibitory (GABAergic) output to the thalamus and brainstem.
parabrachial nucleus: Nucleus of the rostral pons that relays visceral sensory information to the hypothalamus, amygdala, thalamus, and medial prefrontal and insular cortex.
paracentral lobule: Gryal formation on the medial aspect of the cerebral hemisphere formed by the fusion of the pre- and post-central gyri surrounding the medial termination of the central sulcus; comprises the somatic sensorimotor representation of the contralateral foot.
paracrine: Referring to the secretion of hormone-like agents whose effects are mediated locally rather than by the general circulation.
parahippocampal gyrus: Medial-most gyral structure in the inferior temporal lobe; part of the medial temporal lobe memory system that generates cognitive maps in spatial frameworks that facilitate the acquisition of episodic and declarative memory.
parallel fibers: The bifurcated axons of cerebellar granule cells that extend along the length of the folia in the molecular layer of the cerebellar cortex where they synapse on dendritic spines of Purkinje cells.
parallel pathways: Afferent pathways that carry distinct submodalities of sensory information centrally at the same time along anatomically distinct projections.
paramedian circumferential arteries: Shorter branches of the vertebral and basilar arteries that supply lateral aspects of the brainstem.
paramedian pontine reticular formation (PPRF): Neurons in the reticular formation of the pons that coordinate the actions of motor neurons in the abducens and oculomotor nuclei to generate horizontal movements of the eyes; also called the horizontal gaze center.
parasagittal: Standard anatomical planes of section; any vertical plane passing from anterior to posterior that is parallel to the sagittal plane.
parasympathetic: A division of the visceral motor system (division) in which the effectors are cholinergic ganglion cells located near target organs and the central preganglion neurons that innervate them.
parasympathetic ganglia: Locus of primary parasympathetic motor neurons; unlike sympathetic ganglia, which are relatively close to the spinal column, parasympathetic ganglia are further removed and typically embedded in or very near the end organs they innervate.
paravertebral sympathetic chain: Chain of cervical and thoracic sympathetic ganglia located lateral to the spinal column.
parietal lobe: The hemispheric lobe that lies between the frontal lobe anteriorly, and the occipital lobe posteriorly.
parieto-occipital sulcus: Prominent sulcus on the medial surface of the cerebral hemisphere that divides the parietal and occipital lobes.
Parkinson’s disease: Progressive neurodegenerative disease of the substantia nigra pars compacta that results in a characteristic tremor at rest and a general paucity of movement.
parvocellular layers: A component of the primary visual pathway specialized for the detection of detail and color; so named because of the relatively small cells involved.
passive electrical responses: Responses to applied electrical currents that do not require activation of voltage-gated ion channels.
patch clamp: An extraordinarily sensitive voltage clamp method that permits the measurement of ionic currents flowing through individual ion channels.
peptide neurotransmitters: See neuropeptides.
perilymph: The potassium-poor fluid that bathes the basal end of the cochlear hair cells.
peripheral nervous system (PNS): All nerves and neurons that lie outside the brain and spinal cord.
peripheral sensitization: Increased responsiveness of peripheral pain-sensing neurons following tissue damage that is one source of hyperalgesia.
persistent vegetative state: A state that results from profound damage to the brain, perhaps by injury or disease, that is characterized by a lack of awareness. A patient with persistant vegetative state typically can still react to stimuli and exhibit degrees of wakefulness and quiescence.
phenotypic sex: The visible somatic features that define one sex from the other. Phenotypic sex can variably include body hair patterns, body size and musculature, voice, as well as the primary sex characteristics associated with males versus females.
pheromones: Species-specific odorants that play important roles in behavior in some animals, including many mammals.
photopic vision: Vision at high light levels, which is mediated almost entirely by cone cells. Contrast with scotopic vision.
photoreceptors: The specialized neurons in the eye—rods and cones—that are sensitive to light.
phototransduction: The process by which light is converted in electrical signals in the retina.
pia mater: The innermost of the three layers of the meninges; a delicate layer (pia mater means “tender or delicate mother”) closely applied to the surface of the brain.
pineal gland: Midline neural structure lying on the dorsal surface of the midbrain; important in the control of circadian rhythms (and, incidentally, considered by Descartes to be the seat of the soul).
pinna: A component of the external ear.
pitch: The sensory quality roughly corresponding to periodic vibrations of sound stimuli.
pituitary stalk: See infundibulum.
placebo effect: The physiological effects resulting from administration of an inert substance (a placebo).
planum temporale: Region on the superior surface of the temporal lobe posterior to Heschl’s gyrus; notable because it is larger in the left hemisphere in about two-thirds of humans.
PLCβ2: Phospholipase Cβ2, a membrane associated enzyme activated by gustducin, a G-protein specifically associated with G-protein-coupled taste receptor proteins in taste receptors cells. PLCβ2 enzymatically converts the membrane lipid phospho-inositol-biphosphate into second messenger lipids inositol triphosphate (IP₃) and DiacylGlycerol (DAG). IP₃/DAG signaling leads to Ca²⁺ release and activation of TRP channels.
plexus: A complex network of nerves, blood vessels, or lymphatic vessels.
pneumoencephalography: An x-ray based means for brain imaging involving the displacement of cerebrospinal fluid by injection of air into the subarachnoid space to increase signal contrast.
point of fixation: The point in visual space that falls on the fovea of each eye.
polarized epithelial cells: Cells arranged in sheets or layers, that have an apical (top) and basal (bottom) domain that are distinguished by molecular differences. The apical domain of an epithelial cell is specialized for interactions and transduction of signals from the environment. The basal domain is specialized for secretion.
polyneuronal innervation: A state in which neurons or muscle fibers receive synaptic inputs from multiple, rather than single, axons.
pons: One of the three major divisions of the brainstem, lying between the midbrain rostrally and the medulla oblongata caudally; derived from the embryonic metencephalon.
pontine nuclei: Collections of neurons in the base of the pons that receive input from the ipsilateral cerebral cortex and send their axons across the midline to the contralateral cerebellum via the middle cerebellar peduncle.
pontine reticular formation: Collections of neurons in the pons that receive input from the cerebral cortex and send their axons across the midline to the cerebellar cortex via the middle cerebellar peduncle.
pontine-geniculo-occipital (PGO) waves: Characteristic encephalographic waves that signal the onset of rapid eye movement sleep.
pore: Structural feature of an ion channel that allows ions to diffuse through the channel.
pore loop: An extracellular domain of amino acids, found in certain ion channels, that lines the channel pore and allows only certain ions to pass.
positron emission tomography (PET): A technique for examining brain function following injection of unstable, positron-emitting isotopes that are then incorporated into bioactive molecules or metabolites; the emission of positrons are detected by gamma ray detectors and tomographic images are computed that indicate the localization and concentration of the isotopes.
post-tetanic potentiation (PTP): An enhancement of synaptic transmission resulting from high-frequency trains of action potentials. See synaptic potentiation.
postcentral gyrus: The gyrus that forms the posterior bank of the central sulcus; contains the primary somatic sensory cortex.
posterior cerebral arteries: Major vessels derived from the basilar artery that supply the ventral midbrain and the posterior, inferior, and medial aspects of the occipital, parietal, and temporal lobes, including associated deep structures.
posterior chamber: The region of the eye between the lens and the iris.
posterior cingulate cortex (PCC): A functional division of the cerebral cortex located on its midline surface caudal to the central sulcus that surrounds the corpus callosum. It is associated with task-negative cognition, including mind-wandering, and reward.
posterior circulation: Vasculature derived from the vertebral and basilar arteries that supplies blood to the hindbrain and posterior forebrain.
posterior communicating arteries: Small vessels that join the internal carotid arteries to the posterior cerebral arteries, forming the lateral aspects of the circle of Willis.
posterior funiculi: See dorsal columns.
posterior inferior cerebellar artery (PICA): Long circumferential branch of the vertebral artery that supplies dorsolateral aspects of the medulla and posterior and inferior aspects of the cerebellum.
posterior spinal arteries: Principal arteries on the posterior aspect of the spinal cord supplied by the vertebral or posterior inferior cerebellar arteries; supplies blood to the posterior one-third of the spinal cord.
postganglionic axons: Axons that link visceral motor neurons in autonomic ganglia to their targets.
postsynaptic: Referring to the compartment of a neuronal process (typically, a dendritic spine or shaft) or a location on a cell body that is specialized for transmitter reception; downstream at a synapse.
postsynaptic current (PSC): The current produced in a postsynaptic neuron by the binding of neurotransmitter released from a presynaptic neuron.
postsynaptic density: A cytoskeletal junction in developing synapses that may serve to organize postsynaptic receptors and speed their response to neurotransmitter.
postsynaptic dorsal column projection: Axonal projection arising from dorsal horn neurons through the dorsal columns terminating in dorsal column nuclei; conveys mechanosensory information in parallel with central processes of first-order afferents.
postsynaptic potential (PSP): The potential change produced in a postsynaptic neuron by the binding of neurotransmitter released from a presynaptic neuron.
posttraumatic stress disorder (PTSD): A clinical condition that emerges following the experience of one or more traumatic, stressful events. Symptoms include heightened arousal, emotional numbness, avoidance of event reminders, and persistent reexperiencing of the traumatic event(s).
potentiation: An activity-dependent form of short-term synaptic plasticity that enhances synaptic transmission. Potentiation is caused by an increase in the amount of neurotransmitter released in response to presynaptic action potentials and results from persistent calcium actions within presynaptic terminals. Because potentiation acts over a time course of seconds to minutes, it often outlasts the high-frequency trains of action potentials that evoke it, leading to the phenomenon of post-tetanic potentiation.
pre-propeptides: The first protein translation products synthesized in a cell. These polypeptides are usually much larger than the final, mature peptide and often contain signal sequences that target the peptide to the lumen of the endoplasmic reticulum.
precentral gyrus: The gyrus that forms the anterior bank of the central sulcus; contains the primary motor cortex.
precuneus gyrus: Gyral structure on the medial aspect of the parietal lobe between the dorsal ramus of the cingulate sulcus and the parieto-occipital sulcus; a component of the default-mode network.
prefrontal cortex (PFC): Cortical regions in the frontal lobe that are anterior to the primary and association motor cortices; thought to be involved in planning complex cognitive behaviors and in the expression of personality and appropriate social behavior.
preganglionic neurons: Visceral motor neurons in the spinal cord and brainstem that innervate autonomic ganglia.
premotor cortex: Motor association areas in the frontal lobe anterior to the primary motor cortex; involved in planning or programming of voluntary movements and a source of descending projections to motor neurons in the spinal cord and cranial nerve nuclei.
presbyopia: The condition in which aging affects the accommodative ability of the eye.
presynaptic: Referring to the compartment of a neuronal process (typically, a terminal of an axon) at a synapse specialized for transmitter release; upstream at a synapse.
pretectum: A group of nuclei located at the junction of the thalamus and the midbrain; these nuclei are important in the pupillary light reflex, relaying information from the retina to the Edinger–Westphal nucleus.
prevertebral ganglia: Sympathetic ganglia that lie anterior to the spinal column (distinct from the sympathetic chain ganglia).
primary motor cortex: A major source of descending projections to motor neurons in the spinal cord and cranial nerve nuclei; located in the precentral gyrus (Brodmann’s area 4) and essential for the voluntary control of movement.
primary sex characteristics: The distinguishing body features related to chromosomal sex: male versus female gonads and genitalia.
primary somatosensory cortex (SI): Functional division of the cerebral cortex in the postcentral gyrus, corresponding to Brodmann’s areas 3, 1, and 2, that receives somatosensory projections from the ventral posterior complex of thalamic nuclei; processes somatosensory information from the body surface, subcutaneous tissues, muscles, and joints.
primary visual cortex (V1): Brodmann’s area 17 in the medial occipital lobe; major cortical target of the thalamic lateral geniculate nucleus. Also called striate cortex because of the prominence of a heavily myelinated stripe in layer 4 (called the stria of Gennari) that gives this region a striped (striated) appearance.
primary visual pathway: Pathway from the retina via the lateral geniculate nucleus of the thalamus to the primary visual cortex; carries the information that allows conscious visual perception. Also known as the retinogenticulocortical pathway.
priming: A change in the processing of a stimulus due to a previous encounter with the same or a related stimulus, with or without conscious awareness of the original encounter.
primitive pit: An important source of neural inductive signals during gastrulation.
primitive streak: Axial indentation in the gastrulas of birds and mammals that generates the notochord and defines the embryonic midline. At the thickened anterior end of the streak is an indentation, the primitive pit, which is an important source of neural inductive signals during gastrulation.
principal nucleus: Main nuclear division of the trigeminal nuclear complex of the brainstem in the pons (also known as the chief sensory nucleus) that receives mechanosensory afferents from the trigeminal nerve; gives rise to the trigeminal lemniscus the supplies the contralateral ventral posterior medial nucleus.
procedural memory: See nondeclarative memory.
projection neurons: Neurons with long axons that project to distant targets.
promoter: DNA sequence (usually within 35 nucleotides upstream of the start site of transcription) to which the RNA polymerase and its associated factors bind to initiate transcription.
propeptide: Partially processed forms of proteins containing peptide sequences that play a role in the correct folding of the final protein.
prosencephalon: The part of the brain that includes the diencephalon and telencephalon derived from the embryonic forebrain vesicle.
prosody: The normal rhythm, stress, and tonal variation of speech that give it emotional meaning.
prosopagnosia: The inability to recognize faces; usually associated with lesions to the right inferior temporal cortex.
protein kinases: Enzymes that participate in intracellular signal transduction by phosphorylating their target proteins, thereby altering the function of these targets.
protein phosphatases: A family of enzymes that participate in intracellular signal transduction by removing phosphate groups from their target proteins, thereby altering the function of these targets.
protocadherins: A large family of the cadherin class of Ca²⁺ dependent cell adhesion molecules. There are at least 50 or more genes in the mammalian genome that encode protocadherins, and most protocadherin genes are organized so that their exons can be alternatively spliced to generate multiple protocadherin isoforms from the same gene.
pseudounipolar: Morphology of a somatosensory ganglion neuron whose peripheral and central components of an afferent fiber are continuous, attached to the cell body by a single process.
ptosis: A drooping of the upper eyelid.
pupil: The perforation in the center of the iris that allows light to enter the eye. The pupillary light reflex mediates pupillary constriction in full light and expansion (dilation) in dim light; these responses can also be induced by chemicals and by certain emotional states, and thus can be clinically important.
pupillary light reflex: The reduction in the diameter of the pupil that occurs when sufficient light falls on the retina.
putamen: One of the three major components of the striatum (the other two are the caudate and the nucleus accumbens). Also called putamen nuclei.
pyriform cortex: Component of cerebral cortex in the temporal lobe pertinent to olfaction; so named because of its pearlike shape.

R

radial glia: Glial cells that contact both the luminal and pial surfaces of the neural tube, providing a substrate for neuronal migration.
rapid eye movement (REM) sleep: The phase of sleep characterized by low-voltage, high-frequency electroencephalographic activity accompanied by rapid eye movements.
rapidly adapting afferents: Afferents that fire transiently in response to stimulus onset or offset.
ras: The first monomeric G-protein discovered. It is involved in many types of neuronal signaling and also controls differentiation and proliferation of non-neuronal cells.
readiness potential: An electrical potential, recorded from the motor and premotor cortices with EEG electrodes, that signals the intention to initiate a voluntary movement well in advance of actual production of the movement.
receptive aphasia: See Wernicke’s aphasia.
receptive field: The region of a receptive surface (e.g., the body surface, or a specialized structure such as the retina) within which a specific stimulus elicits the greatest action potential response from a sensory cell in a sensory ganglion or within the CNS.
receptive field properties: Neurophysiological characteristics the define the particular responses of sensory neurons to stimuli in their receptive fields; examples for a neuron in the primary visual cortex include the preferred location, orientation, motion direction, and spatial frequency of stimuli that yields the greatest neural response.
receptor molecule: A molecule that binds to chemical signals and transduces these signals into a cellular response.
receptor potentials: The membrane potential change elicited in receptor neurons during sensory transduction. Also called generator potentials. Compare synaptic potentials.
reciprocal innervation: Pattern of connectivity in local circuits of the spinal cord involving excitatory and inhibitory interneurons arranged to ensure that contraction of agonistic muscles produce forces that are opposite to those generated by contraction of antagonistic muscles; thus, reciprocal innervation mediates the simultaneous relaxation of antagonists during contraction of agonists.
red nucleus: Prominent parvocellular nucleus of the midbrain tegmentum involved in regulating activity in the inferior olivary nucleus; integrates input from the cerebral cortex and the contralateral dentate nucleus of the cerebrocerebellum. In non-human mammals, also features a magnocellular division that gives rise to the rubrospinal tract, which participates in upper motor neuronal control of the distal musculature of the upper limbs or forelimbs.
reentrant (or recurrent) neural activation): Following a stimulus or event, a process in which neural activity is fed back to the same brain region activated earlier in the processing sequence.
refractory period: The brief period after the generation of an action potential during which a second action potential is difficult or impossible to elicit.
regenerative: A process that is self-sustaining. For example, action potential propagation is regenerative because an action potential produced at one location depolarizes downstream regions, thereby activating voltage-gated ion channels to generate an action potential in these regions.
restiform body: See cerebellar peduncles.
resting membrane potential: The inside-negative electrical potential that is normally recorded across all cell membranes.
reticular activating system: Region in the brainstem tegmentum that, when stimulated, causes arousal; involved in modulating sleep and wakefulness.
reticular formation: A network of neurons and axons that occupies the core of the brainstem, giving it a reticulated (“net-like”) appearance in myelin-stained material; major functions are modulatory (e.g., regulating states of consciousness) and premotor (e.g., coordinating eye movements, posture, and the regulation of respiratory and cardiac rhythms).
retina: Laminated neural component of the eye that contains the photoreceptors (rods and cones) and the initial processing machinery for the primary (and other) visual pathways.
retinal: A light-absorbing chromophore; the aldehyde form of vitamin A.
retinal waves: A type of oscillatory activity established in the developing mammalian retina, usually prenatally or before eye opening, that is independent of visual input. These oscillations are established by subthreshold activity of subsets of amacrine cells leading to rhythmic firing of subsets of retinal ganglion cells in a spatially distinct manner.
retinogeniculostriate pathway: Another term for the primary visual pathway.
retinohypothalamic pathway: The route by which variation in light levels influences the broad spectrum of functions that are entrained to the day–night cycle.
retinoic acid (RA): A derivative of vitamin A that acts as an inductive signal during early brain development.
retinoid cycle: Process in which retinal is restored to a form capable of signaling photon capture.
retinoid receptors: The family of nuclear transcription factor proteins that form heterodimers and bind isomers of retinoic acid to initiate recognition of response element DNA binding sequences and subsequently influence gene expression. Retinoid receptors are a subclass of the broader steroid/thyroid receptor-transcription factor family.
retrograde: Signals or impulses that travel “backward,” e.g., from the axon terminal toward the cell body, or from the postsynaptic cell to the presynaptic terminal, or from the periphery to the CNS.
retrograde amnesia: The inability to recall existing memories.
reversal potential: Membrane potential of a postsynaptic neuron (or other target cell) at which the action of a given neurotransmitter causes no net current flow.
reward prediction error (RPE): A quantity given by the difference between the reward that was expected and what actually occurs; the activity of some dopaminergic neurons seems to convey this quantity.
reward: A poorly defined term that generally refers to a sense of pleasure following a successful response to some challenge. Often taken to entail dopaminergic neural circuitry.
rhodopsin: The photopigment found in rods.
rhombencephalon: The caudal part of the brain between the mesencephalon and the spinal cord derived from the embryonic hindbrain vesicle; includes the pons, cerebellum, and medulla. Also known as the hindbrain.
rising phase: The initial, depolarizing, phase of an action potential, caused by the regenerative, voltage-dependent influx of a cation such as Na⁺ or Ca²⁺.
robo: The transmembrane receptor for Slit that activates signaling via interactions with non-receptor tyrosine kinases, RhoGTPases and other cytoplasmic signaling molecules to initiate the repulsion/avoidance of an axon or dendrite for a specific direction or location.
ROBO3: An additional ROBO receptor, encoded by a separate gene. ROBO3 is particularly essential for the guidance of axons through the ventral commissure at the midline spinal cord, and ensuring that these axons do not turn around and cross back at the midline.
rods: Photoreceptor cells specialized for operating at low light levels.
roofplate: The thinned dorsal-most medial region of the neural tube, where the two edges of the lateral/alar neuroectoderm fuse during neurulation. This neuroectodermal cells of this region, like the floorplate at the ventral midline, provide secreted signals to specify the neural crest as well as dorsal cell types in the neural tube.
rostral (R): Anterior, or “headward.”
rostral interstitial nucleus: Cluster of neurons in the reticular formation that coordinates the actions of neurons in the oculomotor nuclei to generate vertical movements of the eye; also called the vertical gaze center.
rostral migratory stream: A specific migratory route, defined by a distinct subset of glial cells, that facilitates migration of newly generated neurons from the stem cell niche of the anterior subventricular zone to the olfactory bulb.
rostrotemporal (RT): One of the divisions of the core region of the auditory cortex in non-human primates.
round window: Along with the oval window, a region at the base of the cochlea where the overlying bone is absent.
rubrospinal tract: In non-human mammals, the pathway from the magnocellular divisions of the red nucleus of the midbrain to the spinal cord; participates with the lateral corticospinal tract in governing the distal extremities. In humans, however, the corticospinal tract serves this function and the rubrospinal tract is vestigial (perhaps even nonexistent).
Ruffini afferents: Encapsulated cutaneous mechanosensory receptors in the deep dermis (also found in other tissues) specialized for the detection of cutaneous stretching produced by digit or limb movements.
ryanodine receptor: A ligand-gated ion channel in the endoplasmic reticulum membrane. This receptor binds to the drug ryanodine and elevates cytoplasmic calcium concentration by mediating flux of calcium out of the lumen of the endoplasmic reticulum.

S

saccades: Ballistic, conjugate eye movements that change the point of foveal fixation.
saccule: The otolith organ that detects linear accelerations and head tilts in the vertical plane.
sacral: Caudal region of the spinal cord between the lumbar and coccygeal regions related to the lower extremities and pelvic visceral motor outflow.
sagittal: Standard anatomical plane of section; the vertical plane passing from anterior to posterior through the midline dividing the body (and brain) into right and left sections.
saliency maps: A theoretical construct of visual attention in which the importance of different stimuli in the visual field is set by a combination of top-down processes based on behavioral goals and bottom-up processes resulting from how distinctive the different elements of a stimulus are compared to the background.
salt: One of the five basic tastes; the taste quality produced by the cations of salts (e.g., the sodium in sodium chloride produces the salty taste). Some cations also produce other taste qualities (e.g., potassium tastes bitter as well as salty). The purest salty taste is produced by sodium chloride (NaCl), common table salt. Salt taste is transduced by taste cells via an ameloride sensitive Na⁺ channel.
saltatory: Mechanism of action potential propagation in myelinated axons; so named because action potentials “jump” from one node of Ranvier to the next due to generation of action potentials only at these sites.
scala media: The fluid-filled chamber within the cochlea that sits on the basilar membrane and that lies between the scala vestibuli and the scala tympani.
scala tympani: The fluid-filled chamber within the cochlea at the base of which is located the round window.
scala vestibuli: The fluid-filled chamber within the cochlea at the base of which is located the oval window.
Scarpa’s ganglion: See vestibular nerve ganglion.
Schwann cells: Glial cells in the peripheral nervous system that lay down myelin, which facilitates the efficient generation and rapid conduction of action potentials; also facilitate axon regeneration in damaged nerves (named after the nineteenth-century anatomist and physiologist Theodor Schwann).
sclera: The external connective tissue coat of the eyeball.
SCN genes: suprachiasmatic nucleus. Hypothalamic nucleus lying just above the optic chiasm that receives direct input from the retina; involved in light entrainment of circadian rhythms.
scotoma: A small deficit in the visual field resulting from pathological changes in some component of the primary visual pathway.
scotopic vision: Vision in dim light, where the rods are the operative receptors.
second pain: A category of pain perception described as more delayed, diffuse, and longer-lasting than first pain.
secondary sex characteristics: Anatomical characteristics, such as breasts and facial hair, that generally differ between the sexes but are not necessarily concordant with the chromosomal sex of the individual
secondary somatosensory cortex (SII): Functional division of the cerebral cortex in the parietal operculum just posterior to the postcentral gyrus; processes somatosensory information received from the primary somatosensory cortex.
sedative: Drug that calms patient, in some cases inducing sleep.
segmental nerves: See spinal nerves.
segmentation: The anterior–posterior division of animals into roughly similar repeating units.
selective serotonin reuptake inhibitors (SSRIs): A class of drugs that work by inhibiting the ability of the SERT serotonin transporter to take serotonin up into presynaptic terminals.
selectivity filter: Structure within an ion channel that allows selected ions to permeate, while rejecting other types of ions.
semaphorins: A family of diffusible, growth-inhibiting molecules.
semicircular canals: The vestibular end organs in the inner ear that sense rotational accelerations of the head.
sensitization: Increased sensitivity to stimuli in an area surrounding an injury. Also, a generalized aversive response to an otherwise benign stimulus when it is paired with a noxious stimulus.
sensorineural hearing loss: Diminished sense of hearing due to damage of the inner ear or its related central auditory structures. Contrast with conductive hearing loss.
sensory aphasia: See Wernicke’s aphasia.
sensory systems: Term sometimes used to describe all the components of the central and peripheral nervous system concerned with sensation.
sensory transduction: Process by which the energy of a stimulus is converted into electrical signals by peripheral sensory receptors and then processed by the central nervous system.
sensory–discriminative: The aspect of pain that allows one to distinguish the location, intensity, and quality of a noxious stimulation.
septal forebrain nuclei: Cerebral nuclei at the anterior base of the septum pellucidum; give rise to widespread modulatory projections to diverse targets in the cerebral hemispheres.
septum pellucidum: Non-neural tissue that forms the medial wall along the anterior horns, bodies, and atria of the paired lateral ventricles.
serotonergic neurons: Neurons that contribute to the neuronal basis of wakefulness.
serotonin: A biogenic amine neurotransmitter, derived from the amino acid tryptophan, that is involved in a wide range of behaviors, including emotional states and mental arousal.
sex chromosomes: Either of a pair of chromosomes (XX in female or XY in male mammals) that differ between the sexes.
sexual identity: The subjective perception one feels about one’s phenotypic sex.
sexual orientation: The direction of an individual’s sexual feelings: sexual attraction toward persons of the opposite sex (heterosexual), the same sex (homosexual), or both sexes (bisexual).
sexually dimorphic nucleus of the preoptic area (SDN-POA): A hypothalamic nucleus that in humans and several other mammals differs in size in males versus females: usually the male SDN-POA is larger than that of the female. The SDN-POA is thought to regulate sexual behaviors directly involved with reproduction as well as partner selection.
sham rage: An emotional reaction elicited in cats by electrical stimulation of the hypothalamus, characterized by hissing, growling, and attack behaviors directed randomly toward innocuous targets.
short circumferential arteries: The shortest branches of the vertebral and basilar arteries that supply medial aspects of the brainstem.
short-term memory: Memories held briefly in mind that enable a particular task to be accomplished (e.g., efficiently searching a room for a lost object). Also called working memory.
sigmoid sinuses: Dural venous sinuses that convey venous blood from the transverse sinuses through the jugular foramina and into the jugular veins.
signal amplification: A consequence of intercellular or intracellular signal transduction, resulting from the involvement of reactions that generate a much larger number of products than the number of molecules required to initiate the process. Signal amplification is one of the most important advantages of chemical signaling.
signaling endosome: A vesicular structure, internalized into the cell, usually at an axonal or dendritic process, via the invagination and then constriction of the plasma membrane. These vesicles include transmembrane receptor kinases that have bound their activating ligand while still on the cell surface. The ligand remains bound to the receptor, but as an internalized vesicle the ligand activated kinase domain faces the cytoplasm of the cell. The vesicle can then be transported retrogradely to the cell body and continue to transduce the signal detected at its point of origin throughout the entire cell.
single-photon emission computerized tomography (SPECT): A technique for examining brain function following injection or inhalation of radiolabeled compounds, which produce photons that are detected by a gamma camera moving rapidly around the head and used to generate tomographic images indicating the localization and concentration of the isotopes.
size principle: The orderly recruitment of motor neurons by size to generate increasing amounts of muscle tension.
skin conductance: A stimulus-induced increase in the electrical conductance of the skin due to increased hydration.
sleep spindles: Periodic bursts of activity at about 10 to 12 Hz that generally last 1 to 2 seconds and arise as a result of interactions between thalamic and cortical neurons; intermittent high-frequency EEG spike clusters characteristic of stage II sleep.
slit: A secreted signaling molecule that acts a repulsive cue for growing axons or dendrites.
slow (S) motor units: Small motor units comprising small muscle fibers that contract slowly and generate relatively small forces; but because of their rich myoglobin content, plentiful mitochondria, and rich capillary beds, these small red fibers are resistant to fatigue. S motor units are especially important for activities that require sustained muscular contraction, such as maintaining an upright posture.
slow-wave sleep: The component of sleep characterized by delta waves.
slowly adapting afferents: Afferents that continue to fire, with only modest decrement in firing, in response to the sustained presence of a stimulus.
small clear-core vesicles: A type of synaptic vesicle characterized by a small diameter (typically on the order of 50 nanometers) and an absence of an electron-dense core. These vesicles typically contain and release small-molecular neurotransmitters, such as acetylcholine, glutamate and GABA.
small G-proteins: See monomeric G-proteins.
small-molecule neurotransmitters: The non-peptide neurotransmitters such as acetylcholine, the amino acids glutamate, aspartate, GABA, and glycine, as well as the biogenic amines.
smooth pursuit movements: Slow, tracking movements of the eyes designed to keep a moving object aligned with the foveae.
SNAP-25: A SNARE associated with the plasma membrane. This protein forms a SNARE complex with synaptobrevin and syntaxin that mediates fusion of synaptic vesicles with the presynaptic plasma membrane.
SNAPs: Soluble NSF-attachment proteins. A protein that attaches the enzyme NSF to SNARE complexes, to allow NSF to dissociate the SNARE complexes.
SNAREs: SNAP receptors. Proteins that are found on two membranes and are responsible for fusing the two membranes together.
somatic marker hypothesis: A theory that motivated behavior is influenced by neural representations of body states (the “somatic markers”), whose re-experiencing can shape behavior positively or negatively; the hypothesis that evaluation of one’s own body states makes important contributions to decision making.
somatic motor division: The components of the motor system that support skeletal movements mediated by the contraction of skeletal muscles that are derived from embryonic somites or somitomeres.
somatic motor nuclei: Brainstem nuclei (derived from the basal plate) that give rise to efferent fibers innervating striated muscle fibers derived from embryonic somitomeres; located in the dorsal tegmentum alongside the midline.
somatic stem cells: Cells that can divide to give rise to more cells like itself, but also can divide to give rise to a new stem cell plus one or more differentiated cells of the relevant tissue type (e.g., a hematopoietic stem cell can give rise to all types of blood cells, neural stem cells give rise to all neuronal types, and glial stem cells to glia). Contrast with embryonic stem cell.
somatosensory cortex: Functional division of the cerebral cortex in the postcentral gyrus and anterior parietal lobe that receives somatosensory projections from the ventral posterior complex of thalamic nuclei; processes somatosensory information from the body surface, subcutaneous tissues, muscles, and joints.
somatotopic maps: Cortical or subcortical arrangements of sensory inputs and local circuits that reflect the topological organization of the body.
Sonic hedgehog (Shh): An inductive signaling hormone essential for development of the mammalian nervous system; believed to be particularly important for establishing the identity of neurons in the ventral portion of the developing spinal cord and hindbrain.
sour: One of the five basic tastes; the taste quality produced by the hydrogen ion in acids. Sour tastes are transduced by taste cells via a H⁺ selective TRP channel.
spike timing-dependent plasticity (STDP): Changes in synaptic transmission that depend upon the precise temporal relationship between presynaptic action potentials and postsynaptic responses.
spinal accessory nerve (XI): Cranial nerve XI, a branchial motor nerve that conveys efferents from the rostral cervical spinal cord (spinal accessory nucleus) to the upper trapezius and sternocleidomastoid muscles. Attaches to the rostral cervical cord in a cleft medial to the inferior cerebellar peduncle; enters the cranial vault through the foramen magnum and exits via the jugular foramen.
spinal cord: The caudal (post cranial) portion of the central nervous system (CNS) that extends from the lower end of the brainstem (the medulla) to the cauda equina; mediates the transmission of afferent and efferent neural signals between the CNS and the body.
spinal nerves: Mixed sensory and motor nerves that arise in bilaterally symmetrical pairs from each of 31 segments of the spinal cord.
spinal nucleus: Component of the trigeminal nuclear complex of the brainstem in the caudal pons and medulla oblongata that receives afferents from the trigeminal nerve concerning pain and temperature (also receives collateral of mechanosensory afferents); comprises several subdivisions each of which gives rise to the trigeminothalamic tract that supplies the contralateral ventral posterior medial nucleus.
spinal nucleus of the bulbocavernosus (SNB): Sexually dimorphic collection of neurons in the lumbar region of the rodent spinal cord that innervate striated perineal muscles.
spinal shock: The initial, short-lived period of flaccid paralysis that accompanies damage to upper motor neurons or their descending pathways to lower motor neurons.
spindle cells: Neurons in the insula and anterior cingulate cortex that may support cognition in great apes and humans.
spinocerebellum: Medial part of the cerebellum that receives proprioceptive input from the spinal cord and sends output to the motor cortex via the thalamus and to brainstem upper motor neurons; includes paramedian zones that coordinate movements of distal muscles, and a median zone, called the vermis, that coordinates movements of proximal muscles, including eye movements.
splice variants: Variable messenger RNA transcripts derived from the same gene that are typically produced by including or excluding certain exons from a gene; the result such alternative splicing is the production of a diverse set of related protein products.
split-brain patients: Individuals who have had the cerebral commissures divided in the midline to control epileptic seizures.
Sprague effect: Hemispatial neglect induced by a parietal lesion in humans can be mostly compensated by a lesion of the superior colliculus on the other side.
SRY (sex-reversal gene on the Y chromosome): Gene on the Y chromosome whose expression triggers a signaling and transcriptional regulatory cascade that masculinizes the developing fetus.
stellate cells: Inhibitory interneurons in the cerebellar cortex that receive parallel fiber input and provide inhibitory output to Purkinje cell dendrites.
stereocilia: The actin-rich processes that, along with the kinocilium, form the hair bundle extending from the apical surface of the hair cell; site of mechanotransduction.
stereognosis: Responses to object shapes.
stereopsis: The perception of depth that results from the fact that the two eyes view the world from slightly different angles.
steroid–thyroid nuclear receptors: A large class of receptor transcription factors that selectively bind different members of the steroid-thyroid family of hormones. This class of receptor-transcription factors includes estrogen and androgen receptors.
strabismus: Developmental misalignment of the two eyes; may lead to binocular vision being compromised.
stria vascularis: Specialized epithelium lining the cochlear duct that maintains the high potassium concentration of the endolymph.
striate cortex: See primary visual cortex.
striatum: General term applied to the caudate, putamen, nucleus accumbens and other minor divisions of the ventral basal forebrain. The name derives from the bridges (“striations”) of gray matter that unite the caudate and putamen around which course fibers of the anterior limb of the internal capsule. Principal component of the corpus striatum, an historical term that has been used collectively to refer to the striatum and the globus pallidus.
striola: A line found in both the sacculus and utricle that divides the hair cells into two populations with opposing hair bundle polarities.
Stroop effect: A slowing of response time when a stimulus harbors inherently conflicting information (e.g., responding to the word red printed in green ink versus the word printed in red ink).
submucous plexus: Network of neurons in the enteric division of the visceral motor system just beneath the mucus membranes of the gut and is concerned with chemical monitoring and glandular secretion. Also called Meissner’s plexus.
substance P: An 11-amino acid neuropeptide; the first neuropeptide to be discovered.
substantia nigra: Bipartite gray matter structure in the ventral midbrain; contains a pallidal division, called the pars reticulata, defined by a network (reticulum) of cells that provide inhibitory (GABAergic) output from the basal ganglia to the thalamus and brainstem, and a compact division, called the pars compacta, comprising densely packed neurons that synthesize and release dopamine in the caudate and putamen.
substantia nigra pars compacta: Compact cell division of the substantia nigra in the ventral midbrain featuring densely packed neurons that synthesize and release dopamine in the caudate and putamen.
substantia nigra pars reticulata: Pallidal division of the substantia nigra in the ventral midbrain featuring a network (reticulum) of cells that provide inhibitory (GABAergic) output from the basal ganglia to the thalamus and brainstem.
subthalamic nucleus: A nucleus in the ventral thalamus that receives input from the cerebral cortex and external segment of the globus pallidus and sends excitatory (glutamatergic) projections to the internal segment of the globus pallidus. A component of the indirect pathway from striatum to pallidum.
sulci (sing. sulcus): Spaces between gyri; the largest of these spaces are called fissures.
sulcus limitans: Shallow longitudinal groove in the lateral wall of the lumen of the neural tube that defines a boundary between the alar and basal plates.
summation: The addition in space and time of sequential synaptic potentials to generate a postsynaptic response larger than that produced by a single synaptic potential.
superior cerebellar peduncle: See cerebellar peduncles.
superior colliculus (pl. colliculi): Laminated gray matter structure that forms part of the roof of the midbrain; plays an important role in orienting movements of the head and eyes.
superior division: Referring to the region of the visual field of each eye that corresponds to the top half of the retina.
superior frontal gyri: Superior of three, parallel longitudinal gyri that define the dorsomedial margin of frontal lobe anterior to the precentral sulcus; part of the prefrontal cortex and anterior premotor cortex.
superior oblique muscles: Extraocular muscles that intort the eyeballs when in the primary position (eyes straight ahead) and rotate downward when in adduction.
superior parietal lobules: Gyral formations of the dorsal (superior) parietal lobes that are involved in associating somatosensory, visual, auditory, and vestibular signals and generating a neural construct of the body, the position of its parts, and its movements (body image or schema).
superior rectus muscles: Extraocular muscles that rotate the eyeballs upward.
superior sagittal sinus: Large dural venous sinus along the dorsal aspect of the longitudinal fissure; provides for the drainage of venous blood from the dorsal cerebral hemisphere and the return of cerebrospinal fluid via the arachnoid granulations.
superior salivatory nuclei: Visceral motor nucleus of the rostral pons containing parasympathetic preganglionic neurons that mediate tearing and salivation.
superior temporal gyri: Superior of three, parallel longitudinal gyri that define the dorsolateral margin of temporal lobe; part of the auditory cortex and lateral temporal network that encodes language content.
suprachiasmatic nucleus (SCN): Hypothalamic nucleus lying just above the optic chiasm that receives direct input from the retina; involved in light entrainment of circadian rhythms.
supramodal attention: The focusing of attention on stimulus information across multiple modalities at the same time.
sustentacular cells: The primary support cells of the olfactory epithelium. Sustentacular cells help to maintain appropriate ionic milieu and epithelial integrity for the olfactory sensory neurons and their basal cell precursors throughout life.
sweet: One of the five basic tastes; the taste quality produced by some sugars, such as glucose, fructose, and sucrose. These three sugars are particularly biologically useful to us, and our sweet receptors are tuned to them. Some other compounds (e.g., saccharin, aspartame) are also sweet. Sweet is transduced by taste cells via the T1R class of G-protein-coupled taste receptors.
Sylvian fissure: See lateral fissure.
sympathetic: A division of the visceral motor system (division) in vertebrates comprising, for the most part, adrenergic ganglion cells located relatively far from the related end organs and the central preganglion neurons that innervate them.
sympathetic ganglia: Locus of primary sympathetic motor neurons. See paravertebral sympathetic chain and prevertebral ganglia.
synapse elimination: The developmental process by which the number of axons innervating some classes of target cells is diminished. Also called input elimination.
synapses: The junctions between neurons where information is passed from one to the other; typically refers to chemical synapses where a physical cleft exists between communicating neurons, but could also refer to electrical synapses mediated by gap junctions.
synapsin: A protein which reversibly binds to synaptic vesicles and is responsible for tethering these vesicles within a reserve pool.
synaptic cleft: The space that separates pre- and postsynaptic neurons at chemical synapses.
synaptic depression: A short-term decrease in synaptic strength resulting from the depletion of synaptic vesicles at active synapses.
synaptic facilitation: An increase in synaptic strength that occurs when two or more action potentials invade the presynaptic terminal within a few milliseconds of each other. Facilitation is typically caused by an increase in the amount of neurotransmitter released by a presynaptic action potential and lasts for a fraction of a second.
synaptic potentials: A membrane potential change (or a conductance change) generated by the action of a chemical transmitter agent. Synaptic potentials allow the transmission of information from one neuron to another. Compare receptor potentials.
synaptic transmissions: The chemical and electrical process by which the information encoded by action potentials is passed from a presynaptic (initiating) cell to a postsynaptic (target) cell.
synaptic vesicle cycle: Sequence of budding and fusion reactions that occurs in presynaptic terminals to maintain the supply of synaptic vesicles.
synaptic vesicles: Spherical, membrane-bound organelles in presynaptic terminals that store neurotransmitter molecules and associated molecular machinery that facilitates exocytosis.
synaptobrevin: A SNARE protein located in the membrane of synaptic vesicles. This protein forms a SNARE complex with syntaxin and SNAP-25 that mediates fusion of synaptic vesicles with the presynaptic plasma membrane.
synaptojanin: A protein involved in uncoating of synaptic vesicles. It works by modifying a vesicular lipid, which serves as a cue for vesicle uncoating by Hsc70.
synaptotagmins: A family of calcium-binding proteins found in the membrane of synaptic vesicles and elsewhere. Synaptotagmins 1 and 2 serve as the calcium sensors that trigger the rapid release of neurotransmitters.
synesthesia: A sensory anomaly in which individuals conflate experiences in one sensory domain with those in another.
syntaxin: A SNARE protein found primarily in the plasma membrane. This protein forms a SNARE complex with synaptobrevin and SNAP-25 that mediates fusion of synaptic vesicles with the presynaptic plasma membrane.

T

taste buds: Onion-shaped structures in the mouth and pharynx that contain taste cells.
taste cells: The sensory receptors cells that transduce water or lipid soluble molecules with chemical identities that are perceived as one of the five taste categories: sweet, sour, bitter, salty, and umami (savory, associated with amino acids in foods). Taste cells transduce these five categories of taste stimuli either via homomonomeric or dimeric G-protein coupled taste receptors or through ion-selective channels for Na⁺ (salty) and H⁺ (sour, which reflects acidity in food).
taste papillae: Multicellular protuberances on the tongue, along which taste buds are distributed.
tectorial membrane: The fibrous sheet overlying the apical surface of the cochlear hair cells; produces a shearing motion of the stereocilia when the basilar membrane is displaced.
tegmentum: A general term that refers to the central core of the brainstem.
telencephalon: The part of the brain derived from the anterior part of the embryonic forebrain vesicle; includes the cerebral hemispheres (cerebral cortex and cerebral nuclei).
temporal division: Referring to the region of the visual field of each eye in the direction of the temple.
temporal lobe: The hemispheric lobe that lies inferior to the lateral fissure.
testis-determining factor (TDF): The original name for the gene product of the SRY gene (see SRY).
testosterone: The principal androgen, synthesized in the testes and, in lesser amounts, in the ovaries and adrenal glands.
thalamus: A collection of nuclei that forms the major component of the dorsal diencephalon. Although its subdivisions and functions are many, a primary role of the thalamus is to interact with neural circuits in the cerebral cortex through reciprocal, topographically organized interconnections.
thermosensitive: Ion channels that respond to heat.
third ventricle: Narrow, slit-like ventricle derived from the lumen of the neural tube between the paired diencephalon, which form the lateral wall of the third ventricle.
thoracic: Intermediate region of the spinal cord related to the trunk and sympathetic outflow.
threshold potential: The level of membrane potential at which an action potential is generated.
tinnitus: A pathological condition characterized by spontaneous ringing or rushing noises, which can be either peripheral or central in origin.
tonotopy: The topographic mapping of sound frequency across the surface of a structure, which originates in the cochlea and is preserved in ascending auditory structures, including the auditory cortex.
topographic maps: Point-to-point correspondence between neighboring regions of the sensory periphery (e.g., the visual field or the body surface) and neighboring neurons within the central components of the system (e.g., in the brain and spinal cord).
totipotent: The diploid cells of the early vertebrate embryo that can give rise to all cell classes in the organism including the germ cells (gametes: egg/sperm) are considered to be totipotent.
tracrRNA: Small, trans-encoding RNA that combines with a specific guide RNA species to form an RNA duplex, which then acts to guide a bacterial excision/repair enzyme (endonuclease Cas9) to a genomic location targeted for excision. Following Following Cas9 excision, the DNA may be repaired by non-homologous end joining, yielding a microdeletion mutation; alternatively, a donor DNA sequence can be inserted following Cas9 cleavage via a mechanism similar to homologous recombination.
tracts: Bundles of fasciculated axons in the central nervous system that are gathered into compact structures and typically share a common origin and termination; more or less analogous to nerves in the periphery.
transcranial magnetic stimulation (TMS): Localized, noninvasive stimulation of cortical neurons through the induction of electrical current by the application of strong, focal magnetic fields.
transcription factors: See transcriptional activator proteins.
transcriptional activator proteins: DNA-binding proteins that attach near the start site of a gene, thereby activating transcription of the gene. Also called transcription factors.
transducin: G-protein involved in the phototransduction cascade.
transient receptor potential channel (TRP): The transient receptor potential (TRP) family of ion channels constitute approximately 28 individual genes and the proteins that they encode. All are transmembrane cation selective channels that mediate depolarization in response primarily to various sensory stimuli. These include taste stimuli that interact with the T1R/T2R GPCR taste receptors to activate the TRPM5 channel. There also TRP channels involved in transducing mechanical displacement/stretch across cellular membranes.
transit amplifying cell: A precursor cell, capable of rapid asymmetric divisions, descended from a stem cell. Transit amplifying cells can generate a large number of post-mitotic cells via a series of asymmetric divisions that yield one post-mitotic cell that goes on to differentiate and one precursor. These cells, however, are not self-renewing like stem cells—eventually, there is a terminal symmetric division in which both progeny cease to divide.
translational movements: Linear motion along the X, Y, and Z axes.
transverse: Standard anatomical planes of section that pass through the CNS orthogonal to its long axis.
transverse pontine fibers: Axons of pontine nuclei that cross the midline and form the middle cerebellar peduncles.
transverse sinuses: Dural venous sinuses that convey venous blood in the anterior direction from the confluence of sinuses at the back of the cranium to the sigmoid sinuses.
traveling wave: The sound-evoked propagation of motion from the basal toward the apical end of the basilar membrane.
trichromatic: Referring to the presence of three different cone types in the human retina, which generate the initial steps in color vision by differentially absorbing long, medium, and short wavelength light.
trigeminal brainstem complex: Nuclei of the brainstem that receive or give rise to sensory or motor axons in the trigeminal nerve; comprises the mesencephalic trigeminal nucleus in the midbrain and rostral pons, the principal (chief sensory) nucleus in the pons, the trigeminal motor nucleus in the pons, and the spinal trigeminal nucleus, which itself contains several subdivisions, in the caudal pons and medulla oblongata.
trigeminal (cranial nerve V) ganglion: Cranial nerve conveying somatosensory information from the face to the trigeminal nuclear complex of the brainstem; also conveys motor signals from the brainstem to the muscles of mastication.
trigeminal lemniscus: Axonal projection arising from the principal nucleus of the trigeminal nuclear complex of the brainstem and terminating in the contralateral ventral posterior medial nucleus of the thalamus; conveys mechanosensory signals derived from the face.
trigeminal nerve (V): See trigeminal (cranial nerve V) ganglion.
tripartite synapse: A three-way junction involving a presynaptic terminal, a postsynaptic process, and neighboring glia.
trochlear nerve (IV): Cranial nerve IV, an efferent motor nerve that controls the superior oblique muscle of the eye.
trophic interaction: Referring to the long-term interdependence between nerve cells and their targets.
tuned zero: Optic neurons that respond selectively to points that lie on the plane of fixation.
tuning curves: The function obtained when a neuron’s receptive field is tested with stimuli at different orientations; its peak defines the maximum sensitivity of the neuron in question.
two-point discrimination: Distance between caliper tips needed to distinguish one versus two points of stimulation.
tympanic membrane: The eardrum.
tyrosine kinase receptors (Trk): The receptor proteins for neurotrophins have extracellular domains with specific affinities for one or at most two of the neurotrophin ligands, and intracellular domains with enzymatic activity that can catalyze phosphorylation of a number of target proteins specifically on tyrosine amino acids in those target proteins. There are three neurotrophin tyrosine kinase receptors: TrkA (ligand:NGF), TrkB (ligands: BDNF, NT3, NT4/5), and TrkC (NT-3).

U

umami: The last of five basic tastes: umami is taste detected in response to amino acids in proteins like meat. Umami is also referred to as “savory” taste. It is transduced via the T1R class of G-protein- coupled taste receptors.
uncus: Medial protrusion of the anterior parahippocampal gyrus formed by cortical division of the amygdala.
undershoot: The final, hyperpolarizing phase of an action potential, typically caused by the voltage-dependent efflux of a cation such as K⁺.
upper motor neuron syndrome: Signs and symptoms that result from damage to descending motor pathways; these include weakness, spasticity, clonus, hyperactive reflexes, and a positive Babinski sign.
upper motor neurons: Neurons in the motor cortex or brainstem that give rise to descending projections that govern the activity of lower motor neurons in the brainstem and spinal cord.
utricle: The otolith organ that senses linear accelerations and head tilts in the horizontal plane.
uveal tract: A layer of eye tissue adjacent to the retina that includes three distinct but continuous structures: the choroid, the ciliary body, and the iris.

V

V1Rs: A sub-class of vomeronasal receptors that interact with the G-protein Gαi2 to transduce vomeronasal sensory stimuli.
V2Rs: A sub-class of vomeronasal receptors that interact with the G-protein Gαo to transduce vomeronasal signals.
V4: Region in the extrastriate cortex that contains a high percentage of neurons that respond selectively to color without regard to motion.
vagal crest: The region of the neural crest that arises from the posterior rhombencephalon, migrates into the visceral endoderm and eventually gives rise to the enteric nervous system as well as most of the parasympathetic ganglia.
vagus nerve (X): Cranial nerve X, a mixed sensorimotor nerve that conveys visceral afferents (taste from posterior oropharynx) and somatosensory afferents (mechanosensation from skin on or near pinna) to the brainstem, and branchial motor efferents from the brainstem to muscles of the larynx and pharynx and visceral motor efferents to widely distributed targets in the thorax and upper abdomen. Attaches to the rostral medulla in a cleft between the inferior olive and the inferior cerebellar peduncle, just inferior to the glossopharyngeal nerve.
valence: The degree of pleasantness of a stimulus.
vasocorona: A network of blood vessels on the lateral and ventrolateral margins of the spinal cord connecting circumferential branches of the posterior and anterior spinal arteries.
vection: The sense of self-motion created by visual flow.
ventral (anterior) corticospinal tract: Spinal portion of the corticospinal tract in the anterior–medial column of the spinal cord derived from the ipsilateral motor cortex; contributes to postural control.
ventral anterior nuclei: Nuclei in the ventral tier of the thalamus that receive input from the basal ganglia and cerebellum and project to the motor cortex.
ventral columns: The ventral (anterior) and ventrolateral (anterolateral) regions of spinal cord white matter that convey both ascending information about pain and temperature, and descending motor information from the brainstem and motor cortex concerned with postural control and gain adjustment. Also known as ventrolateral columns or anterolateral columns.
ventral horns: The ventral portion of the spinal cord gray matter derived from the basal plate; populated by interneurons and primary motor neurons.
ventral lateral nuclei: Nuclei in the ventral tier of the thalamus that receive input from the basal ganglia and cerebellum and project to the motor cortex.
ventral pallidum: A structure within the basal ganglia whose fibers project to thalamic nuclei, such as the mediodorsal nucleus.
ventral posterior complex: Group of thalamic nuclei that receive the somatosensory projections from the dorsal column nuclei and the trigeminal nuclear complex of the brainstem.
ventral posterior lateral nucleus (VPL): Component of the ventral posterior complex of thalamic nuclei that receives brainstem projections via the medial lemniscus carrying somatosensory information from the body (excluding the face), and gives rise to projections that pass through the posterior limb of internal capsule and terminate somatotopically in the dorsal two-thirds of the postcentral gyrus.
ventral posterior medial (VPM) nucleus: A component of the ventral posterior complex of thalamic nuclei that receives brainstem projections carrying somatic sensory information from the face including the inputs from the facial, glossopharyngeal and vagal nerve that innervate the taste buds in the tongue peripherally and the gustatory nucleus portion of the solitary nucleus in the brainstem.
ventral roots: The collection of nerve fibers containing motor axons that exit ventrally from the spinal cord and contribute the motor component of each segmental spinal nerve.
ventral tegmental area (VTA): A part of the midbrain that contains many dopaminergic neurons and is important for reward and learning.
ventricles: The spaces in the vertebrate brain that are filled with cerebrospinal fluid and represent the lumen of the embryonic neural tube.
ventrolateral columns: See ventral columns.
ventrolateral prefrontal cortex (VLPFC): A functional division of the prefrontal cortex roughly corresponding to the inferior frontal gyrus and surrounding sulci, as located anterior to motor cortex. Compare dorsolateral prefrontal cortex.
ventromedial prefrontal cortex (vmPFC): The ventral portion of the prefrontal cortex surrounding the hemispheric midline; plays a key role in the control of emotions and social behavior.
vergence movements: Disjunctive movements of the eyes (convergence or divergence) that align the fovea of each eye with targets located at different distances from the observer.
vermis: Median zone of the spinocerebellum that receives proprioceptive input from the spinal cord and sends output to brainstem upper motor neurons; coordinates movements of proximal muscles, including eye movements.
vertebral arteries: Major source of posterior circulation to hindbrain and posterior forebrain.
vertical gaze center: See rostral interstitial nucleus.
vestibular nerve ganglion: Contains the bipolar afferent neurons that innervate the semicircular canals and otolith organs of the auditory vestibule. Also called Scarpa’s ganglion.
vestibular nuclei: Clusters of neurons in the medulla that receive direct innervation from the vestibular nerve.
vestibulo-ocular movements: Involuntary movement of the eyes in response to displacement of the head; this reflex allows retinal images to remain stable during head movement.
vestibulo-ocular reflex (VOR): Involuntary movement of the eyes in response to displacement of the head. This reflex allows retinal images to remain stable while the head is moved.
vestibulocerebellum: Caudal-inferior lobes of the cerebellum, including the flocculus and nodulus, that receives input from the vestibular nuclei in the brainstem and the vestibular nerves; concerned with the vestibulo-ocular reflex and the coordination of movements that maintain posture and equilibrium.
vestibulocochlear nerve (VIII): Cranial nerve VIII, a sensory nerve that conveys vestibular afferents from the various components of the vestibular labyrinth and auditory afferents from the cochlea to the vestibular and cochlear nuclei, respectively, in the dorsolateral caudal pons and rostral medulla. Lateral-most of three cranial nerves that attaches to the brainstem at the junction of the pons and medulla.
visceral motor division: The components of the nervous system (peripheral and central) concerned with the regulation of smooth muscle, cardiac muscle, and glands; organized anatomically and physiologically into sympathetic, parasympathetic, and enteric divisions. Also known as the autonomic nervous system or autonomic motor division.
visceral motor nuclei: Nuclei (derived from the basal plate) in the brainstem and spinal cord that give rise to efferent fibers innervating smooth muscle, cardiac muscle or glands. In the brainstem, located in the dorsal tegmentum just lateral to somatic motor nuclei; in the spinal cord, located in the intermediolateral cell column of thoracic and sacral segments.
visceral motor system: See autonomic nervous system.
visual field: The area in the external world normally seen by one or both eyes (referred to, respectively, as the monocular and binocular fields).
vitreous humor: A gelatinous substance that fills the space between the back of the lens and the surface of the retina.
vocal folds: Source of vocal vibration in the larynx. Synonymous with vocal cords.
voltage clamp method: A technique that uses electronic feedback to simultaneously control the membrane potential of a cell and measure the transmembrane currents that result from the opening and closing of ion channels.
voltage gated: Term used to describe ion channels whose opening and closing is sensitive to membrane potential.
voltage sensor: Charged structure within a membrane-spanning domain of an ion channel that confers the ability to sense changes in transmembrane potential.
voltage-gated ion channels: Ion channels that are are opened or closed in response to changes in the transmembrane potential.
vomeronasal organs (VNO): A pair of chemical sensing organs in the septum (medial process) of the olfactory epithelium that are specialized for the detection and transduction of specific classes of volatile chemicals, pheromones and kairomones. The sensory neurons of the VNO are bipolar vomeronasal sensory receptor neurons that resemble olfactory receptor neurons in the olfactory epithelium. The VNO is the site of expression of a distinct family of GPCR chemosensory receptors that specifically bind pheromones and kairomones.
vomeronasal receptor neurons (VRNs): A class of bipolar chemosensory neurons found in the vomeronasal organ that uniquely express vomeronasal receptors, and whose axons project to the accessory olfactory bulb.
vomeronasal receptors (VRs): A large class of 7-transmembrane G-protein coupled receptor proteins (GPCRs) that bind and transduce phermonal and kairomonal signals. Vomeronasal receptor proteins are localized only to vomeronasal receptor neurons in the vomeronasal organ.
vomeronasal system: A specialized chemical detection system that detects pheromones—volatile chemicals released into the air from conspecifics to regulate social interactions—or kairomones—volatile chemicals from other species indicating predator, prey, or symbiotic status. The vomeronasal system includes the peripheral sensory organ, the vomeronasal organ adkacent to the olfactory epithelium in the nose, and its primary target in the forebrain, the accessory olfactory bulb.

W

Wada test: A procedure sometimes used as a diagnostic tool to determine the location of the speech and language cortex in preparation for neurosurgery. Involves carotid injection of an anesthetic agent.
Wernicke’s aphasia: Difficulty comprehending speech as a result of damage to Wernicke’s language area. Also called sensory or receptive aphasia.
Wernicke’s area: Region of cortex in the superior and posterior region of the left temporal lobe that helps mediate language comprehension. Named after the nineteenth-century neurologist Carl Wernicke.
white matter: A general term that refers to regions of the brain and spinal cord containing large axonal tracts; the phrase derives from the fact that axonal tracts have a whitish cast when viewed in the freshly cut material due to the abundance of myelin.
wide-dynamic-range neurons: Multimodal lamina V neurons that receive converging inputs from nociceptive and non-nociceptive afferents, a quality which makes them a likely substrate for referred pain.
Wisconsin Card Sorting Task: A cognitive test that involves classifying a set of cards, each showing one or more images of a simple shape, into categories based on rules that periodically change throughout the session.
Wnt: A large family of secreted ligands that regulate stem and precursor cell proliferation, transcriptional activation/repression, and differentiation within and beyond the nervous system.

Z

zonule fibers: Radially arranged connective tissue bands that hold the lens of the eye in place.