Chapter 14 Summary

Summary

Sensory receptor cells respond to stimulation by a form of energy. Most sensory cells are specialized to respond to one form of stimulus energy.
Sense organs contain clusters of similar receptor cells as well as nonneural cells.
Receptor cells transduce stimulus energy into an electrical response, usually a depolarizing receptor potential. The transduction depends on specific receptor molecules and can be ionotropic (directly opening ion channels) or metabotropic (triggering a metabolic cascade via a G protein–coupled receptor, or GPCR).
The receptor potential in a sensory neuron can trigger action potentials that propagate to the CNS.
Sensory receptor cells often have cilia or microvilli that increase the area of the membrane surface.

Mechanoreceptors have many sensory functions. In addition to surface mechanoreceptors that convey information about environmental touch and pressure, mechanoreceptors can serve as proprioceptors that monitor body and limb position and muscle length and force. (They can also serve as equilibrium and auditory receptors, as we will see in the next section.)
Mechanoreceptors have stretch-activated ion channels that mediate ionotropic transduction.
Many sensory receptors produce a response that diminishes over time and are said to adapt to sustained stimulation. Tonic (slowly adapting) receptors signal the intensity and duration of a stimulus, whereas phasic (rapidly adapting) receptors signal changes in stimulus intensity.

Hair cells are sensitive and versatile vertebrate mechanoreceptors that transduce displacement of stereocilia into a receptor potential. They are the major receptors of vertebrate hearing and equilibrium sense.
The structure of the vertebrate ear effectively conveys sound-pressure waves into the inner ear. Sounds of different frequencies stimulate hair cells at different locations along the length of the basilar membrane of the cochlea.
Central auditory pathways of vertebrates sort coded information about sounds in order to discriminate and map different sound frequencies and locations. The auditory systems of insects, although less complex, can nonetheless provide them with behaviorally important information.

Most animals possess two types of chemoreceptors for external stimuli: contact or taste chemoreceptors that respond to near-field chemicals at relatively high concentrations, and distance or olfactory chemoreceptors that respond to low concentrations of chemicals from sources over a larger area. This generalization is useful but oversimplifies a greater diversity of external chemical senses, as well as internal chemoreceptors involved in homeostatic regulation.
Taste chemoreceptors of mammals monitor five taste qualities: sweet, sour, salty, bitter, and umami. Insects have taste sensilla that provide at least analogous information.
Transduction mechanisms of chemoreceptors are diverse, both within an animal and across animal phyla. Taste sensory transduction in mammals may involve ionotropic activation of ion channels (salty, sour) or G protein–coupled receptors (sweet, bitter, umami).

Olfactory chemoreceptors of the main olfactory epithelium of vertebrates are neuronal receptor cells with cilia that contain intramembrane receptor proteins. Each receptor cell expresses the gene for one of these membrane receptor proteins, and all the receptor neurons that express that same protein synapse in the same glomerulus of the olfactory bulb. Insect olfactory neurons have broadly similar connection patterns but unrelated receptor proteins.
Vertebrate olfactory receptor proteins are G protein–coupled receptors, which stimulate production of a second messenger, cAMP.
The vomeronasal organ of vertebrates is an accessory olfactory organ that senses pheromonal and other stimuli. Vomeronasal sensory cells are microvillar rather than ciliary, and express GPCR proteins that stimulate production of IP₃ and DAG.

The vertebrate eye is a camera eye that focuses light onto retinal rod and cone photoreceptors. Rods and cones are unusual in that light produces a hyperpolarizing receptor potential.
The photopigment rhodopsin is a GPCR molecule conjugated to retinal. It is contained in membranes of outer segments of vertebrate rods and cones. When rhodopsin absorbs light, it acts via a G protein to decrease the concentration of cGMP in the cytoplasm, leading to closing of cGMP-gated Na⁺ channels that keep the photoreceptor depolarized in the dark. Light-induced closure of these channels hyperpolarizes the photoreceptors.
In arthropods such as Drosophila the photopigment rhodopsin is similar to that of vertebrates and activates a similar G protein, but it is linked to a different intracellular effector and leads to the production of DAG and IP₃, opening ion channels and producing a depolarizing receptor potential.
Rhodopsin is deactivated and ultimately regenerated to 11-cis rhodopsin after activation. In vertebrates, most regeneration is a slow enzymatic process, part of which occurs outside the photoreceptors in the adjacent pigment epithelium.

Neural circuits of the vertebrate retina integrate the responses of retinal photoreceptors to excite and inhibit retinal ganglion cells. Ganglion cell receptive fields may be excited or inhibited by light at the center of the field, whereas light in the surround antagonizes the effect of light in the center.
Straight-through pathways (photoreceptor → bipolar cell → ganglion cell) produce the center (on- or off-center) of a ganglion cell’s receptive field. Lateral pathways through horizontal cells and amacrine cells produce the antagonistic surround.
Axons of ganglion cells make up the optic nerve, relaying visual information to several brain areas. The geniculostriate pathway projects to the lateral geniculate nucleus (LGN) and from there to the primary visual cortex.
Simple and complex cells in the primary visual cortex respond to light or dark bars or edges oriented at particular angles.
Parallel pathways in the visual cortex convey information about different aspects of a visual stimulus, such as details of visual form, movement, color, and binocular determination of object distance.
Color vision depends on the ratio of activation of three classes of cone photoreceptors sensitive to different wavelengths of light. Retinal circuitry integrates color contrasts based on red–green and blue–yellow opponencies.