7. Chemoreceptors enable invertebrates to detect food, escape danger, and communicate.
8. In humans, olfaction occurs when odorant molecules bind to receptors the olfactory epithelium of the nasal passages. The brain perceives a smell by evaluating the pattern of olfactory receptor cells that bind to odorant molecules.
10. Invertebrate photoreceptors contain pigments (usually rhodopsin),associated with membranes. Light stimulation alters the pigment and changes the charge across the membrane, which may generate an action potential. Visual systems range from simple eye cups to compound eyes composed of ommatidia, to lens systems.
11. Humans perceive vision with complex lens system. The human eye contains three layers. The outer layer, the sclera, protects. It forms the transparent cornea in the front of the eyeball. The next layer, the choroid coat, is pigmented, located toward the rear of the eye, and absorbs light. In the front of the eye, the choroid coat forms the ciliary body, which controls the shape of the lens that focuses light on the photoreceptors, and the opaque iris. The pupil constricts or dilates to adjust to the amount of light entering the eye.
12. The innermost eye layer is the multilayered retina. Beneath a pigment layer lie photoreceptors: rods for black-and-white vision in dim light and cones for color vision in brighter light. These cells synapse with bipolar cells that form the middle retinal layer. The bipolar cells, in turn, synapse with ganglion cells whose fibers leave the retina as the theoptic nerve, which carries the neural messages to the brain for interpretation. Light activates rhodopsin, altering retinal, which ultimately alters ion permeability of the receptor cell membrane. Three types of cones each contain a pigment that maximally absorbs light of a range of particular wavelengths. The brain interprets the ratio of the activities of the three cone types as a color.
13. Mechanoreceptors bend in response to sound, to movement, or touch. Invertebrate hearing depends on setae and tympanal organs, and balance and equilibrium depend on statolith crystals within statocysts.
14. In human hearing, sound enters the auditory canal, vibrating the tympanic membrane. These vibrations are transmitted through the middle ear and amplified by three bones, the malleus, incus, and stapes. The movement of these bones changes the pressure in fluid within the cochlea, which, in turn, vibrates the basilar membrane. As the basilar membrane moves, it pushes hair cells against the tectorial membrane, which signals the brain to perceive the pitch of the sound through the location of the moving hair cells. The brain determines the sound's loudness from the frequency of action potentials and the number of stimulated hair cells.
15. The semicircular canals and the vestibule in the inner ear sense body position and movement. Fluid movement within these areas stimulates sensory hair cells, and the brain interprets this information, providing a sense of equilibrium.
16. Pacinian corpuscles, Meissner's corpuscles, and free nerve endings are skin mechanoreceptors. Pacinian corpuscles respond to vibration; Meissner's corpuscles to gentle pressure; and free nerve endings to touch, hot, cold, and pain.