|32.1 Sensory Systems Are Adaptive
1. Sensory receptors detect, transduce, and
amplify stimuli, allowing and animal to perceive its environment.
2. Natural selection has molded sensory systems.
32.2 Sensory Systems, Although Diverse, Operative by the Same General Principles
3. A sensory receptor selectively responds to a single form of energy and converts it to receptor potentials, which change membrane potential in proportion to stimulus strength.
4. Sensory receptors amplify stimuli.
5. If a stimulus remains constant, a sensory
receptor ceases to respond (sensory adaptation).
6. Sensory receptors are classified by the type of energy they detect: chemicals, light, or mechanical energy.
7. Invertebrates have chemoreceptors 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 ororant molecules.
9. Humans perceive taste (gustation) when chemicals
stimulate receptors within taste buds.
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. Invertebrate hearing depends on setae and
tympanal organs, and balance and equilibrium depend on
statolith crystals within statocysts.
14. Mechanoreceptors bend in response to sound, to
movement, or touch. 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 throughout 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, Meissners corpuscles, and free nerve endings are mechanoreceptors that detect touch. Pacinian corpuscle respond to intense pressure, Meissner's corpuscles to gentle pressure, and free nerve endings
to touch, pressure, and pain.|