Lewis/Life - Review of Key Concepts

Review of Key Concepts - Chapter 32

Sensory receptors detect, transduce, and amplify stimuli, allowing an animal to perceive its environment. Senses are adaptive. Sensory receptors may form sense organs.
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. If a stimulus remains constant, a sensory receptor ceases to respond (sensory adaptation).
Sensory receptors include chemoreceptors, photoreceptors, and mechanoreceptors.
Invertebrates have chemoreceptors to detect food and perform other functions. Invertebrate photoreceptors contain pigments 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 ocelli to compound eyes to complex lens systems. Invertebrate hearing depends on setae and tympanal organs, and balance and equilibrium depend on statolith crystals within statocysts.
Vertebrate sensory systems are similar in structure and function in different species, but they may be located in different parts of the body. Some species have senses humans do not have.
Humans perceive smell when odorant molecules bind receptors in the olfactory epithelium of the nasal passages. The brain perceives a smell by evaluating the pattern of olfactory receptor cells that bind odorant molecules. Humans perceive taste when chemicals stimulate receptors within taste buds. Humans perceive vision with a complex lens system in the eye.
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 the amount of light entering the eye.
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 optic nerve. This nerve carries the neural messages to the brain for interpretation. Light activates rhodopsin in a rod cell in a way that alters ion permeability of the cell membrane, generating action potentials. Three types of cones each contain a pigment that maximally absorbs light of a particular wavelength. The brain interprets the ratio of the activities of the three cone types as a color.
Mechanoreceptors bend in response to sound, body movement, or touch. In human hearing, sound enters the auditory canal, vibrating in the tympanic membrane. These vibrations are transmitted through the middle ear and amplified by three bones, the malleus, incus, and stapes. The movements of these bones change 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.
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.
Pacinian corpuscles, Meissner's corpuscles, and free nerve endings are mechanoreceptors that detect touch. Pacinian corpuscles respond to intense pressure, Meissner's corpuscles to gentle pressure, and free nerve endings to touch, pressure, and pain.