1. Sensory transduction is the transmission of a sensory nervous impulse. A receptor potential is a change in membrane potential that occurs in a sensory receptor. In most sensory receptors (except photoreceptors), the receptor potential exists as the ability of both sodium and potassium ions to pass through voltage-gated ion channels.
2. Muscle spindle receptors respond to stretching in muscles; Golgi tendon organs respond to tension in tendons when they are being stretched.
3. Otoliths in the saccule and utricle of the inner ear respond to changes in body position relative to gravity. The semicircular canals in the inner ear detect changes in angular acceleration. There are three to detect changes in all three dimensions perpendicular to one another. This works as long cilia of sensory cells move against short cilia in the same direction as fluid moves, altering firing of neurons of short cilia. The lateral line of fishes functions the same way: cilia are deflected by the flow of water over them, depolarizing the sensory membrane and sending a signal to the brain.
4. Aortic and carotid bodies detect changes in concentration of blood gases. The receptors are particularly sensitive to carbon dioxide levels. Based on sensory information from these receptors, respiration rate and depth is regulated.
5. Sound waves vibrate the tympanic membrane; the hammer, anvil, and stirrup vibrate correspondingly; the stirrup vibrates the oval window. The sense of hearing actually takes place in the basilar membrane of the cochlea. Recognition of individual frequencies is accomplished as the basilar membrane, which is differentially flexible, vibrates in a portion according to the frequency of the sound wave, stimulating hair cells in that region.
6. The lens focuses light rays. Amphibians and fishes use muscles to move the lens position relative to the retina, whereas other vertebrates use muscles to alter the shape of the lens. The iris controls the amount of light entering the eye.
7. Rods contain rhodopsin and cones contain photopsin; both contain cis-retinal as part of the molecule. When a photoreceptor absorbs light, a chain reaction occurs that is ultimately responsible for slamming shut the sodium channels, at a rate of about 1000/second. This causes the membrane to hyperpolarize.
8. Rods and cones are not evenly distributed because most cones are at the central region of the retina, especially at the point of best focus, the fovea. A point-to-point image is formed at foveal cone cells. At this point each cone cell is connected to a single visual ganglion cell with an axon in the optic nerve; outside the fovea many receptor cells are connected to a single ganglion cell. This latter region provides information about movement and boundaries.
9. Two eyes provide images at slightly different angles, providing sensitive depth perception. Predators have eyes facing forward on the head, whereas prey have eyes located to the sides of the head. Predators have better stereoscopic vision, whereas prey have better vision over a broad receptive field.
10. Fish have developed a sense of electrical perception, but terrestrial animals have not because air does not readily conduct electricity but water does.