In addition to possessing the classical senses of vision, hearing, touch, smell, and taste, humans respond to a variety of other stimuli. For example, the vestibular apparatus of the internal ear provides continuous feedback about linear and angular accelerations and allows us to maintain an upright posture. Pressure receptors in our vascular systems and osmoreceptors in our brains help maintain a constant blood volume. Other species employ still more exotic sensory modalities. Many migratory species, especially birds, can orient themselves by reference to the earth's magnetic field. Pit vipers and boas use thermal imaging to strike warm mammalian prey; bats and cetaceans employ sonar to locate respectively insects and fishes.
Despite the wide variety of physical stimuli to which various senses are responsive, there are consistent principles in their operation. A sensory response commences with an antenna, an apparatus for accumulating sensory energy and concentrating it at the receptor cells. In the key step of sensory transduction, the physical stimulus engenders an electrical response that represents the magnitude, duration, and other properties of the input. Most responses are then amplified to assure that their sizes exceed those of any noise sources. Many sensory systems use tuning to enhance responsiveness to stimuli at behaviorally important frequencies and to suppress those at less significant frequencies. In some instances, a sensory receptor bears an axon along which information flows into the central nervous system; in other cases the receptor must forward information across a chemical synapse to excite the nerve fiber. In either event the final step in a peripheral sensory response is the encoding of relevant information in the firing pattern of an afferent nerve fiber.
