means by which animals (animal) detect and respond to stimuli in their internal and external environments. The senses of animals are most usefully described in terms of the kind of physical energy, or modality, involved. There are four main modalities: the light senses ( photoreception; i.e., vision), the mechanical senses ( mechanoreception; i.e., touch, balance, and hearing), the chemical senses ( chemoreception; i.e., taste and smell), and the electric sense (electroreception) of certain fish. In addition to these external senses there are also internal senses that use the same modalities. For example, there are proprioceptors, which are mechanical sensors that measure the lengths of muscles (muscle) and positions of joints (joint), and there are interoceptors that monitor blood pressure. Similarly, there are chemoreceptors that monitor blood CO₂ ( carbon dioxide) and pH levels, as well as a variety of receptors (receptor) that respond at the cellular level to the presence of hormones (hormone) and metabolites.

vision is used by animals to determine the layout of their surroundings, and thus this sense is particularly important for locomotion. In animals with eyes (eye, human) that have good resolution, vision can be used to identify objects from their geometric appearance; however, this requires a sophisticated brain of the kind found in vertebrates (vertebrate), cephalopod mollusks such as octopus, and higher arthropods (arthropod), such as bees (bee) and jumping spiders (jumping spider). All vision, or photoreception, relies on photoreceptors that contain a special light-detecting molecule known as rhodopsin. Rhodopsin detects electromagnetic radiation—light with wavelengths (wavelength) in the range 400–700 nanometres (1 nm = 10⁻⁹m). There are some animals that can detect infrared radiation (wavelengths greater than 700 nm); for example, some snakes (snake) use infrared radiation to locate warm-blooded (warm-bloodedness) prey, and certain beetles (beetle) can use it to sense forest fires (forest fire). However, animals that detect wavelengths in the infrared do this with receptors that sense heat or mechanical expansion, rather than with photoreceptors.

Many invertebrates have the capacity to see and analyze polarized light. polarization arises from atmospheric scattering and reflection at smooth surfaces such as water. In polarized light all the photons have their electrical fields vibrating in the same plane; this can be detected by photoreceptors if the molecules are appropriately aligned. The projecting microvillus structure of invertebrate receptors makes this possible. Many insects (insect) use polarization to work out the Sun's direction when the sky is overcast, and others use it to detect water surfaces.

There are a great many varieties of mechanical receptors in animals, but best known are the receptors that mediate touch (touch reception), the variety of hair cell receptors in vertebrates that mediate hearing (the acoustico-lateralis system), and the muscle spindle proprioceptors that monitor the state of muscle contraction. The basic mechanism by which a stimulus is converted to an electrical signal in cells is known as transduction. An example of mechanical transduction, worked out in studies of fruit fly receptors, consists of channels in the membrane that are triggered to open by stretch, which allows cations (cation) to enter the cell.

In all vertebrates there is a type of mechanically sensitive cell known as a hair cell. The outer surface of these cells contains an array of tiny hairlike processes, including a kinocilium (not present in mammals), which has a typical internal fibre skeleton, and stereocilia, which do not have fibre skeletons. Stereocilia decrease in size with distance from the kinocilium and are functionally polarized. When the stereocilia are bent toward the kinocilium, the hair cell is excited, and the nerve fibre that contacts the cell fires action potentials. In contrast, bending the hairs away from the kinocilium inhibits firing.

Hair cells have many uses. In fishes (fish) the cells are part of the lateral line system, a series of canals in the skin that are open to the surrounding water and that are used to monitor water currents caused by the fish itself and by other fish. The canals are equipped at intervals with clusters of hair cells, each with a jellylike cap known as a cupula. The cupula is displaced by water movement, thus bending the hairs beneath it, resulting in activity in the nerve. In the inner ear of higher vertebrates there are three variants of this basic design, responsible for detecting the direction of gravity, angular rotation, and sound waves (sound). In the utricle and saccule of the inner ear there are patches of hair cells known as maculae. Within each maculae, the stereocilia are embedded in a gelatinous mass known as the otolithic membrane, which contains small stonelike calcium carbonate particles called otoconia. The otolithic membrane and otoconia bend the hairs in the direction of gravity, providing the animal with a vertical reference direction; similar organs of balance, known as statocysts, are common in invertebrates. Also in the inner ear of vertebrates are the three semicircular canals. Each consists of an almost circular tube, with a bulge at one point containing a cluster of hair cells with a gelatinous cupula attached. When the head rotates, the fluid in the tube lags behind the tissue surrounding it. This displaces the cupula, which causes the stereocilia to bend, providing a signal that is proportional to the rate of head rotation in the plane of the stimulated canal. One of the main functions of the semicircular canals is to drive the vestibulo-ocular reflex, which enables the eyes to counter-rotate and maintain a steady gaze when the head turns.

In mammals the ear (ear, human) consists of the outer sound-collecting pinna (auricle); the middle ear, which contains ossicles that function to match the mechanics of sound in air to sound in water; and the inner ear, which contains the cochlea. The cochlea is a complex coiled structure. It consists of a long membrane, known as the basilar membrane, which is tuned in such a way that high tones vibrate the region near the base and low tones vibrate the region near the apex. Sitting on the basilar membrane is the organ of Corti, an array of hair cells with stereocilia that contact a gelatinous membrane called the tectorial membrane. Sound entering the inner ear stimulates different regions of the basilar membrane, depending on sound frequency. Hair cells in the stimulated regions are excited by the resulting shearing action between the stereocilia and the tectorial membrane. There are two kinds of hair cells in the organ of Corti. The inner hair cells are sensory, and the nerves extending from them send acoustic information to the brain. In contrast, the outer hair cells are motile and have a role in amplifying and modifying the movement of the basilar membrane.

The external chemical senses are usually divided into taste, or gustation (for dissolved chemicals that inform about the palatability of food), and smell, or olfaction (for airborne chemicals that inform about events at a distance). The sense of taste in humans is confined to the mouth region, especially the tongue. In contrast, catfish have taste buds covering their whole body surface. There are five accepted Aristotelian sub-modalities of taste—salt, acid, sweet, bitter, and savory (umami)—that are segregated to some extent in different regions of the mouth. Each has a different transduction mechanism. Salt receptors simply respond to the increase in sodium ions entering them. In acid receptors, the H⁺ ions inactivate potassium channels, resulting in an increase in excitation of the cell. Sweet, bitter, and savory receptors have special proteins in the membrane that detect appropriate molecules. When stimulated, these proteins set off a chain of biochemical events that lead to the production of an action potential. Taste mechanisms require relatively high concentrations; for example, salt and sweet tastes have thresholds of around 0.01 molar. However, bitter tastes, whose function is to prevent organisms from eating toxic substances, have lower thresholds; quinine is detectable in a concentration of only 8 micromolar.

Two families of fish, the mormyrids (mormyrid) of Africa and the gymnotids of South America, have independently developed a unique sense for the detection of objects in their surroundings and for communication. These fish usually inhabit murky rivers, such as the Amazon (Amazon River) or the Nile (Nile River), where vision is impossible. They have an organ in the tail, derived from nerve or muscle, that sends weak electrical discharges into the surrounding water. They also have an array of receptors, derived from lateral line organs and situated over the front part of the body, that detect the electric field produced by the tail organs. Objects in the surroundings of the fish distort this field, and the changes are detected and interpreted in terms of the locations and electrical properties of the objects. This makes navigation possible over a range of a few metres. The sense is also used in both aggressive and sexual communication. Other fish such as sharks (shark) have electroreceptors but no electric organs, and they use electroreception in a passive sense to detect the electric fields that result from the neuromuscular activity of buried prey.

Useful introductory accounts of sensory function include Fred Delcomyn, Foundations of Neurobiology (1997); and Robert F. Schmidt and Helmut Altner (eds.), Fundamentals of Sensory Physiology, 3rd ed. (1986). Coverage of sensory physiology, with a bias toward the human senses, is provided by C.U.M. Smith, Biology of Sensory Systems, 2nd ed. (2008). An interesting account of the senses in terms of information gathering is given in David B. Dusenbury, Sensory Ecology (1992). A six-volume reference, with chapters by different authors, that provides extensive information on various facets of sensory neuroscience is A.I. Basbaum et al., The Senses: A Comprehensive Reference (2008).Michael Land