CHEMORECEPTION

process by which organisms respond to chemical stimuli. The process begins when chemical stimuli come in contact with chemoreceptors, specialized cells in the body that convert (transduce) the immediate effects of such substances directly or indirectly into nerve impulses. A nerve cell (neuron) that makes a direct conversion is called a primary receptor; a cell that is not a neuron but that responds to stimulation by inducing activity in an adjacent nerve cell is called a secondary receptor. the process by which all organisms respond to chemical stimuli. The physiological mechanisms of chemoreception and their role in patterns of adaptation to the environment have been little studied except in humans, a few other mammalian species, and some groups of insects. The precise mode in which chemoreceptive tissues respond to stimuli is at present only partially understood. In more complex animals, neural responses are produced by direct contact with specialized nerve cells, called primary receptors, or indirectly through the stimulation of neurons by nearby cells, referred to as secondary receptors. In nearly all species two different chemoreceptive functions are controlled by these correspondingly distinct types of receptor cells. In humans and most other mammals the olfactory response (smell) is initiated by primary receptors in the nasal lining that react to stimuli in gaseous form, while the gustatory response (taste) is channeled through secondary receptors in the tongue called taste buds. The sensation of flavour, a term often employed as a synonym for taste, is really a response to a combination of different stimuli, including texture, temperature, taste, and smell and is common to all mammals. Distinct olfactory and gustatory receptors are also found in fishes and many invertebrates, though in these organisms they are often referred to as distance and contact chemoreceptor cells, respectively. Mucous membranes and mucosal epiderma in land animals and the external layers of aquatic species also contain nerve endings linked directly to the spinal system that are highly responsive to chemical stimuli. One of the principal adaptive functions of chemoreception is the location and selection of foods. The particular odours of potential food sources may draw organisms toward them (attractants) or evoke an avoidance response (repellents) or may stimulate feeding behaviour (phagostimulants) or inhibit it (antifeedants). Some of these chemical agents in plants and animals are different substances from those that supply nutrients to the organisms that feed on them. Other chemical stimulants produced by many animals, called pheromones, are important regulators of social behaviour. They are often crucial among local biological populations in identifying members and excluding outsiders, and they frequently serve to establish territorial boundaries. Odorous substances secreted when there is danger may help to alert the community, as among bees. In addition to contributing to communication and identification, pheromones are also important for organisms in locating specific regions for mating or habitation, especially in certain fishes such as salmon, and in the regulation of reproductive behaviour. Chemical secretions in insects and most mammals serve to attract mates, to distinguish potential mates of the same species from those of other species, and to initiate mating patterns. In many aquatic species, such as mollusks and coelenterates, reproductive activity is induced in colonies by chemical stimulants released by one or more members into the surrounding water. Even the unicellular protozoa react to reproductive chemical stimuli (in species that propagate by conjugation). They may locate food sources by responding to levels of carbon dioxide concentration, as do some species of roundworms (Nematoda). In many crustaceans, specialized chemoreceptive organs called esthetascs, located near the antennae and the smaller antennules, are responsive to the sea-mineral content of their habitat. In addition to these distance chemoreceptive organs, contact chemoreception in crustaceans can take place in a number of regions along the legs, tail, and gills; some fish, similarly, have chemoreceptors along their tails and fins. Chemoreceptors in insects are found primarily on the antennae, but among the other organs that serve this function are small external structures called hair sensilla. These receive chemical stimuli through micropores in their surface layer that contain minute branches of neurons located at their base. Another specialized chemoreceptive structure, Jacobson's organ-particularly active in lizards and snakes but not found in birds, fish, and some mammals, including humans-establishes a direct link between the front of the mouth region and the olfactory portion of the brain. Among birds, taste buds and well-developed olfactory brain centres indicate a need for reassessment of their chemoreceptive faculties, which until recently were thought to be deficient and entirely subordinate to organs of visual and aural perception. Olfactory receptors in vertebrates consist of basal, supporting, and receptor cells, which terminate in minute cilia. It has been found that there are synaptic connections among olfactory receptors that allow for coordination and flexibility in responses among a number of sensory cells. Though the exact structure of the vertebrate taste buds is not known, current theories suggest that they are made up of four or five different types of cells; one type, the sensory cell, is believed to be replaced frequently at regular intervals. It is no longer thought that gustatory reception in vertebrates can be described in terms of four primary taste characteristics-sweet, sour, salty, and bitter-with distinct types of taste buds that respond specifically to one of these and not to others. Rather, electrophysiological evidence indicates that although some cells are especially responsive to specific types of stimulus, they also respond to other taste stimuli to varying degrees as well. None of the several theories that have been proposed to explain the chemical mechanisms of gustatory and olfactory reception have as yet been verified, and the factors that trigger sensory responses and enable cells to discriminate among different stimuli have not yet been conclusively determined. Additional reading J.E. Amoore, Molecular Basis of Odor (1970), a technical discussion of molecular shapes and odours; M. Beroza (ed.), Chemicals Controlling Insect Behavior (1970), technical reports at a symposium on pheromones and defensive secretions of insects; T.H. Bullock and G.A. Horridge, Structure and Function in the Nervous Systems of Invertebrates, 2 vol. (1965), a monumental review of invertebrate sensory physiology and neurophysiology, with extensive bibliographies; V.G. Dethier, The Physiology of Insect Senses (1963), a technical review, with sections on chemoreception; H. Frings and M. Frings, Animal Communication (1964), a semipopular survey, including sections on chemical signaling in the animal kingdom; R. Harper, E.C. Bate Smith, and D.G. Land, Odour Description and Odour Classification (1968), a technical review of odour theory and practical schemes of classification; T. Hayashi (ed.), Olfaction and Taste II (1967), technical reports at a symposium on vertebrate chemoreception, especially electrophysiological and electron microscope studies, and discussion of theories; J.W. Johnston, D.G. Moulton, and A. Turk (eds.), "Communication by Chemical Signals," Advances in Chemoreception, vol. 1 (1970), a technical discussion of the field; M.R. Kare and O. Maller (eds.), The Chemical Senses and Nutrition (1967), technical reports at a symposium, mostly on human chemoreception, with an extensive bibliography on taste for the years 1566-1966; W.W. Kilgore and R.L. Doutt (eds.), Pest Control: Biological, Physical, and Selected Chemical Methods (1967), technical reviews by specialists, including chapters on pheromones, repellents, and antifeedants; H. Kleerekoper, Olfaction in Fishes (1969), a semipopular review especially on orientation by odours; L. Milne and M. Milne, The Senses of Animals and Men (1962), a popular survey of senses and behaviour; R.W. Moncrieff, The Chemical Senses, 3rd ed. (1967), a standard technical reference on chemoreception in vertebrates, particularly humans; G.H. Parker, Smell, Taste, and Allied Senses in the Vertebrates (1922), a classic summary of earlier research and theories; H.W. Schultz, E.A. Day, and L.M. Libbey (eds.), Symposium on Foods: The Chemistry and Physiology of Flavors (1967), technical reports at a symposium, particularly on chemical analysis for odorants in foods; see Scientific American for excellent semipopular articles on many aspects of chemoreception (February 1964, August 1964, June 1967, May 1968, and February 1969); T.A. Sebeok (ed.), Animal Communication (1968), technical reviews by specialists, with chapters on chemical signaling; E. Sondheimer and J.B. Simeone (eds.), Chemical Ecology (1970), technical reviews by specialists on effects of environmental chemicals on animals, including chapters on plant feeding stimulants, communication signals, defense chemicals, and fish orientation; T.H. Waterman (ed.), The Physiology of Crustacea, vol. 2 (1961), technical reviews by specialists, including chapters on senses and behaviour; V.B. Wigglesworth, The Principles of Insect Physiology, 6th ed. (1965), a standard textbook in the field, including a chapter on chemoreception; K.M. Wilbur and C.M. Yonge (eds.), The Physiology of Mollusca, vol. 2 (1966), technical reviews by specialists, including chapters on chemoreception and behaviour; G.E.W. Wolstenholme and J. Knight (eds.), Taste and Smell in Vertebrates (1970), technical reports at a symposium, particularly on morphology of receptors, electrophysiology, and theories; D.L. Wood, R.M. Silverstein, and M. Nakajima (eds.), Control of Insect Behavior by Natural Products (1970), technical reports at a symposium particularly concerned with methods of research on feeding stimulants, deterrents, and pheromones; R.H. Wright, The Science of Smell (1964), a semitechnical discussion of odour theories, particularly the molecular vibration theory; Y. Zotterman (ed.), Olfaction and Taste (1963), technical reports at a symposium, particularly on morphology, electrophysiology, and theories. Later works include H. Acker and R.G. O'Regan (eds.), Physiology of the Peripheral Arterial Chemoreceptors (1983); Dietland Mller-Schwarze and Robert M. Silverstein (eds.), Chemical Signals in Vertebrates: Proceedings of the Third International Symposium (1983); D. Michael Stoddart (ed.), Olfaction in Mammals: Proceedings of a Symposium of the Zoological Society of London (1980); A.D. Hasler, A.T. Scholz, and R.W. Goy, Olfactory Imprinting and Homing in Salmon (1983); Klaus Reutter, Taste Organ in the Bullhead (Teleostei) (1978); R.H. Wright, The Sense of Smell (1982). Hubert William Frings