The capability of detecting chemical substances and of monitoring their concentration. The distinction between the gustation and olfaction cannot be made in many animals. Chemoreceptors occur both externally and internally. Strictly, every nerve cell acts as a chemoreceptor in that it reacts specifically to substances released by other nerve cells, but the term is usually reserved for those specialized sensory cells that are designed to detect chemical substances in the environment (exteroceptors) and in the body fluids (interoceptors).
The mechanisms of chemoreception involve the recognition of specific molecules by receptor sites on cell membranes. Whether this recognition occurs on the basis of chemical action, molecular shape, or both, is not fully understood. Thus we do not know what sugar and saccharine have in common that makes them both taste sweet to blowflies, rats, monkeys, and humans. Both taste and smell depend upon chemoreceptors. In the traditional sense, smell is concerned with the detection of low concentrations of airborne substances, while taste results from direct contact with relatively high concentrations of chemical substances. In both cases, however, the chemicals are presented to the receptor in solution, and the distinction is difficult to justify in some animals, especially those that live in water. Nevertheless, in many animals there is a neurological distinction, in that some nerves are concerned with relaying olfactory messages, and others gustatory messages. In the blowfly (Phormia regina), for example, chemoreceptors in the antennae detect small quantities of airborne substances, and chemoreceptors in the tarsi (feet) are capable of detecting salt, sugar, and pure water. In vertebrates, the sense of taste is relayed via the facial (VII) and glossopharyngeal (IX) cranial nerves, while the sense of smell is transmitted by the olfactory nerve (I).
Subjects: Zoology and Animal Sciences.