chemoreception

chemoreception

Sensory process by which organisms respond to external chemical stimuli, by employing specialized cells (chemoreceptors) that convert the stimuli directly or indirectly into nerve impulses. Chemoreceptors that are components of sensory neurons that directly convert chemical stimuli are called primary receptors. Secondary receptors are not part of neurons but respond to stimulation by inducing activity in an adjacent neuron. Most mammals possess two classes of chemoreceptors: the primary receptors involved in smell and located in the epithelium of the nasal cavity and the secondary receptors involved in taste and located in the tongue's taste buds. Aquatic animals and terrestrial species with mucus-secreting skins typically possess chemoreceptors all over the body. For many animals, chemoreception is the most important means of receiving information about their surrounding environment. Chemoreception plays roles in finding appropriate food and in reproductive behaviour (see pheromones). It serves additional purposes in some animals, such as distinguishing members of the same community from outsiders.

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chemoreception [‚kē·mō·ri′sep·shən]

(physiology)

Reception of a chemical stimulus by an organism.

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Chemoreception

The ability of organisms to detect changes in the chemical composition of their exterior or interior environment. It is a characteristic of every living cell, from the single-celled bacteria and protozoa to the most complex multicellular organisms. Chemoreception allows organisms to maintain homeostasis, react to stimuli, and communicate with one another. See Homeostasis

At the single-cell level, bacteria orient toward or avoid certain chemical stimuli (chemotaxis); algal gametes release attractants which allow sperm to find oocytes in a dilute aqueous environment; and unicellular slime molds are drawn together to form colonial fruiting bodies by use of aggregation pheromones. See Cellular adhesion, Taxis

In multicellular organisms, both single cells and complex multicellular sense organs are used to homeostatically maintain body fluids (interoreceptors) as well as to monitor the external environment (exteroreceptors). The best-studied interoreceptors are perhaps the carotid body chemoreceptors of higher vertebrates, which monitor the levels of oxygen, carbon dioxide, and hydrogen ions in arterial blood. The best-studied exteroreceptors are those associated with taste (gustation) and smell (olfaction). Internal communication is also effected by chemical means in multicellular organisms. Thus both hormonal and neural control involve the perception, by cells, of control chemicals (hormones and neurotransmitters, respectively). See Chemical senses, Olfaction, Sense organ, Taste

The basic mechanism underlying chemoreception is the interaction of a chemical stimulus with receptor molecules in the outer membrane of a cell. These molecules are believed to be proteins which, because of their three-dimensional shapes and chemical properties, will have the right spatial and binding “fit” for interaction with only a select group of chemicals (the same basic mechanism by which enzymes are specific for various substrates). The interaction between a chemical stimulus and a receptor molecule ultimately leads to structural changes in membrane channels. The net result is usually a change in membrane conductance (permeability) to specific ions which changes both the internal chemical composition of the cell and the charge distribution across the cell membrane. In single-celled organisms, this may be sufficient to establish a membrane current which may elicit responses such as an increase or decrease in ciliary movement. In multicellular organisms, it usually results in changes in the rate of release of hormones or the stimulation of neurons. See Cell membranes

The basic characteristics of all chemoreceptors are specificity (the chemicals that they will respond to); sensitivity (the magnitude of the response for a given chemical stimulus); and range of perception (the smallest or largest level of stimulus that the receptor can discriminate). Specificity is a consequence of the types of proteins found in the membrane of a receptor cell. Each cell will have a mosaic of different receptor molecules, and each receptor molecule will show different combinations of excitatory or inhibitory responses to different molecules. In an excitatory response, there is a net flux of positive ions into the cell (depolarization); for an inhibitory response, there is a net flux of negative ions into the cell (hyperpolarization). The stronger the stimulus—that is, the more of the chemical present—the more receptors affected, the greater the change in conductance, and the larger the membrane current. In animals with nervous systems, these changes in conductance of primary sensory cells can lead to one of two events. In some receptors, if the current is excitatory and sufficient in magnitude (threshold), an action potential will be generated at a spike-initiating zone on the neuron. Other receptors respond by releasing a neurotransmitter that acts on a second-order neuron which is excitable and therefore can generate action potentials. See Biopotentials and ionic currents

The sensitivity of a chemoreceptor reflects both the amount of chemical substance required to initiate a change in membrane potential or discharge of the receptor cell, and the change in potential or discharge for any given change in the level of the chemical stimulus. There are real limits as to the extent of change in membrane conductance or firing frequency. Thus, for more sensitive cells, there is a smaller range over which they can provide information about the change in concentration of any given chemical before it has reached its maximum conductance or discharge rate and has saturated.

In animals, the responsiveness of some chemoreceptors can be either enhanced or attenuated by other neural input. These influences come in the form of efferent inputs from the central nervous system, from neighboring receptors, or even from recurrent branches of the chemoreceptor's own sensory axons. The net effect is either (1) to increase the acuity of the receptors (excitatory input brings the membrane potential of the receptor cell closer to threshold, requiring less chemical stimulus to elicit a response); or (2) to extend the range of responsiveness of the receptors (inhibitory input lowers the membrane potential of the receptor cell, requiring more chemical stimulus to bring the cell to threshold). For example, chemical sensitivity is greatly heightened in most animals when they are hungry.

Any given chemoreceptor cell can have any combination of receptor proteins, each of which may respond to different chemical molecules. Thus, chemoreceptor cells do not exhibit a unitary specificity to a single chemical substance, but rather an action spectrum to various groups of chemicals. The ability of animals to distinguish such a large number of different, complex, natural chemical stimuli resides in the ability of higher centers in the nervous system to “recognize” the pattern of discharge of large groups of cells. Sensory quality does not depend on the activation of a particular cell or group of cells but on the interaction of cells with overlapping response spectra.

Despite the common, basic mechanism underlying chemoreception in all organisms, there is a great diversity in the design of multicellular chemoreceptive organs, particularly in animals. The complex structures of most of these organs reflect adaptations that serve to filter and amplify chemical signals. Thus, the antennae in many insects, and the irrigated protective chambers, such as the olfactory bulb of fishes and nasal passages of mammals, increase the exposure of chemoreceptor cells to the environment. At the same time, they allow the diffusion distances between chemoreceptive cells and the environment to be reduced, thereby increasing acuity. In terms of filtering, they may serve to convert turbulent or dispersed stimuli into temporal patterns that can be more easily interpreted. The extent to which such structural adaptations are seen in various organisms tends to reflect the relative importance of chemoreception to the organism, which, to a large extend, reflects the habitat in which the organism lives. See Chemical ecology