neuron(redirected from peripheral motor neuron)
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(or nerve cell), the basic structural and functional unit of the nervous system.
A neuron receives signals that enter from receptors and other neurons and processes and transmits the signals in the form of nerve impulses to the effector nerve endings, which control the activities of the corresponding organs of response (muscles, gland cells, or other neurons). Neurons differentiate from neuroblasts, which arise in the neurula stage of embryonic development.
In the process of differentiation a neuron develops specialized structures to ensure the performance of the various neuronal functions. Branched outgrowths, or dendrites, are specialized to receive information; these structures have a receptive membrane and are sensitive to specific physiological stimuli. The excitatory and inhibitory processes that are localized in the receptive membrane accumulate and act on the stimulus region, the most excitable area of the surface membrane of the neuron; this serves as the origin for the spreading bioelectric potentials. The longest outgrowth, the axon (or axis cylinder), is covered by an electrically excitable conducting membrane that serves to transmit the potentials. Having reached the terminal sections of the axon, the nerve impulse excites the secretory membrane; as a result of this, a physiologically active substance, either a mediator substance (chemical transmitter) or a neurohormone, is secreted from the nerve endings.
In addition to structures associated with the performance of specific functions, a neuron has a nucleus (as do all living cells) that, together with the perinuclear cytoplasm, forms the cell body, or perikaryon. It is here that synthesis of macromolecules takes place. Some of these are transported along the axoplasm (the cytoplasm in the axon) to the nerve endings.
The structure, dimensions, and shape of neurons vary widely. Neurons of the cerebral cortex, cerebellum, and some other areas of the central nervous system have complex structures. Multipolar neurons are characteristic of the brain of vertebrates. In such neurons, several dendrites and one axon emerge from the cell-body; the initial section of the axon serves as the excitatory region. Numerous nerve endings from the outgrowths of other neurons converge on the cell body and dendrites of a multipolar neuron. The ganglia of invertebrates usually consist of unipolar neurons; the cell body only fulfills a trophic function and is connected with the axon at the axon hillock. It would appear that such a neuron does not necessarily have true dendrites, and reception of synaptic signals is effected by specialized areas on the surface of the axon. Neurons with two outgrowths are called bipolar; they occur most often as peripheral sensory neurons having one axon and one dendrite, which impinges on the cell surface.
Neurons are classified according to their position in a reflex arc: afferent, or sensory, neurons receive information from the external environment or from receptor cells; interneurons, or internuncial neurons, connect one neuron with another; efferent neurons transmit impulses to the organs of response (for example, motoneurons innervate muscles).
Neurons are also classified according to their chemical specificity, that is, according to the nature of the physiologically active substance that is secreted by the nerve endings of a given neuron. For example, a cholinergic neuron secretes acetylcholine and an adrenergic neuron secretes adrenaline. The number of neurons present in a nervous system determines the variety and complexity of functions that an organism can perform; for example, there are 102 neurons in the Rotatoria and more than 1010 in man.
REFERENCESEccles, G. Fiziologiia nervnykh kletok. Moscow, 1959. (Translated from English.)
Hyden, H. “Neiron.” (Translated from English.) In the collection Funktsional’naia morfologiia kletki. Moscow, 1963.
Mekhanizmy deiatel’nosti tsentral’nogo neirona. Moscow-Leningrad, 1966.
Nervnaia kletka: sb. st. Edited by N. V. Golikov. Leningrad, 1966.
D. A. SAKHAROV