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the aggregation of nerve cells, or neurons, in the nervous system that continuously participate in the execution of reflexes and in the regulation of a particular function or some aspect of that function. The neurons that make up a nerve center are for the most part strictly localized.
The simplest nerve center consists of a few neurons that constitute an isolated ganglion. For example, in some crustaceans the cardiac ganglion, which comprises only nine neurons, controls the heartbeat. In highly organized animals nerve centers are part of the central nervous system (CNS), and may consist of thousands and even millions of neurons.
Each nerve center receives information by way of nerve fibers from the sense organs or from other nerve centers. This input is in the form of nerve impulses. The information is analyzed by those nerve center neurons whose outgrowths—or axons—do not extend beyond the center. The final link is made by the neurons whose outgrowths do leave the nerve center; command impulses from the center are delivered along these efferent pathways to the peripheral organs or to other nerve centers. The neurons that constitute a nerve center are linked together by excitatory and inhibitory synapses to form nerve nets (not to be confused with the term “nerve net” as applied to invertebrates). In addition to consisting of neurons that are excited only in response to incoming nerve signals or to the action of various serum chemical stimuli, a nerve center may also consist of pacemaker neurons, which are automatic. Pacemaker neurons have the capacity to generate periodic nerve impulses.
The concept of the nerve center implies that the various functions of the body are regulated by different segments of the nervous system. The location of a nerve center is experimentally determined by stimulation or destruction of certain parts of the brain and spinal cord. Ablation studies and studies involving discrete lesions are also used. If stimulation of a certain segment of the CNS elicits a physiological reaction or reflex that disappears when the affected portion of the CNS is removed or destroyed, then it is safe to assume that the experimenter is dealing with a nerve center that is actively involved in a particular physiological reaction or reflex.
The concept of localization of function in the nervous system is not universally agreed upon; at best, it is accepted with reservations. The objections are based on two lines of experimental evidence. The first underscores the plasticity of certain portions of the nervous system. It is this capacity for functional reconstruction that compensates, for example, for the loss of the medulla. The second line of evidence proves that structures that are located in different segments of the nervous system are interconnected and may affect the performance of a single function. This latter line of evidence has led some physiologists to completely reject the localization of function and others to broaden the concept of the nerve center to include all the structures that affect the performance of a given function. Modern neurophysiology has reconciled this difference of opinion by invoking the idea of a functional hierarchy in a nerve center. According to this conceptualization, the separate components of a single function are controlled by nerve centers located at different levels of the nervous system. The coordinated activity of the nerve centers that constitute this hierarchical system guarantees the uniform performance of a complex function. This is the basis for any given function’s adaptive character.
The concept of nerve center dominance, one of the most important principles underlying the activity of a nerve center, was formulated during the years 1911 to 1923 by A. A. Ukhtomskii.
REFERENCESObshchaia i chastnaia fiziologiia nervnoi sistemy. Leningrad, 1969.
Fiziologiia cheloveka, 2nd ed. Edited by E. B. Babskii. Moscow, 1972.
D. A. SAKHAROV