The CENTERS of the NERVOUS SYSTEM —
functionally connected sets of neurons located in one or several structures of the central nervous system, providing implementation of complete reaction of an organism or regulation of this or that function.
The first attempts to localize in brain structures nek-ry functions of an organism, especially mental, belong to anatomists.
The ideas formulated by F. Gall at the end of 18 century about «think-tanks» almost for century defined approach to a problem of localization of functions in various structures of c. N of page (see the Cerebral cortex). In the subsequent fiziol. M. Fluran-s's experiments with destruction of big cerebral hemispheres at birds and a wedge, Zh. Buyo's observations brought a scientific basis under the idea about the centers of a nervous system. In the 70th
G. Th. Fritsch and E. Hitzig established 19 century that the electric irritation of front departments of a cerebral cortex of dogs causes reduction of certain groups of skeletal muscles. These experiments showed existence in a cerebral cortex of so-called motoriums that in the latest researches was confirmed in experiences on monkeys. Further experiments with irritation and destruction of structures of a brain, and also a wedge, overseeing by patients with local defeats of a nervous system
the CENTERS of the NERVOUS SYSTEM 171
completed the formation of ideas of existence of the centers of a nervous system. The analysis of integrative activity of a spinal cord which is carried out by Ch. Sherrington, destruction of microsites of a myelencephalon (K. Bernard, N. A experiences. Mislav-sky, F. V. Ovsyannikov) allowed to describe the center of a nervous system as the limited accumulation of nervous cells providing implementation of strictly certain function of an organism (breath, salivation, the movement, etc.). Development of the reflex theory (see), especially ideas of a reflex and a reflex arc (see the Reflex), formed a structural and functional basis for the analysis and interpretation of work of the centers of a nervous system. From these positions in a three-membered reflex arc transfer of vozbuzhdeniye from its afferent part on efferent is carried out in the corresponding nerve center. Irritating and destroying various sites of a head or spinal cord and observing at the same time disturbance or manifestation of various reflex acts, physiologists could localize in this or that structure of c. N of page centers of the corresponding functions or reflexes (e.g., tussive center, vasomotor center). Transition from acute experiments preferential to hron. to experiences on the whole organism led to emergence of ideas of the dynamic functional organization of the centers of a nervous system with involvement of structures at various levels of c. N of page. I. M. Sechenov, I. P. Pavlov, A. A. Ukhtomsky's researches proved the principles and mechanisms of merging of structures of c. N of page during the formation of complete behavioural acts of animals and the person. An example of the dynamic organization of processes in a nervous system are conditioned reflexes (see). In 1911 I. P. Pavlov formulated idea of the space organization of a nerve center in this connection it began to be considered not as anatomic and as functional education. The similar point of view was stated also by A. A. Ukhtomsky in his ideas of a dominant (see) as konstellyation of nerve centers, and existence of a dominant can change functional specifics of the centers. Ideas of the functional organization of nerve centers nevertheless were based on space communication between the specific structures including both back and a myelencephalon, and bark of big hemispheres with subcrustal educations. At the same time in the 30th the concept was widely adopted 20 century And. To a beta (1931) about existence of huge amorphous network of neurofibrilla, edges are provided by distribution of vozbuzhdeniye in a nervous system in any directions and, therefore, does not form specific nerve centers. Its experiences with cross-linking of nerves of different extremities showed ample opportunities of homogeneous neurofibrillar network to «reeducation». Similar views on homogeneity of brain structures were developed by Leshli (To. S. La-shley, 1929) in the ideas
of equipotentiality of bark of big hemispheres. Successful development of ideas of the dynamic organization of the centers of a nervous system was promoted by P. K. Anokhin's works, E. A. Asratyana, etc. Use of a technique of creation of a heterogeneous nervous anastomosis by P. K. Anokhin allowed to study dynamic reorganizations of nerve centers in the conditions of their communication with inadequate peripheral bodies. In the next decades ideas of the mezhneyronalny organization of nerve centers found reflection in works of a number as domestic (I. S. Beritashvili, M. N. Livanov, P. G. Kostiuk, etc.), and foreign (J. Ekkls, X. Megun, J. Moruzzi, etc.) scientists.
Uniform classification of the centers of a nervous system does not exist. On localization distinguish the cortical, subcrustal and spinal centers. In a brain diencephalic, mezentsefalny, bulbar, hypothalamic, thalamic allocate also the centers. More often nerve centers divide on functions (e.g., a vasomotor center, the center of heat generation, the secretory center) or on afferent perception (the centers of sight, hearing, sense of smell, etc.). Allocate also nerve centers, to-rye create motivational conditions of an organism (see Motivations), being pacemakers (see) motivational vozbuzhdeniye (the centers of hunger, thirst, saturation, etc.), and also nerve centers, to-rye on the basis of integration of vozbuzhdeniye carry out complete reactions of an organism (the centers of swallowing, sneezing, defecation, a respiratory center, the sexual center, etc.). Sometimes a nerve center the scientist, for the first time designates by last name it described (Brock's centers, Vernike, etc.).
At certain stages of embryonic and post-natal development of a nervous system in its centers between neurons continuous communications, architecture form to-rykh is defined by genetic programs of development. Broad acquisition by an organism of new skills, training in new forms of reaction is characteristic of early post-natal development that leads to formation of dynamic functional linkages between the nervous cells combining in the form of a nerve center. Complexity of its organization in many respects depends on complexity of adjustable functions. Bonds of nerve centers with the respective receptive fields are genetically caused. The elementary nerve center consists of several neurons forming a ganglion. At the highest animals and the person nerve centers include tens of thousands of nervous cells. At the same time fiziol. the concept about the center of a nervous system differs from structural idea of a kernel where close located neurons are characterized by existence of the general morphological features.
Mechanisms of mezhneyronalny interactions in nerve centers in many respects are defined by the general principles of work of a head and spinal cord. As functional association of neurons is carried out by means of mezhneyronalny synapses (see), properties of the last define specifics and features of integration of vozbuzhdeniye in the centers of a nervous system (see Excitement). Use in researches of a nervous system of microelectrode technics (see the Mikroelek-trodny method of a research) allowed to establish the nek-ry general principles in work of nerve centers. Among them the greatest value has the principle of convergence of heterogeneous vozbuzhdeniye to separate neurons. Registration of pulse activity of the separate nervous cells which are a part of a nerve center shows reorganization of their pulse flow at impacts on an organism of irritants of various touch modality. Similar convergence of vozbuzhdeniye received the name of multisen-weed and is observed practically in any nerve center. At the same time specifics of each center are defined by a complex of the afferent vozbuzhdeniye going from certain receptive zones. E.g., to neurons of a respiratory center (see) will converge excitement from mechanioreceptors of lungs, chemoceptors of vessels, receptors of a mucous membrane of respiratory tracts; to neurons of the hypothalamic centers of hunger and saturation converging excitement come from receptors went. - kish. path and chemoceptors of vessels. Can form a structural basis of convergence of heterogeneous vozbuzhdeniye to separate neurons of the center of a nervous system described in a brain of a sinapsoarkhitektonik, formed by the terminations of a large number of afferent axons on the same dendrite and akso-axonal synapses between terminalyam of axons. The similar organization of inter-neural bonds not only provides convergence of impulses in the centers of a nervous system, but also creates conditions for integrative activity of neurons. The phenomena of protorhenium, simplification, braking and occlusion can be result of interaction of the converging vozbuzhdeniye on neuron. Protoreniye consists in reduction of time of a synaptic delay in transfer of excitement at the expense of temporary summation of impulses, the following on an axon. The effect of simplification is shown when a series of impulses of excitement causes a condition of subthreshold excitement in the synoptic field of neuron, a cut in itself it is not enough for emergence on a postsynaptic membrane of action potential, but in the presence of the subsequent impul-sation coming on any other axons and reaching the same synoptic field becomes threshold and in neuron emergence of excitement is possible. In case of simultaneous arrival of various afferent vozbuzhdeniye to synoptic fields of several neurons total quantity of the excited cells decreases (occlusion) that is shown by decrease in functional activity of executive body. Reduction or complete cessation of functional activity of body is possible also as a result of development in a nerve center of processes of braking (see). Electronic microscopic examinations of the synoptic organization of a nerve center showed that the single large afferent termination contacts to a large number of dendrites of separate neurons. The similar ultrastructural organization can form a basis for the wide divergence of impulses of excitement leading to irradiation of excitement within the center. Irradiation is directed when excitement covers a certain group of neurons. Association on one neuron of synoptic entrances from many next cells creates conditions for animation (multiplication) of impulses of excitement on an axon, and the model of a neural trap offered Laurent de No (R. Lo-rente de N6) allows to understand a phenomenon of prolongation of vozbuzhdeniye in nerve centers. Similar functional linkages can ensure long functioning of effector neurons at trace amount of the afferent impulses coming to the center. Elektrofiziol. researches showed existence of a constant rate of impulses of excitement from a nerve center to effectors that provides a nek-swarm constant tonic excitement of the center. Mick-roelektrodnye researches of bioelectric activity of nervous cells allow to estimate a tone of a nerve center based on the ratio of number of the discharged and so-called silent cells. The tone of the center is provided not only the afferent impulses, but also humoral influences (hormones, metabolites, biologically active agents) arriving from peripheral receptors. Nek-ry nerve centers have property of the automaticity (see. Respiratory center). This property is connected not only with features of synoptic bonds between neurons, but also features of metabolism of the cells entering into structure of the center. One of philosophy of work of a nerve center established by A. A. Ukhtomsky is his ability to domination in activity of a nervous system (see the Dominant). In the dominant center of a nervous system neurons are characterized by a long hyperexcitability, against the background of a cut efficiency of temporary and spatial summation of vozbuzhdeniye increases. The dominating center of a nervous system, suppressing activity of other centers, can become predominating in formation of the purposeful behavioural act of animals and the person (see Motivations).
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Important property of nerve centers are their plasticity, i.e. ability to reorganization of the functional impacts on body depending on the changed requirements of the whole organism. Similar «reeducation» is possible at damage of various sites of a brain or to cases of need of compensation of the broken function on the periphery (see. Compensatory processes). Mechanisms of plastic reorganizations of nerve centers became clear after creation in an experiment at an animal heterogeneous anastomosis of nervous trunks. As showed experiences of pupils of P. K. Anokhin, a determinal factor of processes in the centers of a nervous system is change of quality of a flow of afferent impulses from the periphery. At the same time in the changed conditions the nerve center loses the specifics due to the lack of an adequate afferent flow of impulses from the periphery, at the same time exerting the trophic and regulating impact on peripheral body, changes the level of its functioning. Thus, in the central peripheral relations the major role belongs to the afferent flows of vozbuzhdeniye signaling about results of reorganization in work of body under the influence of activity of a nerve center. The determined consistent patterns formed a basis for development in 1935 by P. K. Anokhin to the theory of functional system of an organism (see. Functional systems). From system positions the nerve center represents the selective integration of structures of a brain of various levels of the organization necessary and sufficient for obtaining result, useful to an organism. So, in the functional system determining the level of nutrients, optimum for metabolism, the main and leading structure of the food center is the hypothalamic area, edges at interaction with a reticular formation, limbic structures and bark of big hemispheres carries out thin integration of vegetative functions of an organism (see. Food center). Merging of these structures is based not only on the afferent impulses arriving to them from interoceptors of an organism, but also on the afferent influences from the environment which are selected by a motivational condition of an organism (see. Afferent synthesis).
In comparative and physiological aspect development of the centers of a nervous system in invertebrates went from a setevidny nervous system to emergence of single-pole neurons. Such cell through the same shoot both receives, and transfers impulses of excitement. The single-pole neurons united in a ganglion can function as the elementary nerve centers. Transformation of single-pole neuron in bipolar, to-ry through dendrites perceives excitement, and transmits them through an axon, at vertebrata led to formation of a nerve center. Further evolution of nerve centers was carried out on the way of increase in amount of the integrated neurons and complication of the function regulated by the center, and also merging of structures of a brain of various level in the course of formation of functional systems of an organism.
For a research of the centers of a nervous system use the methods applied in neurophysiology (see). Registration in a nerve center of the caused bioelectric potential (see Bioelectric potential, the Potential caused) in response to irritation of receptors or the central structures allows to find out the nature of bonds of the centers with the periphery and among themselves. The Neyronalny organization is studied by means of a microelectrode method of a research. Quite often electro-fiziol. methods of a research are combined with histochemical, biochemical or radiological methods of the analysis of neyronalny structures.
See also Brain, the Nervous system, the Spinal cord.
Bibliography: Asratyane. A. Fiziolo
Giya of the central nervous system, page 247, M., 1953; Fundamentals of physiology of functional systems, under the editorship of K. V. Sudakov, M., 1983; P and in l about in I. P. Complete works, t. 3, book 1, page 147, M. — JI., 1951; The Problem of the center and the periphery in физиологии^ nervous activity, under the editorship of P. Anokhin, Gorky, 1935; Private physiology of a nervous system, under the editorship of P. G. Kostiuk, etc., JI., 1983; Sherrington Ch. S. The integrative action of the nervous system, N. Y., 1906; Sperry E, W. Problem of central nervous reorganization after nerve regeneration and muscle transposition, Quart. Rev. Biol., v. 20, p. 311, 1945. Yu. A. Fadeyev.