METAMERISM (grech, meta for, following, after + meros a part) — one of patterns of a structure of a body inherent to an animal with bilateral (bilateral) symmetry consisting in a partition (segmentation) of a body of nek-ry animals on a number of sites (segments, metameres) repeating consistently along a longitudinal axis.
The metameric arrangement and repetition of bodies in segments for a longitudinal axis corresponds to the nature of the active movement inherent to bilateral animals, i.e. progress in the direction of a longitudinal axis of a body forward. The animal leading the attached life or swimming in suspension (coelenterates, erinaceouses), svoystven other type of symmetry — radial, at Krom the repeating parts of a body (anti-measure) are located around a longitudinal axis on radiuses. The vast majority of animals, including and all the highest (arthropod, chordate), possess a bilateralism with more or less accurately expressed to M. The progressive differentiation of the head end of a body is inherent to this animals.
A classical example of M. is the partition of a body of annlides (Annelides). It consists of several tens joints, in to-rykh internals repeat as outside (bristles, parapodiya, gills), and (secretory, circulatory, nervous systems, and in the forefront of a body — generative organs). Such M. is called gomonomny. At arthropods gomonomny M. is replaced by geteronomny: somites are not identical on the structure and function (outgrowths of one form short moustaches, jaws, others — extremities, appendages of generative organs, etc.), often several next segments grow together and form complexes (the head, a breast, an abdomen). A peculiar form M. is the strobilization at flat worms — lentets, providing distribution of eggs at falling away of mature joints from the general strobiles.
The body of vertebrate animals has the metameric structure which is most expressed at the lowest (fish) and remaining at mammals including the person. Disturbance of a metamerism in the course of evolution of vertebrata is connected with isolation of the head, extremities and shift of internals. The functional tasks of front department of a trunk connected with perception and processing of information from the outside world caused the shift and uneven development of a number of front metameres where distantny and contact receptors, initial departments of digestive and respiratory paths, the leading department of c developed. N of page — a brain. The exit of vertebrata to the land and the necessity of movement on solid substrate caused the shift and uneven development of chest and lumbar metameres which led to development of extremities.
Studying of embryonic development of the person allows to establish precisely a metameric origin of all structures and body organs. At the adult the metameric structure fullestly remained in structure of a backbone, intercostal muscles and that is especially important, in a segmented structure of a spinal cord, an arrangement of spinal nodes and nodes of century of N of page. Patterns of an innervation of not metameric parts of a body of the person, napr, extremities, directly show their origin from certain metameres. Knowledge of patterns — is metameric go structures of somatic and vegetative parts of the nervous system — has big a wedge, value. Between a sensitive innervation and segments of a spinal cord there is very exact ratio (dermatomeriya) allowing to judge on the basis of disorders of sensitivity of certain sites of skin the level of damage of a spinal cord.
The sympathetic innervation of internals also has the metameric centers in side horns of segments of a spinal cord. Exactly here impulses on afferent fibers from various internals arrive and here the efferent sympathetic fibers which are carrying out centrifugal impulses to smooth muscles and glands of internals originate.
Clinical value of a metamerism
Clinical value of a metamerism is especially essential in topical diagnosis of nek-ry damages of a spinal cord, peripheral nerves, partly a brain trunk. A metamere of a spinal cord consider the site it along a long axis consisting of a back and front horn with two couples of back (sensitive) and ventral (motive) roots. Back roots represent axons of nervous cells of the spinal nodes having strictly metameric structure. Long dendrites of these neurons participate in creation of textures and peripheral nerves, to-rye contain also motive fibers — axons of cells of front horns. Set of all these bunches of nerve fibrils is called a neuromere. The muscles innervated by motive cells of one segment of a spinal cord form a myomere. Thus, each segment of a spinal cord is related to a certain group of muscles. On this basis at detection at sick sluggish atrophic paresis with the corresponding structure of affected muscles it is possible to establish to a topic of defeat (see. Spinal cord ). Damage of cells of front horns and ventral roots differs from defeat of textures and peripheral nerves, at to-rykh other groups of muscles suffer and, besides, disorders of sensitivity join. Defeats of motor-neurons of front horns meet at a number of diseases (poliomyelitis, tick-borne encephalitis, Dyushenna a disease, etc.). In recent years apply to differentiation of segmented and neural defeat electromyography (see).
Between a sensitive innervation of skin and segments of a spinal cord there is also an exact ratio. On distribution of disorder of sensitivity both at its loss, and at increase it is possible to differentiate segmented defeat from neural, to define what segment, the root or a nerve is struck. The same treats also sensitivity of face skin. The kernel of a spinal way of a trifacial is a homolog of back horns. At defeat of this kernel of disorder of sensitivity on a face have segmented character - so-called zones of Zelder (see. Trifacial ).
It should be noted that the zones of a metameric innervation of skin specified on schemes are connected not only with one segment of a spinal cord. Usually each dermatomere or a myomere is innervated from the basic and from two next — above - and underlying segments. E.g., the umbilical line belongs to the Th10 level, however anesthesia at this level arises when defeat covers not only a Th10 segment of a spinal cord, but partially and Th9 and Th11. The specified pattern is of great importance at topical diagnosis of a tumor of a spinal cord, discogenic radiculitis, especially when the issue of need of surgical treatment is resolved.
These M. matter also in diagnosis of diseases of internals. Impulsation of pain from the struck body through sympathetic cells of side horns of a spinal cord can extend to somatic conductors of painful sensitivity. So-called synalgias in the corresponding dermatomeres and scleromeasures result (see. Zakharyina — Geda of a zone ).
Bibliography: Beklemishev V. N. Bases of a comparative anatomy of invertebrates, t. 1, page 176, M., 1964; B and N of of River. The management to topical diagnosis of diseases of a head and spinal cord, the lane with it., M. — L., 1929; D about of e of l V. A. Oligomerization of homologous bodies as one of highways of evolution of animals, D., 1954; X about d about with X. G. Nervous diseases, page 19, M., 1974; Brain W. R. Brain’s diseases of the nervous system, p. 30, Oxford, 1977; Traite de zoologie (anato-mie, systhematique, biologie), publ. par P. P. Grasse, t. 16, P., 1971.
B. H. Pavlova; D. K. Bogorodinsky (not BP.).