The aim of the investigation was to study changes of some parameters of nervous fibers in the ontogenesis. It was determined and established that the increase of nerve length myelin segments protract, that corresponds to the length internodal segments with increase of length and body weight of animal. The length of myelin segment is accompanied by increase of caliber of nervous fiber but these parameters do not have proportional dependence. Thus length of myelin segment in the nervous fibers of large caliber was relatively shorter, that it is determined by the coefficient of g. This tendency is present during all period of postnatal ontogenesis. It is proved, that the thickness of myelin sheath is determined by the coefficient of g, thus the nervous fibers of young rats had relatively the less thickness of myelin shell in axon of less diameter by comparison to adult rats. The changes of thickness of myelin sheath correspond to the increase of sizes of myelin segment. It agrees with the results of previous researches indicative on lengthening of myelin segment with the increase of caliber of nervous fibers in default of a similar conformity to the law in regard to the thickness of their myelin sheath. 50 white rats of seven age groups from new-born to adult age were involved in the research. During histological analysis it was measured length and middle diameter of myelin segment and it was calculated on results measurings on either side from the kernel of neyrolemmocitis. Measurings were conducted on the direct areas nervous fibers, in which both internal and external surfaces of myelin sheath come to light as equidistant and parallel segments proper to the generally accepted thickness myelin sheath and long to a few of mcm. During ultrathin cuts it was measured the thickness of myelin sheath between the internal and external surfaces of myelin sheath and it was determined the index of form as factor of disparity and deviation from the ideal form of circle and counted all information for the arc measurings based on measurings of circumference of object. Statistical treatment was produced by the methods of not parametric statistics. It was established that on histograms is multiplied length of nervous fibers and myelin segments with age, proportional to length of body of animal and it was halted at completion of growth of animals, that specifies on the permanent amount of myelin segments during all life and makes on the average 140 myelin segments on one myelin fiber. Thus, diameter of nervous fiber and length of myelin segment was studied during all postnatal ontogenesis of in the same accordance. Dispersion of length of myelin segments is reflected however much a function as the clusters of fibers of certain caliber has dependence of type of linear regress, that reflects ontogenetic adaptation of an organism to corresponding age, increase of correlation of length and caliber of myelin nervous fibers and refutes existence of linear regression of geometrical proportions of myelin segment. Progressive diminishment of myelin segment with the size of diameter of myelin nervous fibers from new-born to the adult organism is proved, that depends on variation of caliber of nervous fiber. It is multiplied the diameter of myelin sheath accordingly to increase of caliber of axons at minimum variation of information accordingly different age periods of ontogenesis and is explained by a tendency to gradual diminishment of coefficient of correlation accordingly in every age group of animals. Thus, thickness of myelin sheath in the nervous fibers of large diameter is less in young animals, than in mature ones. It is shown that with the increase of thickness of myelin fibers length of myelin segment is relatively shorter, that is determined by the coefficient of “g” and is reserved during all period of postnatal ontogenesis. The thickness of myelin sheath is determined by the coefficient of “g”, therefore in young rats the less diameter of myelin sheath in axons of fewer diameters is determined relatively.

Keywords: nervous fibers, myelin sheath, myelin segments, postnatal ontogenesis, rats

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