Myelinization

MYELINIZATION, the process of myelin deposition on nerve fibers during the period of organism development (see the separate plate, figures 1–3). Myelinization begins in the embryo at the 5th month of intrauterine life; different parts of the brain become myelinated not simultaneously, but in a known regular order. Fiber systems having the same complexity of function become myelinated simultaneously; the more complex the function of a given system, the later its fibers are coated with myelin. The deposition of myelin serves as an indication that the fiber has become active. At the child's birth, myelinization is far from complete: while some parts of the brain are already fully myelinated and ready for function, others have not yet finished their development and cannot serve for either physical or mental functions. In the newborn child, the spinal cord is very rich in myelinated fibers; fibers not coated with myelin are found only in its inner parts and in the region of the pyramidal tract. The fibers of the brainstem and cerebellum are covered with a significant amount of myelin sheath. Of the subcortical ganglia, the fibers of the globi pallidi are already myelinated, whereas the fibers of the caudate nucleus and putamen are covered with myelin only by the 5th–6th month of extrauterine life. The cerebral hemispheres in many of their parts lack myelin and have a grayish appearance on section: in the normal newborn child, centripetal (sensory) fibers, part of the pyramidal tracts, part of the olfactory, auditory, and optic pathways and centers, and separate areas in the corona radiata are supplied with myelin; however, the greater part of the parietal, frontal, temporal, and occipital lobes, as well as the cerebral commissures, still lack myelin. Association systems destined for higher mental functions are coated with myelin later than other systems, as a result of which the cortical zones of projection centers and fibers remain isolated and unconnected with one another; during this period, all sensations received by the child from the outside remain isolated, all its movements are reflex and appear only as a result of external or internal stimuli. Gradually, the development of myelin sheaths occurs in all parts of the brain, whereby a connection is established between various centers and, in connection with this, the child's intellect develops: the child begins to recognize objects and understand their meaning. The myelinization of the main systems of the hemisphere ends in the eighth month of extrauterine life, and from this moment it continues only in individual fibers for many more years (according to some data, the outer layers of the cerebral cortex finally become myelinated only by the 45th year of life and perhaps even later). Depending on the time of appearance of myelin in the cerebral hemispheres, Flechsig divides them into different regions: those parts where the fibers become covered with myelin early he calls early regions (Primordialgebiete), while those in which myelin appears later he calls late regions (Spätgebiete). On the basis of these researches, Flechsig distinguishes two kinds of centers in the cerebral cortex: some are connected by projection tracts with underlying formations—these are the projection centers; others, having no connection with the underlying parts of the brain, but connected by association fibers with the projection centers of the cortex, are the association centers (see Brain, vol. VII, pp. 533–534). In the study of the brain, myelinization is utilized as a method—the myelinogenetic method or Flechsig's method.