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Myelin is formed by oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system.Therefore, the first stage of myelinogenesis is often defined as the differentiation of oligodendrocyte progenitor cells (OPCs) or Schwann cell progenitors into their mature counterparts, [4] followed by myelin formation around axons.
Myelin was first described in 1854 by Rudolf Virchow, [17] although it was over a century later, following the development of electron microscopy, that its glial cell origin and its ultrastructure became apparent. [18] In vertebrates, not all axons are myelinated. For example, in the PNS, a large proportion of axons are unmyelinated.
His great service to science was his so-called method. This is based the fact that the time at which various systems of fibers making up the brain become covered with myelin varies. For instance, the sensory root fibers become myelinated before the motor; the upper branch of the vestibular nerve becomes myelinated before the lower.
The myelin sheath that surrounds and protects nerve cells is made by cells called oligodendrocytes. In a person with MS, these cells are lost, so damaged myelin sheaths cannot be repaired.
The maturation of myelination in intracortical layers coincides with critical period closure in mice, which has led to further research on the role of myelination on critical period duration. [ 37 ] Myelin is known to bind many different axonal growth inhibitors that prevent plasticity seen in critical periods. [ 38 ]
While in Japan, he studied vertebrate nerve fibers and discovered the insulating function of the myelin sheath, a material that speeds the conduction of nerve impulses.He also was the first to show that electrical impulses traveling along myelinated nerve cells actually "jump" between the breaks in the myelin wrapping, called nodes of Ranvier.
Experiments in Wallerian degeneration have shown that upon injury oligodendrocytes either undergo programmed cell death or enter a state of rest. Therefore, unlike Schwann cells, oligodendrocytes fail to clean up the myelin sheaths and their debris. In experiments conducted on rats, [18] myelin sheaths were found for up to 22 months. Therefore ...
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