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| Myelin Sheath
Myelin in the central nervous system is produced by oligodendrocytes. The myclin sheath is formed by the spiral wrapping of the cytoplasmic processes of the myelinating cells around axons, and the subsequent extrusion of the cytoplasm leads to a compact, tightly spiraled, multi layered envelope. Electron microscopy reveals the characteristic structure of the myelin sheath: major dense lines alternate with thinner intraperiod lines to form the repeating units. The major dense line results from the fusion of the thicker, inner leaflet of the oligodendrocytic plasma membrane, whereas the intraperiod line is formed by the apposition of the thinner, outer leaflet of this membrane. Unlike Schwann cells ( the myelinating cells of the peripheral nervous system which are concerned with the myelination of a single axon ), each oligodendrocyte may provide myelin for many axons. From this it follows that the motion by which the sheath is produced by the myelinating cell may be different in the central and peripheral nervous Systems. Schwann cells can, in principle, lay down myelin by rotating around a single axon, the oligodendrocyte, being in connection with many axons, cannot perform a revolving motion. Consequently, ensheathment takes place by the progressive lengthening of the oligodendrocytic process which encircles the axon completely and an internal mesaxon is formed by apposition of the free edges of the myclinating process. The outer most lamella of the sheath encloses the outer tongue and this, together with the internal mesaxon, represents the tenuous connections which remain between the fully formed myclin and the myelinating cell.
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