Role Of Myelin

Myelin provides insulation for axons and is necessary for saltatory conduction. It is composed of tightly wrapped lipid bilayers with specialized protein constituents. Peripheral nervous system (PNS) myelin is formed by the extension of Schwann cells, and central nervous system (CNS) myelin is produced by oligodendrocytes. As the cell processes wrap around the axon, cytoplasm is extruded, leaving the inner (cytoplasmic) and outer (extracellular) plasma membrane leaflets in close apposition. Electron microscopy reveals several dozen spirals containing dense lines. The darker, major dense line is formed by the apposition of the inner leaflets, and the lighter, intraperiod line is formed by the close approximation of outer leaflets (Fig. 48-1 (Figure Not Available) ). '5 The myelin coating is interrupted at regular intervals (nodes of Ranvier) where the axon membrane with its concentration of voltage-gated sodium channels is exposed to the extracellular environment.

CNS and PNS myelin differ in a number of important ways. Schwann cells myelinate only one internodal segment from a single PNS axon, whereas oligodendrocytes myelinate multiple CNS axons. The proteins also differ. Proteolipid protein (PLP) accounts for approximately 50 percent of the CNS myelin proteins. PLP is an integral membrane protein responsible for holding the outer

_TABLE 48-1 -- PRIMARY (IDIOPATHIC) INFLAMMATORY DEMYELINATING DISORDERS OF THE CENTRAL NERVOUS SYSTEM_

Acute disseminated encephalomyelitis

Monophasic

Multiphasic

Relapsing (controversial) Monosymptomatic syndromes Optic neuritis

Acute transverse myelitis (partial and complete)

Brain stem demyelination

Multiple sclerosis

Neuromyelitis optica

Marburg's disease

Schilder's myeloclastic diffuse sclerosis (controversial) Balo's concentric sclerosis

Figure 48-1 (Figure Not Available) Major constituent components of CNS myeli(Modified from Raine CS: Morphological aspects of myelin and myelination. In Morrell P [ed]: Myelin. New York, Plenum Press, 1984, p 26.)

leaflets together at the intraperiod line. Mutations in this highly conserved protein cause Pelizaeus-Merzbacher disease. Protein zero (P0) is the major PNS myelin protein and performs a function similar to PLP in compacting the intraperiod line. Myelin basic protein (MBP) makes up 30 percent of CNS and 10 percent of PNS myelin proteins. MBP is not an integral protein but binds to the cytoplasmic surface and is responsible for compaction at the major dense line. Myelin associate glycoprotein (MAG) accounts for about 1 percent of both peripheral and central myelin. Myelin oligodendrocyte glycoprotein (MOG) and cyclic nucleotide phosphodiesterase (CNP) are minor constituents of CNS myelin and are not found in the PNS. Peripheral myelin protein 22 (PMP22) is a minor component of PNS myelin.

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