MYELIN SHEATH & COMPOSITION

MYELIN SHEATH:

Myelin was discovered in 1854 by Rudolf Virchow (Virchow, 1854). The multilayered myelin sheath electrically insulates axons and thus restricts the generation of action potentials to myelin-free regions of axons, i.e., the nodes of Ranvier, resulting in a saltatory and rapid propagation of nerve impulses. The formation of myelin around axons proceeds in various steps, requiring complex and reciprocal interactions between axons and myelin-forming oligodendrocytes and Schwann cells in the central nervous system (CNS) and peripheral nervous system (PNS), respectively.

First, myelin-forming glial cells have to recognize those axons that are competent to become myelinated. The processes of myelin-forming glial cells have then to adhere to these axons and to enwrap them. Subsequently, the extracellular space between the spiraling loops is reduced and the cytoplasm is extruded from the glial processes to yield compact myelin with its intraperiod and major dense lines. Only relatively small portions of the myelin sheath remain uncompacted, including the periaxonal cytoplasmic collar, paranodal loops, and the Schmidt- Lanterman incisures of PNS myelin (Hildebrand et al., 1993; Peters et al., 1991).

COMPOSITION:

Myelin is the essential constituent of white matter in the CNS which contains; 40–50% myelin on a dry weight basis (Norton and Poduslo, 1973; Norton and Poduslo, 1973). Myelin is a poorly hydrated structure containing 40% water in contrast to gray matter (80%). Myelin dry weight consists of 70% lipids and 30% proteins. This lipid-to-protein ratio is very peculiar to the myelin membrane.  Some of the proteins that make up myelin are myelin basic protein (MBP), myelin oligodendrocyte glycoprotein (MOG), and proteolipid protein (PLP). It is generally the reverse in other cellular membranes. The insulating properties of the myelin sheath, which favour rapid nerve conduction velocity, are largely due to its structure, its thickness, its low water content, and its richness in lipids. The specific constituents of myelin, glycolipids, and proteins are formed in the oligodendrocytes.