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from Greek: myelon - marrow
German: Myelin

1 Definition

Myelin is a fatty substance produced by Schwann cells (PNS) or oligodendrocytes (CNS). It forms the biochemical basis for myelin sheaths.

2 Biochemistry

Myelin has a water content of about 40%. The dry matter is made up mainly of lipids (80%; cholesterol, phospholipids, cerebrosides and others) and proteins. Those percentages, however differ according to its location (PNS or CNS).

Two of those proteins are MOG (myelin oligodendrocyte glycoprotein) and MBP (myelin basic protein). The proportional contents of different components seem to have a crucial influence on the structural integrity of myelin sheaths. Important lipids making up myelin are glykolipids and galactocerebrosides. Their hydrocarbon chains interact, strengthening the myelin sheaths.

3 Function

Myelin decreases electrical capacity of nerve cell membranes and increases their electrical resistance which prevents leaking of electrical excitation from the axon to the surroundings. This drastically improves transmission rates. Nerve fibers surrounded by myelin sheaths show saltatory conduction, allowing rapid transmission of nerve impulses over long distances.

In addition, myelin sheaths act as guide structures for the regeneration of axons after nerve damage.

4 Pathology

Proteins embedded in the myelin sheaths that surround nerve fibers can be targeted by autoimmune responses. The resulting conditions are called demyelinating diseases, e.g.

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