Myelin

Synonyms

Axon sheath

Definition

Myelin is a protein and lipid-rich substance, which is produced by oligodendrocytes in the central nervous system and by Schwann cells in the peripheral nervous system.

Current Knowledge

Myelin is elaborated into a sheath and wrapped in a concentric fashion around an axon. The deposition of myelin in a spiraling pattern around an axon generates two morphological features of the myelin sheath, that is, the major dense line (DL) and the intraperiod line (IL). The DL is formed when the cytoplasm within the myelin process is lost and the opposing plasma membranes come together. As the myelin sheath spirals around the axon, the outer faces of the plasma membrane of each wrap oppose each other to form the intraperiod line. The ensheathment of an axon with myelin is not continuous along its length, but is laid down as segments of myelin (internodes) that are interrupted, at regular intervals, by areas devoid of myelin, which are termed the nodes of Ranvier. The...