Myelinated Nerve Fibre, Myelin

Abstract

Nerve fibres designed for particularly rapid and efficient conduction of action potentials are equipped with a myelin sheath, a lipid-enriched layer, produced by specialised glial cells, the oligodendrocytes in the central nervous system, and the Schwann cells in the peripheral nervous system. The myelin sheath isolates the axon from the surrounding compartments. It reduces the current flow across the axonal membrane allowing a saltatory conduction along the nerve fibre from one to the next area lacking a myelin sheath at the sites of the cell borders, called the nodes of Ranvier (cf. Fig. 163). A myelin sheath is absent from the axon hillock and the terminal ramifications of the axon where synapses are established. In the central nervous system, oligodendrocytes produce myelin sheaths for more than one axon, and the oligocyte cell body remains outside of the myelin sheaths. By contrast, in the peripheral nervous system the Schwann cells provide myelin only for one axon, and the entire cell including the nucleus becomes part of the myelinated nerve fibre.