Last update: Dec 2nd, 2017

Unmyelinated and Myelinated Axons

• Larger diameter axons tend to be myelinated (covered by a myelin sheath), which allows them to conduct action potentials at greater velocities than smaller diameter, unmyelinated axons.

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• Myelin sheaths are formed by Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system.

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• The sheaths consist of a series of myelinated segments, each extending about 1 millimeter along the outer surface of the axon.
• The segments are separated by small gaps (about 1 micrometer) called nodes of Ranvier (enlarged for this illustration).

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• Schwann cells and oligodendrocytes form coats of myelin by wrapping the axon with concentric layers of their cell membranes, which are mostly lipid.

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• The layer of insulation created by the myelin coat inhibits current loss from the axon by blocking the movement of ions through the axon membrane.
• Ion channels in myelinated neurons are therefore mostly concentrated in the nodes of Ranvier.

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• In comparison, ion channels in unmyelinated neurons occur along the the entire length of the axon.

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