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.

Learn more about the anatomy of the nervous system with this interactive quiz and labeling exercises guide.

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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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