Axons are in effect the primary transmission lines of the nervous system, and as bundles they help make up nerves.

Individual axons are microscopic in diameter - typically about one micrometre across - but may extend to macroscopic lengths.

The longest axons in the human body, for example, are those of the sciatic nerve, which run from the base of the spine to the big toe of each foot.

These single-cell fibers may extend a meter or even longer.

In vertebrates, the axons of many neurons are sheathed in myelin, which is formed by either of two types of glial cells: Schwann cells ensheathing peripheral neurons and oligodendrocytes insulating those of the central nervous system.