An axon, or "nerve fiber," is a long slender projection of a nerve cell, or "neuron," which conducts electrical impulses away from the neuron's cell body or soma. 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 micrometer 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 generally, 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. Along myelinated nerve fibers, gaps in the sheath known as nodes of Ranvier occur at evenly spaced intervals, enabling an especially rapid mode of electrical impulse propagation called saltation.
The axons of some neurons branch to form axon collaterals, along which the bifurcated impulse travels simultaneously to signal more than one other cell.