Axon

An axon is the long, slender fiber that grows out of a neuron's cell body and carries its output signal. Most neurons have just one axon, which leaves the cell body at a tapered region called the axon hillock. The hillock and the first stretch of the axon, the initial segment, together form the trigger zone where action potentials begin.

Once started, an action potential sweeps along the axon to its far end, where the axon divides into many branches. Each branch ends in a small swelling, the axon terminal, that releases neurotransmitter onto a target cell. Many axons are wrapped in myelin, a fatty insulating layer made by glial cells and broken by short bare gaps called nodes of Ranvier. The impulse leaps from node to node, so myelinated axons, like wider ones, conduct faster.

Axons can be remarkably long: a motor neuron whose cell body lies in the lower spinal cord can send its axon about a meter to the muscles of the foot. Outside the brain and spinal cord, bundles of axons make up nerves; inside, they form tracts, which look pale as white matter because of their myelin.