Axon (noun): the long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron’s cell body.
The axon is the part of the nerve cell through which messages pass from the cell body to other cells. The axon is typically a single, thin extension of the nerve cell, though some types of neurons have multiple axons. The word “axon” comes from Greek ἄξων (áxōn), meaning “axis”.
Axons are classified according to their diameter into three groups: large (A), medium (B), and small (C). The vast majority of neurons in the human nervous system are small-diameter type C fibers. Axons conduct electrical impulses called action potentials away from the neuron’s cell body toward target cells. These impulses are generated by special structures called voltage-gated ion channels embedded in the plasma membrane of an axon’s initial segment. When an action potential arrives at an axon terminal, it triggers release of neurotransmitters that bind to and activate receptors on target cells; this process is known as synaptic transmission.
Most axons in vertebrates are insulated with myelin, a white fatty substance produced by glial cells wrapped around them in segments. Myelin provides electrical insulation for fast conduction of action potentials and also helps to protect against infection and mechanical injury. However, some types of unmyelinated fibers do exist; these include sensory nerves that detect pain and temperature changes, as well as motor nerves that control muscle contraction.