Axon is one of the three basic parts of a nerve cell or neuron. The axon, also called the nerve fiber, is a tubelike extension of a neuron cell body. The axon’s specialized function is to carry messages. An axon, which is part of one neuron, may have branches to make contact with as many as 1,000 other neurons.
Most axons in the central nervous system (brain and spinal cord) are less than 1/25 inch (1 millimeter) long. However, many axons in the nervous system are longer, and some are much longer. For example, the axons that extend from the spinal cord to the muscles in the feet may be 30 to 40 inches (75 to 100 centimeters) long.
The structures commonly called nerves are actually bundles of axons lying next to one another in a cordlike formation. Nerves can be made up of the axons of motor neurons, sensory neurons, or both.
Some axons are covered by a sheath of a white, fatty substance called myelin. The myelin increases the speed of impulses along the axons. Myelin is responsible for the distinction between the gray matter and white matter in the nervous system. Gray matter consists largely of unmyelinated axons (axons without myelin sheaths) and neuron cell bodies. White matter is made up mostly of axons that have white sheaths of myelin. Myelin is formed by special supporting cells called glia. Glial cells called oligodendrocytes produce the myelin that surrounds the axons of many neurons within the central nervous system. In the peripheral nervous system, made up of the nerves that connect the central nervous system with every part of the body, myelin is produced by other glial cells called Schwann cells.
See also Nervous system (How the nervous system works) .
