The axon, also called the nerve fibre, is a tubelike extension of a neuron cell body. The axon is specialized to carry messages. An axon of one neuron may have enough branches to make contact with as many as 1,000 other neurons. Most axons in the central nervous system are less than 1 millimetre long. However, many axons in the peripheral 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 75 to 100 centimetres 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 or sensory neurons, or of 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 also causes the distinction between the grey matter and white matter in the nervous system. Grey 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 in the peripheral nervous system by Schwann cells, which are special supporting cells that surround the axons. In the central nervous system, supporting cells called glia produce myelin.