Myelination is the process by which an electrically insulating layer known as a myelin sheath develops over neurons, the nerve cells responsible for transmitting electrical signals throughout the body. Specifically, myelination forms a myelin layer over the axon, the long, fibrous section of a neuron along which electrical signals are transmitted. The myelin sheath greatly increases the speed at which electrical impulses travel throughout the body. Many processes that are initiated, maintained, or terminated by rapidly-propagated nerve signals could not occur at a sufficient rate without the myelin sheath.
Signal transduction along neurons lacking myelin tends to be quite slow, as the signals are forced to travel at a constant rate down uninsulated axons. Myelination, however, greatly increases the speed of transduction. It does not occur along the entire length of the neuron but instead leaves tiny gaps referred to as "nodes of Ranvier." Signals, instead of traveling at a constant rate down an uninsulated axon, are instead able to "jump" from node to node in a process called "saltatory conduction."
The process of myelination, by which the myelin sheath is formed, begins early in fetal development and continues for many years, generally into adolescence. Usually, myelination affects the neurons throughout the body more than those in the brain during fetal development. By adolescence, however, myelin sheaths exist on the axons of most of the neurons in the body, including those in the brain. Such sheaths are white in appearance and make up the "white matter" component of the brain, in contrast to the grey-brown cell bodies, blood vessels, and other cellular components that make up the "grey matter" component of the brain.
Myelin is primarily composed of water, lipids, and proteins. Myelination occurs very rapidly during early infancy before slowing somewhat and finally stopping during adolescence. It is important that infants receive diets relatively high in fat, as fats are necessary to supply the lipid content of the myelin sheath during the process of myelination.
The debilitating results of various diseases and disorders that cause demyelination can illustrate the importance of myelination. Different diseases can slow signal transduction down different sets of neurons and cause a wide array of unpleasant effects such as loss of sensation, impaired cognition, and reduced motor function. Demyelination can be caused by a range of different factors, including genetics, vitamin deficiency, infection, and other unknown causes. One of the most prominent examples of a demyelinating disease is multiple sclerosis, or MS, in which the myelin sheaths in the brain and spinal cord degrade. This degradation can cause a massive range of neurological symptoms negatively impacting both sensory and motor function.