<P> Myelin is a fatty white substance that surrounds the axon of some nerve cells, forming an electrically insulating layer . It is essential for the proper functioning of the nervous system . It is an outgrowth of a type of glial cell . </P> <P> The production of the myelin sheath is called myelination or myelinogenesis . In humans, myelination begins early in the 3rd trimester, although little myelin exists in the brain at the time of birth . During infancy, myelination occurs quickly, leading to a child's fast development, including crawling and walking in the first year . Myelination continues through the adolescent stage of life . </P> <P> Schwann cells supply the myelin for the peripheral nervous system, whereas oligodendrocytes, specifically of the interfascicular type, myelinate the axons of the central nervous system . Myelin is considered a defining characteristic of the (gnathostome) vertebrates, but myelin - like sheaths have also been seen in some invertebrates, although they are quite different from vertebrate myelin at molecular level . Myelin was discovered in 1854 by Rudolf Virchow . </P> <P> Myelin comprises different cell types and varies in chemical composition and configuration but performs the same insulating function . Myelinated axons are white; hence, the "white matter" of the brain . Myelin insulates axons from electrically charged atoms and molecules . These charged particles (ions) are found in the fluid surrounding the entire nervous system . Under a microscope, myelin looks like strings of sausages . </P>

The cells that surround an axon creating a myelin sheath in the pns are called