myelin

Myelin is a dielectric material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system.The production of the myelin sheath is called myelination.

Myelin is considered a defining characteristic of the (gnathostome) vertebrates, but myelin-like sheaths have also arisen by parallel evolution in some invertebrates, although they are quite different from vertebrate myelin at the molecular level.

Myelin was discovered in 1854 by Rudolf Virchow.