Myelin is the lipid substance that covers long axons to speed up action potentials, and is responsible for the appearance of the white matter. A myelinated axon consists of nodes of ranvier, which are supported by glial cells in the white matter and look like small tubes covering the axon with small gaps in between them. Myelin is created by oligodendrocytes in the central nervous system, and by schwann cells in the peripheral nervous system.
Demyelinating disease, such as multiple sclerosis, are characterized by loss of myelin. Genetic diseases involving a defect of the coding for peripheral nerve myelin protein 22, such as Charcot-Marie-Tooth disease, also display abnormal myelin.