page_title: Myelin
section_title: Composition
hierarchical_section_title: Myelin / Composition
caption_attribution_description: Transmission electron micrograph of a myelinated axon. The myelin layer (concentric) surrounds the axon of a neurone, showing cytoplasmathic organs inside. Generated and deposited into the public domain by the Electron Microscopy Facility at Trinity College.
context_page_description: Myelin is a lipid-rich substance that surrounds nerve cell axons to insulate them and increase the rate at which electrical impulses are passed along the axon. The myelinated axon can be likened to an electrical wire with insulating material around it. However, unlike the plastic covering on an electrical wire, myelin does not form a single long sheath over the entire length of the axon. Rather, each myelin sheath insulates the axon over a single long section and, in general, each axon comprises multiple long myelinated sections separated from each other by short myelin sheath-gaps called nodes of Ranvier.
  Myelin is formed in the central nervous system by glial cells called oligodendrocytes and in the peripheral nervous system by glial cells called Schwann cells. In the CNS, axons carry electrical signals from one nerve cell body to another. In the PNS, axons carry signals to muscles and glands or from sensory organs such as the skin. Each myelin sheath is formed by the concentric wrapping of an oligodendrocyte or Schwann cell process around the axon. Myelin reduces the capacitance of the axonal membrane.
context_section_description: CNS myelin differs slightly in composition and configuration from PNS myelin, but both perform the same "insulating" function (see above). Being rich in lipid, myelin appears white, hence the name given to the "white matter" of the CNS. Both CNS white matter tracts (e.g. the optic nerve, corticospinal tract and corpus callosum) and PNS nerves (e.g. the sciatic nerve and the auditory nerve, which also appear white) each comprise thousands to millions of axons, largely aligned in parallel. Blood vessels provide the route for oxygen and energy substrates such as glucose to reach these fibre tracts, which also contain other cell types including astrocytes and microglia in the CNS and macrophages in the PNS.
  In terms of total mass, myelin comprises approximately 40% water; the dry mass comprises between 60% and 75% lipid and between 15% and 25% protein. Protein content includes myelin basic protein (MBP), which is abundant in the CNS where it plays a critical, non-redundant role in formation of compact myelin; myelin oligodendrocyte glycoprotein (MOG), which is specific to the CNS; and proteolipid protein (PLP), which is the most abundant protein in CNS myelin, but only a minor component of PNS myelin. In the PNS, myelin protein zero (MPZ or P0) has a similar role to that of PLP in the CNS in that it is involved in holding together the multiple concentric layers of glial cell membrane that constitute the myelin sheath. The primary lipid of myelin is a glycolipid called galactocerebroside. The intertwining hydrocarbon chains of sphingomyelin strengthen the myelin sheath. Cholesterol is an essential lipid component of myelin, without which myelin fails to form.
caption: Transmission electron micrograph of a cross-section of a myelinated PNS axon, generated at the Electron Microscopy Facility at Trinity College, Hartford, Connecticut
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