Myelin (/ˈmaɪ.əlɪn/ MY-ə-lin) is a lipid-rich material that in most vertebrates surrounds the axons of neurons to insulate them and increase the rate at which electrical impulses (called action potentials) pass along the axon. The myelinated axon can be likened to an electrical wire (the axon) with insulating material (myelin) 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. Myelin ensheaths part of an axon known as an internodal segment, in multiple myelin layers of a tightly regulated internodal length.

The ensheathed segments are separated at regular short unmyelinated intervals, called nodes of Ranvier. Each node of Ranvier is around one micrometre long. Nodes of Ranvier enable a much faster rate of conduction known as saltatory conduction where the action potential recharges at each node to jump over to the next node, and so on till it reaches the axon terminal. At the terminal the action potential provokes the release of neurotransmitters across the synapse, which bind to receptors on the post-synaptic cell such as another neuron, myocyte or secretory cell.

Myelin is made by specialized non-neuronal glial cells, that provide insulation, and nutritional and homeostatic support, along the length of the axon. In the central nervous system, myelination is formed by glial cells called oligodendrocytes, each of which sends out cellular extensions known as foot processes to myelinate multiple nearby axons. In the peripheral nervous system, myelin is formed by Schwann cells, which myelinate only a section of an axon. In the CNS, axons carry electrical signals from one nerve cell body to another. The "insulating" function for myelin is essential for efficient motor function (i.e. movement such as walking), sensory function (e.g. sight, hearing, smell, the feeling of touch or pain) and cognition (e.g. acquiring and recalling knowledge), as demonstrated by the consequence of disorders that affect myelination, such as the genetically determined leukodystrophies; the acquired inflammatory demyelinating disease, multiple sclerosis; and the inflammatory demyelinating peripheral neuropathies. Due to its high prevalence, multiple sclerosis, which specifically affects the central nervous system, is the best known demyelinating disorder.

Myelin was first described as white matter fibres in 1717 by Vesalius, and first named as myelin by Rudolf Virchow in 1854. Over a century later, following the development of electron microscopy, its glial cell origin, and its ultrastructure became apparent.

Myelin is found in all vertebrates except the jawless fish. Myelin in the central nervous system (CNS) differs slightly in composition and configuration from myelin in the peripheral nervous system (PNS), but both perform the same functions of insulation and nutritional support. Being rich in lipid, myelin appears white, hence its earlier name of white matter of the CNS. Both CNS white matter tracts such as the corpus callosum, and corticospinal tract, and PNS nerves such as the sciatic nerve, and the auditory nerve, which also appear white, comprise thousands to millions of axons, largely aligned in parallel. In the corpus callosum there are more than 200 million axons. 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.

Myelin-associated glycoprotein (MAG) is a critical protein in the formation and maintenance of myelin sheaths. MAG is localized on the inner membrane of the myelin sheath and interacts with axonal membrane proteins to attach the myelin sheath to the axon. Mutations to the MAG gene are implicated in demyelination diseases such as multiple sclerosis.

The main purpose of myelin is to increase the speed at which electrical impulses (known as action potentials) propagate along the myelinated fiber. In unmyelinated fibers, action potentials travel as continuous waves, but, in myelinated fibers, they "hop" or propagate by saltatory conduction. The latter is markedly faster than the former, at least for axons over a certain diameter. Myelin decreases capacitance and increases electrical resistance across the axonal membrane (the axolemma). It has been suggested that myelin permits larger body size by maintaining agile communication between distant body parts.