Myelin is a specialized, multilayered membrane that surrounds segments of the axons of many neurons. Composed largely of fatty substances and proteins, it forms an electrically insulating sheath that allows nerve impulses to travel rapidly and efficiently. Myelin is produced by supporting cells called glia, rather than by the neurons themselves. Damage to this sheath can slow or interrupt communication within the nervous system. (medlineplus.gov)
Structure and myelin-forming cells
A myelin sheath consists of repeated layers of a glial cell membrane wrapped around an axon. During formation, much of the cytoplasm between successive layers is displaced, producing tightly packed, or compact, myelin. Other regions retain cytoplasm and contain machinery involved in maintaining the sheath. Myelin therefore remains part of a living cell rather than being an inert coating deposited around a nerve fiber. (ncbi.nlm.nih.gov)
In the central nervous system, which includes the brain and spinal cord, myelin is formed by oligodendrocytes. Each oligodendrocyte can extend processes that make separate sheaths around several axons. In the peripheral nervous system, Schwann cells produce myelin; each myelinating Schwann cell forms one segment around one axon. Thus, a long peripheral axon requires a succession of Schwann cells along its length. (pmc.ncbi.nlm.nih.gov)
The myelinated segments are called internodes. Between them are short gaps known as nodes of Ranvier, where the axonal membrane is exposed and specialized for electrical signaling. Myelin is consequently discontinuous along an axon, and these interruptions are essential to its function. The sheath, the underlying axon, and the nodal regions together form an organized signaling unit. (ncbi.nlm.nih.gov)
Molecular composition
Myelin is unusually rich in lipids compared with many other biological membranes. Its lipid and protein components provide both insulation and structural organization. Central and peripheral myelin have broadly similar functions, but their protein compositions differ substantially, reflecting the different cells that produce them. (ncbi.nlm.nih.gov)
Myelin basic protein is an important component of compact myelin. Other characteristic proteins include proteolipid protein in central myelin and myelin protein zero in peripheral myelin. Additional proteins occupy particular regions rather than being distributed uniformly throughout the sheath. For example, myelin-associated glycoprotein occurs near the axon, whereas myelin oligodendrocyte glycoprotein is found on the outer surface of central myelin. This molecular organization supports membrane adhesion and interactions between glia and axons. (ncbi.nlm.nih.gov)
Electrical and metabolic functions
Nerve impulses propagate as action potentials, transient changes in electrical voltage across the axonal membrane. In an unmyelinated axon, excitation spreads sequentially along adjacent membrane regions. In a myelinated axon, the sheath increases electrical resistance and reduces effective capacitance, allowing local electrical currents to spread rapidly between nodes. Voltage-gated sodium channels concentrated at the nodes regenerate the action potential. (ncbi.nlm.nih.gov)
This mechanism is called saltatory conduction. The description that an impulse “jumps” between nodes refers to the sites where action potentials are regenerated; electrical current still travels through the intervening axonal segment. Restricting excitation mainly to nodes also reduces the amount of ion movement that must subsequently be reversed to restore concentration gradients. Myelin therefore improves the efficiency as well as the speed of signaling. (ncbi.nlm.nih.gov)
Myelin-forming cells also contribute to axonal metabolism. Experimental studies show that oligodendrocytes can supply metabolites such as lactate and pyruvate that axons use to generate adenosine triphosphate. Studies in mice demonstrate that oligodendrocyte metabolic activity can support long-term axonal integrity. These functions distinguish the myelin–axon relationship from a simple analogy with insulation around an electrical wire. (nature.com)
Development and plasticity
Myelination is an important feature of nervous-system development. Human imaging studies show substantial changes associated with myelin growth during infancy, with different pathways developing at different rates. Development continues through childhood and into early adulthood rather than occurring simultaneously throughout the brain. (nature.com)
Myelin formation is not restricted to early development. New oligodendrocytes are generated in the healthy adult brain, and animal experiments connect this process with neuroplasticity. In mice learning to run on a wheel with irregularly spaced rungs, production of new oligodendrocytes increased. Blocking their formation impaired acquisition of the skill without removing pre-existing myelin. These findings establish a contribution of new myelin-forming cells to this experimental form of motor learning, rather than a universal explanation of learning. (pubmed.ncbi.nlm.nih.gov)
Damage and repair
Demyelination is the loss or destruction of myelin. It can disrupt conduction and compromise the relationship between an axon and its supporting glia. In multiple sclerosis, inflammation associated with the immune system damages myelin and can also injure axons in the central nervous system. Symptoms depend partly on where the damage occurs. Inherited disorders called leukodystrophies can also affect myelin; examples include Krabbe disease and metachromatic leukodystrophy, which involve disturbances of lipid processing. (ninds.nih.gov)
Remyelination is the formation of new sheaths around previously demyelinated axons. In the central nervous system, it commonly involves oligodendrocyte precursor cells that proliferate and differentiate into myelin-forming oligodendrocytes; surviving mature oligodendrocytes can also contribute. Repair may occur spontaneously, but it can be incomplete, and experimental studies associate aging with reduced precursor-cell differentiation. Research distinguishes restoring myelin from suppressing inflammation or replacing axons that have already been lost. (nature.com)