Myelin
Lipid-rich insulator enabling rapid nerve impulse conduction in vertebrates.
Myelin 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. Unlike the plastic covering on an electrical wire, myelin does not form a single long sheath over the entire length of the axon. Instead, it ensheaths specific segments known as internodal segments, in multiple tightly regulated layers. These ensheathed segments are separated by short, unmyelinated gaps called nodes of Ranvier, each about one micrometre long. The nodes enable saltatory conduction, where the action potential recharges at each node and jumps to the next, greatly increasing conduction speed until it reaches the axon terminal, where it triggers neurotransmitter release across a synapse. Myelin is produced by specialized glial cells: in the central nervous system (CNS), oligodendrocytes extend foot processes to myelinate multiple nearby axons; in the peripheral nervous system (PNS), Schwann cells myelinate only a single section of an axon. Myelin is essential for efficient motor function, sensory function, and cognition, as demonstrated by disorders that affect it, such as the genetically determined leukodystrophies, the acquired inflammatory demyelinating disease multiple sclerosis (the most common demyelinating disorder), and inflammatory demyelinating peripheral neuropathies. Myelin was first described as white matter fibres in the 16th century by Vesalius, but the word myelin was coined by Rudolf Virchow in 1854, from the Greek *myelos* meaning bone marrow. Its glial cell origin and ultrastructure became apparent after the development of electron microscopy. Myelin is found in all vertebrates except jawless fish. It comprises about 40% water; the dry mass is 60–75% lipid and 15–25% protein. Key proteins include myelin basic protein (MBP), myelin oligodendrocyte glycoprotein (MOG), proteolipid protein (PLP) in the CNS, and myelin protein zero (MPZ) in the PNS. The primary lipid is galactocerebroside, and cholesterol is essential for myelin formation. Myelin-associated glycoprotein (MAG) helps attach the myelin sheath to the axon and is implicated in demyelination diseases.
- first_described
- 16th century by Vesalius as white matter fibres
- composition
- Approximately 40% water; dry mass 60-75% lipid and 15-25% protein
- found_in
- All vertebrates except jawless fish
- key_functions
- Insulation, saltatory conduction, axonal support and regeneration track
Lore & Background
Over a century later, electron microscopy revealed its glial cell origin and ultrastructure. Myelin is produced by specialized glial cells: oligodendrocytes in the central nervous system, which myelinate multiple axons via foot processes, and Schwann cells in the peripheral nervous system, which myelinate only a section of an axon. The myelin sheath is segmented into internodal lengths separated by nodes of Ranvier, where action potentials recharge and propagate via saltatory conduction. Myelin composition differs slightly between the CNS and PNS but both perform insulation and nutritional support. Key proteins include myelin basic protein, proteolipid protein, and myelin oligodendrocyte glycoprotein in the CNS, and myelin protein zero in the PNS. Cholesterol is essential for myelin formation, and myelin-associated glycoprotein helps attach the sheath to the axon.
Reader's Guide
Myelin's significance lies in its fundamental role in enabling rapid and efficient neural communication, which underpins movement, sensation, and cognition. The discovery of its structure and function, particularly saltatory conduction, revolutionized understanding of nervous system physiology. Disorders affecting myelin, such as multiple sclerosis—the best known demyelinating disease—highlight its critical importance; damage leads to functional deficits. Myelin also provides a track for peripheral nerve regeneration after injury, though regeneration is imperfect. In the CNS, myelinated axons do not regenerate. The process of myelination begins in humans around week 26 of gestation and is driven by axonal signals. Myelin's lipid-rich composition gives white matter its characteristic appearance, and its presence in all vertebrates except jawless fish suggests an evolutionary advantage for larger body size and agile communication. Ongoing research continues to uncover additional roles of myelinating cells, including axonal support and energy provision.
Did You Know?
- Myelin was first described as white matter fibres in the 16th century by Vesalius.
- The word myelin comes from the Greek word myelos, meaning bone marrow.
- Myelin comprises approximately 40% water; its dry mass is 60-75% lipid.
- In the central nervous system, myelin is formed by oligodendrocytes, each of which myelinates multiple nearby axons.
Frequently Asked Questions
What are Myelin's powers/role?
Myelin electrically insulates axons so that action potentials jump between unmyelinated gaps in a process called saltatory conduction, boosting signal speed dramatically. It also provides structural scaffolding to the axon and lays down a physical track that guides axonal regrowth after injury.
How does Myelin's story end?
In disease, Myelin's integrity is stripped away or degraded, as seen in multiple sclerosis and the leukodystrophies. Once the sheath is lost, impulses slow or halt, and motor control, sensation, and cognition all suffer.
Why is Myelin important?
Without Myelin the nervous system would be far too slow and metabolically wasteful to support coordinated movement, fine sensory discrimination, or complex thought. Its dry mass is roughly 60–75% lipid and 15–25% protein (with about 40% water by total weight), a composition perfectly tuned for electrical insulation.
Where does Myelin appear in the body?
Myelin lines axons throughout both the central and peripheral nervous systems of all vertebrates except jawless fish like lampreys and hagfish. In the brain and spinal cord oligodendrocytes build the sheath, while in peripheral nerves Schwann cells take on the wrapping role.
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