| Myelin sheath |
Dendrite
Soma
Axon
Nucleus
Node of
Ranvier
Axon Terminal
Schwann cell
Myelin sheath
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| Structure of a typical neuron |
Myelin is an electrically insulating phospholipid layer that surrounds the axons of many neurons. It is an outgrowth of glial cells: Schwann cells supply the myelin for peripheral neurons while oligodendrocytes supply it to those of the central nervous system. Myelin is considered a defining characteristic of the (gnathostome) vertebrates, but it has also arisen by parallel evolution in some invertebrates.[1] Myelin was discovered in 1878 by Louis-Antoine Ranvier.
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Contents
- 1 Composition of myelin
- 2 Function of myelin layer
- 3 Demyelination
- 4 See also
- 5 External links
- 6 References
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Composition of myelin
Myelin produced by different cell types varies in chemical composition and configuration, but performs the same insulating function. Myelinated neurons are white in appearance, hence the "white matter" of the brain.
Myelin is composed of about 80% lipid fat and about 20% protein. Some of the proteins that make up myelin are Myelin basic protein (MBP), Myelin oligodendrocyte glycoprotein (MOG) and Proteolipid protein (PLP). Myelin is made up primarily of a glycolipid called galactocerebroside.The intertwining of the hydrocarbon chains of sphingomyelin serve to strengthen the myelin sheath.
Function of myelin layer
The main consequence of a myelin layer (or sheath) is an increase in the speed at which impulses propagate along the myelinated fiber. Along unmyelinated fibers, impulses move continuously as waves, but, in myelinated fibers, they hop or "propagate by saltation". Myelin increases resistance across the cell membrane by a factor of 5,000 and decreases capacitance by a factor of 50.verification needed] Myelination also helps prevent the electrical current from leaving the axon. When a peripheral fiber is severed, the myelin sheath provides a track along which regrowth can occur. Unmyelinated fibers and myelinated axons of the mammalian central nervous system do not regenerate.
Demyelination
Demyelination is the act of demyelinating, or the loss of the myelin sheath insulating the nerves, and is the hallmark of some neurodegenerative autoimmune diseases, including multiple sclerosis, transverse myelitis and adrenoleukodystrophy. The immune system may play a role in demyelination associated with such diseases. Heavy metal poisoning may also lead to demyelination. When myelin degrades, conduction of signals along the nerve can be impaired or lost, and the nerve eventually withers. Even very small amounts of mercury have been shown to be particularly destructive to nerve sheaths. (see How Mercury Causes Brain Neuron Degeneration )
Research to repair damaged myelin sheaths is ongoing. Techniques include surgically implanting oligodendrocyte precursor cells in the central nervous system and inducing myelin repair with certain antibodies. While there have been some encouraging results in mice, it is still unknown whether this technique can be effective in humans.
