Myelin
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In neuroscience, myelin is an electrically insulating fatty layer that surrounds the axons of many neurons, especially those in the peripheral nervous system. 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. The myelin produced by the different cell types varies in its chemical composition or configuration, but performs the same function. Myelinated neurons are white in appearance, hence the well known "white matter" of the brain.
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"). Myelination also helps the electrical current from leaving the axon and causing a short circuit in the brain. When a fiber is severed, the myelin sheath provides a track along which regrowth can occur. Unmyelinated fibers do not regenerate.
Demyelination is a loss of myelin and is the root cause of symptoms experienced by patients with diseases such as multiple sclerosis and transverse myelitis. The immune system may play a role in demyelination associated with such diseases. Heavy metal poisoning may also lead to demyelination. When an axon's myelin degrades due to these diseases, conduction can be impaired or lost.
Research is currently being undertaken to repair damaged myelin sheaths. These techniques include surgically implanting oligodendrocytes in the central nervous system and inducing myelin repair with certain antibodies.
External links
- Athabasca University Biological Psychology Website (http://psych.athabascau.ca/html/Psych402/Biotutorials/2/myelin.shtml)
- The MS Information Sourcebook, Myelin (http://www.nationalmssociety.org/Sourcebook-Myelin.asp)de:Myelin fr:Myéline he:מיאלין