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"). 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. Heavy metal poisoning may lead to demyelination. When an axon's myelin degrades due to these diseases, conduction can be impaired or lost.
Myelination in the human brain continues from before birth until at least 20 years of age.
Up to the age of 20, large areas of the frontal lobes are not yet myelinated.1 Myelination begins in thedevelopmentally oldest parts of the brain, like the brain stem, moving to the areas of the nervous system that have developed more recently, like the prefrontal lobe and cortex.
Myelin spreads throughout the nervous system in stages, which vary slightly in each individual.
Myelinatedneurons are white in appearance, hence the well-known "white matter" of the brain.
Myelin is made up primarily of a sphingolipid called sphingomyelin, and it is thought that the intertwining of the hydrocarbon chains of sphingomyelin serve to strengthen the myelinsheath.
Myelin increases resistance by a factor of 5,000 and decreases capacitance by a factor of 50.
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