Myelin is a substance that makes up the tube-shaped
covering of various nerves fibers (such as a delta fibers)
in the body and helps them transmit messages fast.
Myelin mostly contains thin layers of various fats (such
as cholesterol), in addition to some protein. The fat
contained in myelin gives it a white, cream-colored
appearance. This is why white matter in the brain (such
as periventricular white matter) appears white. White
matter is a group of white nerve fibers that conduct nerve
impulses quickly.

White matter is important for muscle movements and
quick thinking. This is why diseases that damage myelin
can slow down motor functioning and reduce thinking
Top: Myelin (yellow) covering
an axon (nerve fiber).
Bottom: Myelin destruction
due to disease.
Perhaps the most well know disease that causes this is multiple sclerosis. Multiple
sclerosis is a condition in which multiple areas of abnormal patches (known as plaques)
develop in the brain and/or spinal cord (depending on the stage of the illness) as a result
of myelin damage. Nerve fibers that do not contain myelin are known as unmyelinated.

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Myelin is also a term used for droplets of fat that are created during the process of cell
destruction or bodily decay. A cell is the smallest, most basic unit of life, that is capable
of existing by itself. Myelin comes from the Greek word "myelos," meaning "marrow."
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