Myelin sheaths
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Myelin is a fatty white substance that surrounds the axon of some nerve cells, forming an electrically insulating layer. It is essential for the proper functioning of the nervous system. It is an outgrowth of a type of glial cell.
The production of the myelin sheath is called myelination or myelinogenesis. In humans, myelination begins early in the 3rd trimester, although little myelin exists in the brain at the time of birth. During infancy, myelination occurs quickly, leading to a child's fast development, including crawling and walking in the first year. Myelination continues through the adolescent stage of life.
Schwann cells supply the myelin for the peripheral nervous system, whereas oligodendrocytes, specifically of the interfascicular type, myelinate the axons of the central nervous system. Myelin is considered a defining characteristic of the (gnathostome) vertebrates, but myelin-like sheaths have also been seen in some invertebrates, although they are quite different from vertebrate myelin at the molecular level. Myelin was discovered in 1854 by Rudolf Virchow.
Myelin is made by different cell types, and varies in chemical composition and configuration, but performs the same insulating function. Myelinated axons are white in appearance, hence the "white matter" of the brain. Myelin helps to insulate the axons from electrically charged atoms and molecules. These charged particles (ions) are found in the fluid surrounding the entire nervous system. Under a microscope, myelin looks like strings of sausages.
Cholesterol is an essential constituent of myelin. Myelin is about 40% water; the dry mass is about 70–85% lipids and about 15–30% proteins. Some of the proteins are myelin basic protein (MBP, which makes up ~30% of myelin protein), myelin oligodendrocyte glycoprotein, and proteolipid protein (PLP, which makes up ~50% of myelin protein). The primary lipid of myelin is a glycolipid called . The intertwining hydrocarbon chains of sphingomyelin serve to strengthen the myelin sheath. In brain, the myelin sheath covers the fibers of corpus callosum which constitute the inner part of cerebral hemisphere.
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