Myelin is a dielectric (electrically insulating) material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system. Myelin is an outgrowth of a glial cell. The production of the myelin sheath is called myelination. The production of myelin occurs in the fourteenth week of fetal development, while very little amounts of myelin exist in the brain at the time of birth. During infancy myelination occurs quickly and does not stop until the adolescent stages of life. Because of this rapid myelination, it is essential that children under the age of two receive a diet higher in fats than one of an adult.
Schwann cells supply the myelin for peripheral neurons, 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 it has also arisen by parallel evolution in some invertebrates. Myelin was discovered in 1854 by Rudolf Virchow.
Composition of myelin
Myelin made by different cell types 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 is composed of about 70 - 80 % lipids and about 20 - 30 % proteins (by dry weight). 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 hydrocarbon chains of sphingomyelin serve to strengthen the myelin sheath.
Function of myelin layer
The main purpose of a myelin layer (or sheath) is to increase 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 electrical resistance across the cell membrane by a factor of 5,000 and decreases capacitance by a factor of 50. Thus, myelination helps prevent the electrical current from leaving the axon.
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