According to Wikipedia, 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. It is an outgrowth of a type of glial cell. The production of the myelin sheath is called myelination. In humans, the production of myelin begins in the 14th week of fetal development, although little myelin exists in the brain at the time of birth. During infancy, myelination occurs quickly and continues through the adolescent stages of life.
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 myelin-like sheaths have also arisen by parallel evolution in some invertebrates, although they are quite different from vertebrate myelin at the molecular level. Myelin was discovered in 1854 by Rudolf Virchow.
Composition of myelin
Myelin is made up 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. The fat 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. Myelin is also a part of the maturation process leading to a child’s fast development, including crawling and walking in the first year.
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, myelin oligodendrocyte glycoprotein, and proteolipid protein. The primary lipid of myelin is 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 decreases capacitance across the cell membrane, and increases electrical resistance. Thus, myelination helps prevent the electrical current from leaving the axon.
When a peripheral fiber is severed, the myelin sheath provides a track along which regrowth can occur. Unfortunately, the myelin layer does not ensure a perfect regeneration of the nerve fiber. Some regenerated nerve fibers do not find the correct muscle fibers and some damaged motor neurons of the peripheral nervous system die without regrowth. Damage to the myelin sheath and nerve fiber is often associated with increased functional insufficiency.
Unmyelinated fibers and myelinated axons of the mammalian central nervous system do not regenerate. Some studies have revealed optic nerve fibers can be regenerated in postnatal rats. This regeneration depends upon two conditions: axonal die-back has to be prevented with appropriate neurotrophic factors, and neurite growth inhibitory components have to be inactivated. These studies may lead to further understanding of nerve fiber regeneration in the central nervous system.