Composition and Formation of Myelin Sheath
Myelin may be made by different cell types, varies in chemical composition and configuration, but performs the same insulating function. Myelin is about 40% water; the dry mass of myelin is about 70 - 85% lipids and about 15 - 30% proteins. Some of the proteins that make up myelin are myelin basic protein (MBP), myelin oligodendrocyte glycoprotein (MOG), and proteolipid protein (PLP). The primary lipid of myelin is a glycolipid called galactocerebroside (GalC). The intertwining hydrocarbon chains of sphingomyelin serve to strengthen the myelin sheath. A many-layered white, lipid and protein covering produced by the neuroglia that is called the myelin sheath surround the axons of most neurons. The sheath electrically insulates the axon of a neuron and increases the speed of nerve impulse conduction. Axons with such a covering are said to be myelinated whereas those without it are said to be unmyelinated. Two types of neuroglia produce myelin sheaths: neurolemmocytes and oligodendrocytes. In the PNS, neurolemmocytes form myelin sheaths around the axons during fetal development. In forming a sheath, a neurolemmocyte wraps around the axon in a spiral fashion in such a way that its nucleus and cytoplasm end up in the outside layer. The inner portion, up to 100 layers of neurolemmocyte membrane, is the myelin sheath. The outer nucleated cytoplasmic layer of the neurolemmocyte, which encloses the myelin sheath, is called the neurolemma (sheath of Schwann). A neurolemma is found only around the axons in the PNS. When an axon is injured, the neurolemma aids in the regeneration by forming a regeneration tube that guides and stimulates regrowth of the axon. At intervals along an axon, the myelin sheath has gaps called neurofibral nodes (nodes of Ranvier). In the CNS, an oligodendrocyte myelinates many axons in somewhat the same manner that neurolemmocytes myelinate PNS axons. Oligodendrocytes...
Please join StudyMode to read the full document