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Sem Ii Cc 3 2020 SEM II CC 3 2020 MYELINOGENESIS • The process by which myelin (composed of multiple concentric layers of phospholipid membrane) sheath is formed or deposited on a nerve fiber (axon) is called myelinogenesis. • Neural tissue secrets very little extracellular matrix and glial cells provide structural stability to neurons by wrapping around them. In addition to providing support, the myelin acts as insulation around the axons and speeds up their signal transmission. • In the CNS, oligodendrocytes and in the PNS, Schwann cells take part in the formation of myelin sheath. • In the CNS, one oligodendrocyte branches and forms myelin sheaths, around portions of several nerve fibers (axons). • In the PNS, one Schwann cell is associated with one axon covering only 1-1.5 mm area. (from Gannon) MYELINATION IN PERIPHERAL NERVOUS SYSTEM (PNS) • In the PNS, one Schwann cell is associated with myelination of one axon. • Each Schwann cell wraps around only 1-1.5 mm segment of an axon. • A single axon may have as many as 500 different Schwann cells along its length. • Each Schwann cell wraps several times (about 100 times), forming a covering of 8-10 µm thickness. • The nerve fiber (axon) to be myelinated, is first invaginated by the cell membrane of the Schwann cell → thus a double layered mesaxon is formed , Each membrane of mesaxon is composed of two lipid layers sandwiched by layers of protein. → mesaxon rotates round the axon (1-1.5 mm segment) → several layers of mesaxons (about 100 times wrappings) are wrapped around the nerve fiber. → Thus these layers of mesaxons closely packed, form the myelin sheath → the cytoplasm of Schwann cell along with its nucleus remains outside, beneath the outermost layer of Schwann cell membrane i.e. in nurilemma (sheath of Schwann), to form an additional sheath. Stages of formation of Myelin sheath by Schwann cell. (from GKP) • Along the myelinated axon, there are some tiny gaps between two myelin insulated areas, called Nodes of Ranvier. • At each node of Ranvier, a tiny section of axon membrane remains in direct contact with the extracellular fluid. The nodes, permeable to ions, play an important role in transmission electrical signal along the axon. The portion of a myelinated nerve fiber, between two nodes of Ranvier is internodal segment or internode. Composition of myelin sheath Myelin contains protein, lipids, and water. The main lipids present include cholesterol, phospholipid, and glycosphingolipids. Other lipids are present in smaller amount. Objectives of Myelination Myelination serves following four purposes. 1. It increases the speed of conduction. 2. It reduces energy expenditure by the cell. 3. It provides a protective covering to the axon. 4. It is responsible for the color of the white matter of the brain and spinal cord. Timing of Myelination during Development Myelination of different types of nerve fibers takes place at different times. 1. The sensory fibers of the dorsal column system first get myelinated, which occurs at 4th-5th month of intrauterine life. 2. The corticospinal tract fibers start myelinating at two months of age and the process gets completed at about 2nd year of life, when the child has learned to walk. Unmyelinated Axons Unmyelinated axons do not have myelin sheath. 1. The Schwann cells are present near these axons, but their mesaxons do not completely spin around them. 2. Another difference is that several such axons may invaginate into the cytoplasm of a single Schwann cell. 3. Somatic nerve fibers of very small diameter, postganglionic sympathetic neurons of the autonomic nervous system, dorsal root fibers and most of the fibers in invertebrates are unmyelinated ( non-medullated). 4. The speed of conduction of impulse is slower in unmyelinated nerve fibers. Unmyelinated axons- invagination into one Schwann cell - DISORDERS ASSOCIATED WITH MYELINATION Multiple sclerosis, neuromyelitis optica spectrum disorders, inflammations due to autoimmune reactions, chronic inflammatory demyelinating polyneuropathy. .
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