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Myelin

Myelin is a lipid-rich (fatty) substance formed in the central nervous system (CNS) by glial cells called oligodendrocytes, and in the peripheral nervous system (PNS) by Schwann cells. Myelin insulates nerve cell axons to increase the speed at which information (encoded as an electrical signal) travels from one nerve cell body to another (as in the CNS) or, for example, from a nerve cell body to a muscle (as in the PNS). The myelinated axon can be likened to an electrical wire (the axon) with insulating material (myelin) around it. However, unlike the plastic covering on an electrical wire, myelin does not form a single long sheath over the entire length of the axon. Rather, each myelin sheath insulates the axon over a single section and, in general, each axon comprises multiple long myelinated sections separated from each other by short gaps called Nodes of Ranvier. Each myelin sheath is formed by the concentric wrapping of an oligodendrocyte or Schwann cell process around the axon. Myelin is a lipid-rich (fatty) substance formed in the central nervous system (CNS) by glial cells called oligodendrocytes, and in the peripheral nervous system (PNS) by Schwann cells. Myelin insulates nerve cell axons to increase the speed at which information (encoded as an electrical signal) travels from one nerve cell body to another (as in the CNS) or, for example, from a nerve cell body to a muscle (as in the PNS). The myelinated axon can be likened to an electrical wire (the axon) with insulating material (myelin) around it. However, unlike the plastic covering on an electrical wire, myelin does not form a single long sheath over the entire length of the axon. Rather, each myelin sheath insulates the axon over a single section and, in general, each axon comprises multiple long myelinated sections separated from each other by short gaps called Nodes of Ranvier. Each myelin sheath is formed by the concentric wrapping of an oligodendrocyte or Schwann cell process around the axon. More precisely, myelin speeds the transmission of electrical impulses called action potentials along myelinated axons by insulating the axon and reducing axonal membrane capacitance. On a molecular level, it increases the distance between the cations on the outside of the axon and the Na⁺-ions that move through the axoplasm during an action potential, thereby greatly reducing the magnitude of the repulsive forces (which are inversely proportional to the square of the distance, as per Coulomb's law) between them that would otherwise act to inhibit the movement of the Na⁺-ions. The discontinuous structure of the myelin sheath results in saltatory conduction whereby the action potential 'jumps' from one node of Ranvier, over a long myelinated stretch of the axon called the internode, before 'recharging' at the next node of Ranvier, and so on, until it reaches the axon terminal. Nodes of Ranvier are the short (~1 micron) unmyelinated regions of the axon between adjacent long (~0.2 mm - >1 mm) myelinated internodes. Once it reaches the axon terminal, this electrical signal provokes the release of a chemical message or neurotransmitter that binds to receptors on the adjacent post-synaptic cell (e.g. nerve cell in the CNS or muscle cell in the PNS) at specialised regions called synapses. This 'insulating' role for myelin is essential for normal motor function (i.e. movement such as walking), sensory function (e.g. hearing, seeing or feeling the sensation of pain) and cognition (e.g. acquiring and recalling knowledge), as demonstrated by the consequences of disorders that affect it, such as the genetically determined leukodystrophies; the acquired inflammatory demyelinating disorder, multiple sclerosis; and the inflammatory demyelinating peripheral neuropathies. Due to its high prevalence, multiple sclerosis, which specifically affects the central nervous system (brain, spinal cord and optic nerve), is the best known disorder of myelin. The process of generating myelin is called myelination or myelinogenesis. In the CNS, cells called oligodendrocyte precursor cells (OPCs; the precursors of oligodendrocytes) differentiate into mature oligodendrocytes, which form myelin. In humans, myelination begins early in the 3rd trimester, although only little myelin is present in either the CNS or the PNS at the time of birth. During infancy, myelination progresses rapidly, with increasing numbers of axons acquiring myelin sheaths. This corresponds with the development of cognitive and motor skills, including language comprehension, speech acquisition, crawling and walking. Myelination continues through adolescence and early adulthood and although largely complete at this time, myelin sheaths can be added in grey matter regions such as the cerebral cortex, throughout life. Myelin is considered a defining characteristic of the jawed vertebrates (gnathostomes), though axons are ensheathed by a type of cell, called glial cells, in invertebrates. Although these glial-wraps are quite different from vertebrate compact myelin, formed, as indicated above, by concentric wrapping of the myelinating cell process multiple times around the axon. Myelin was first described in 1854 by Rudolf Virchow, although it was over a century later, following the development of electron microscopy, that its glial cell origin and its ultrastructure became apparent. In vertebrates, not all axons are myelinated. For example, in the PNS, a large proportion of axons are unmyelinated. Instead, they are ensheathed by non-myelinating Schwann cells known as Remak SCs and arranged in Remak bundles. In the CNS, non-myelinated (or intermittently myelinated axons; meaning having long non-myelinated regions between myelinated segments), intermingle with myelinated ones and are entwined, at least partially, by the processes of another type of glial cell called the astrocyte. CNS myelin differs slightly in composition and configuration from PNS myelin, but both perform the same 'insulating' function (see above). Being rich in lipid, myelin appears white; hence, the name given to the 'white matter' of the CNS. Both CNS white matter tracts (e.g. the optic nerve, corticospinal tract and corpus callosum) and PNS nerves (e.g. the sciatic nerve and the auditory nerve; which also appear white) each comprise thousands to millions of axons, largely aligned in parallel. Blood vessels provide the route for oxygen and energy substrates such as glucose to reach these fibre tracts, which also contain other cell types including astrocytes and microglia in the CNS and macrophages in the PNS. In terms of total mass, myelin comprises approximately 40% water; the dry mass comprises between 60% and 75% lipid and between 15% and 25% protein. Protein content includes myelin basic protein (MBP), which is abundant in the CNS where it plays a critical, non-redundant role in formation of compact myelin; myelin oligodendrocyte glycoprotein (MOG), which is specific to the CNS; and proteolipid protein (PLP,) which is the most abundant protein in CNS myelin, but only a minor component of PNS myelin. In the PNS, myelin protein zero (MPZ or P0) has a similar role to that of PLP in the CNS in that it is involved in holding together the multiple concentric layers of glial cell membrane that constitute the myelin sheath. The primary lipid of myelin is a glycolipid called galactocerebroside. The intertwining hydrocarbon chains of sphingomyelin strengthen the myelin sheath. Cholesterol, is an essential lipid component of myelin, without which myelin fails to form.

[ "Central nervous system", "Psychiatry", "Pathology", "Neuroscience", "Nogo Receptors", "Myelin fragmentation", "Myelinogenesis", "Dejerine–Sottas disease", "GJC2" ]
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