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07-SEPTEMBER-2008 03:17:44 - Myelin Structure of a typical neuron Myelin sheath Dendrite Soma Axon Nucleus Node of Ranvier Axon Terminal Schwann cell Myelin sheath Myelin is an electrically-insulating dielectric phospholipid layer that surrounds only the axons of many neurons. It is an outgrowth glial cell: Schwann cells supply the myelin for peripheral neurons, whereas oligodendrocytes supply it to those 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.1 Myelin was discovered in 1854 by Rudolf Virchow 2. Contents 1 Composition of myelin 2 Function of myelin layer 3 Demyelination and Dysmyelination 4 Symptoms of Demyelination 5 See also 6 References 6.1 Also see 6.1.1 Relating to diabetes 6.1.2 Relating to myelin's geometry, and its fibre-optic potentiality 7 External links 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 80% lipid and about 20% protein. 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 of the hydrocarbon chains of sphingomyelin serve to strengthen the myelin sheath. Function of myelin layer Transmission electron micrograph of a myelinated axon. Generated at the Electron Microscopy Facility at Trinity College, Hartford, CT Transmission electron micrograph of a myelinated axon. Generated at the Electron Microscopy Facility at Trinity College, Hartford, CT The main consequence of a myelin layer or sheath is an increase in 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 resistance across the cell membrane by a factor of 5,000 and decreases capacitance by a factor of 50.citation needed Myelination also 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. Unmyelinated fibers and myelinated axons of the mammalian central nervous system do not regenerate. Demyelination and Dysmyelination Demyelination is the act of demyelinating, or the loss of the myelin sheath insulating the nerves, and is the hallmark of some neurodegenerative autoimmune diseases, including multiple sclerosis, acute disseminated encephalomyelitis, transverse myelitis, Alexander's disease, chronic inflammatory demyelinating polyneuropathy, Guillain-Barré Syndrome and central pontine myelinosis. Sufferers of pernicious anaemia can also suffer nerve damage if the condition is not diagnosed quickly. Sub-acute combined degeneration of the cord secondary to pernicious anaemia can lead to anything from slight peripheral nerve damage to severe damage to the central nervous system affecting speech, balance and cognitive awareness. When myelin degrades, conduction of signals along the nerve can be impaired or lost and the nerve eventually withers. The immune system may play a role in demyelination associated with such diseases, including inflammation causing demyelination by overproduction of cytokines via upregulation of tumor necrosis factor TNF3 or interferon. Heavy metal poisoning may also lead to demyelination. Even very small amounts of mercury have been shown to be particularly destructive to nerve sheaths.4 Research to repair damaged myelin sheaths is ongoing. Techniques include surgically implanting oligodendrocyte precursor cells in the central nervous system and inducing myelin repair with certain antibodies. While there have been some encouraging results in mice via stem cell implant, it is still unknown whether this technique can be effective in humans.5 Dysmyelination on the other hand is different from the lesions producing process of active demyelination and is characterized by defective structure and function of myelin sheaths. Such defective sheaths often arise from genetic mutations affecting the biosynthesis and formation of myelin. Examples of human diseases where dysmyelination has been implicated include leukodystrophies Pelizaeus-Merzbacher disease, Canavan disease, Phenylketonuria and schizophrenia.678 Symptoms of Demyelination Demyelination destruction or loss of the myelin sheath typically results in diverse symptoms. The symptoms are determined by the functions normally contributed by the affected neurons. Damage to the myelin sheath disrupts signals between the brain and other parts of the body producing a range of symptoms. Symptoms are often heterogeneous - dependent on pathophysiology of demyelination - differing from patient to patient, and have different presentations upon clinical observation and in laboratory studies. Blurriness in the central visual field that affects only one eye; may be accompanied by pain upon eye movement Double vision Odd sensation in legs, arms, chest, or face, such as tingling or numbness neuropathy Weakness of arms or legs Cognitive disruption including speech impairment, memory loss Heat sensitivity symptoms worsen, reappear upon exposure to heat such as a hot shower Loss of dexterity Difficulty coordinating movement or balance disorder Difficulty controlling bowel movements or urination Fatigue See also Multiple Sclerosis MS, caused by loss of myelin The Myelin Project, project to re-generate myelin Myelinogenesis, order of myelination of central nervous system Myelin Repair Foundation, non-profit medical research foundation accelerating drug discovery in myelin repair for multiple sclerosis References ^ Invertebrate Myelin ^ Virchow R 1854 Ãœber das ausgebreitete Vorkommen einer dem Nervenmark analogen Substanz in den tierischen Geweben. Virchows Arch. Pathol. Anat. 6:562-572. ^ 1 Ledeen R.W., Chakraborty G., Cytokines, Signal Transduction, and Inflammatory Demyelination: Review and Hypothesis Neurochemical Research, Volume 23, Number 3, March 1998, pp. 277-28913 ^ 2 University of Calgary: How Mercury Causes Brain Neuron Degeneration ^ 3 FuturePundit January 20, 2004 ^ Krämer-Albers at al., 2006 ^ Matalon et al., 2006 ^ Tkachev et al., 2007 Krämer-Albers EM, Gehrig-Burger K, Thiele C, Trotter J, Nave KA. 2006 Nov 8. Perturbed interactions of mutant proteolipid protein/DM20 with cholesterol and lipid rafts in oligodendroglia: implications for dysmyelination in spastic paraplegia. J Neurosci. 2645:11743-52.PMID: 17093095 Matalon R, Michals-Matalon K, Surendran S, Tyring SK. 2006. Canavan disease: studies on the knockout mouse. Adv Exp Med Biol.; 576:77-93.PMID: 16802706 Tkachev D, Mimmack ML, Huffaker SJ, Ryan M, Bahn S. 2007 Aug. Further evidence for altered myelin biosynthesis and glutamatergic dysfunction in schizophrenia. Int J Neuropsychopharmacol. 104:557-63.PMID: 17291371 Also see Relating to diabetes Vlassara H, Brownlee M, Cerami A. 1985 Jun; Recognition and uptake of human diabetic peripheral nerve myelin by macrophages. Diabetes. 346:553-7. PMID: 4007282 Thornalley PJ. 2002; Glycation in diabetic neuropathy: characteristics, consequences, causes, and therapeutic options. Int Rev Neurobiol. 50:37-57. PMID: 12198817 Relating to myelin's geometry, and its fibre-optic potentiality Donaldson, H.H. Hoke, G.W. 1905. The areas of the axis cylinder and medullary sheath as seen in cross sections of the spinal nerves of vertebrates. Journal of Comparative Neurology. 15, 1- - Early evidence of approximately-constant ratio of myelin-thickness to axon diameter. Duncan, D. 1934. A relation between axone diameter and myelination determined by measurement of myelinated spinal root fibres. Journal of Comparative Neurology. 60, 437-471. - another historic paper on the myelin/axon ratio. Rushton, W.A.H. 1951. A theory of the effects of fibre size in medullated nerve. J.Physiology, 115, 101-122. Calculation of best geometry for saltatory conduction. Traill, R.R. 1977/1980/2006 Toward a theoretical explanation of electro-chemical interaction in memory-use. Monograph #24, Cybernetics Department, Brunel University.4, or as Part B of Thesis.5 - showing that other extra signal-modes are possible for such coaxials, which could make myelin even more important. Traill, R.R. 1988. The case that mammalian intelligence is based on sub-molecular memory-coding and fibre-optic capabilities of myelinated nerve axons. Speculations in Science and Technology. 113, 173-181. Traill, R.R. 2005. Strange regularities in the geometry of myelin nerve-insulation - a possible single cause. Ondwelle: Melbourne6 - or in Gen.Sci.J.7. - Offers explanation for the myelin/axon ratio, and other details. optic nerve, physiology subsection; - applies some of this theory. External links The Myelin Project Athabasca University Biological Psychology Website The MS Information Sourcebook, Myelin The Myelin Repair Foundation H E Histology Luxol Fast Blue: Modified Kluver's Method to stain for Myelin Sheath v d e Histology: nervous tissue Neurons gray matter soma - axon axon hillock, axoplasm, axolemma, neurofibril/neurofilament dendrite Nissl body, dendritic spine, apical dendrite, basal dendrite types: bipolar - pseudounipolar - multipolar - pyramidal - Purkinje - granule Afferent nerve/Sensory nerve/Sensory neuron GSA - GVA - SSA - SVA - fibers Ia, Ib or Golgi, II or Aβ, III or Aδ or fast pain, IV or C or slow pain Efferent nerve/Motor nerve/Motor neuron GSE - GVE - SVE - Upper motor neuron - Lower motor neuron α motorneuron, γ motorneuron Synapses neuropil - synaptic vesicle - neuromuscular junction - electrical synapse - Interneuron Renshaw Sensory receptors Meissner's corpuscle - Merkel nerve ending - Pacinian corpuscle - Ruffini ending - Muscle spindle - Free nerve ending - Olfactory receptor neuron - Photoreceptor cell - Hair cell - Taste bud Glial cells Astrocyte Radial glia - Oligodendrocyte - Ependymal cells Tanycytes - Microglia Myelination white matter CNS: Oligodendrocyte PNS: Schwann cell - Neurolemma - Nodes of Ranvier/Internode - Schmidt-Lanterman incisures Related connective tissues epineurium - perineurium - endoneurium - nerve fascicle - meninges v d e Structures of the cell membrane Caveolae/Coated pits - Cell junctions - Glycocalyx - Lipid raft/microdomains - Myelin sheath - Nodes of Ranvier - Nuclear envelope - Phycobilisomes - Porosomes Retrieved from http://en..org/wiki/Myelin Categories: Nervous system | NeuronsHidden categories: All articles with statements | Articles with statements since April 2007 Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages Català Deutsch Español Euskara Français Ido Bahasa Indonesia Ã?slenska Italiano עברית Nederlands 日本語 ‪Norsk bokmÃ¥l‬ Polski Português РуÑ?Ñ?кий Suomi Svenska ä¸æ–‡ This page was last modified on 26 August 2008, at 20:51
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