WordNet

(of neurons) covered with a layer of myelin (同)medullated
loss of the myelin covering of some nerve fibers resulting in their impaired function

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2012/09/06 17:32:58」(JST)

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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. Myelin 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 fourteenth 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.^[1] Myelin was discovered in 1854 by Rudolf Virchow.^[2]

Composition of myelin

Myelin may be 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.The fat helps to insulate the axons from electrically charged atoms and ions. These electrically charged particles are found in the fluid that surrounds the entire nervous system. Under a microscope myelin look like strings of sausages. Myelin are also a part of the maturation process that leads to a child's fast development, this includes crawling and walking in the first year. The myelin sheath is permeable to some chemicals such as glucose, allowing an external fuel supply for myelinated neurons.^[3]

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.

Function of myelin layer

Transmission electron micrograph of a myelinated axon. Generated at the Electron Microscopy Facility at Trinity College, Hartford, CT.

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 (PNS) die without re-growth. 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 reveal that optic nerve fibers can be regenerated in postnatal rats. This optic nerve 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. This study may lead to further understanding of nerve fiber regeneration in the central nervous system.^{[citation needed]}

Disorders of the myelin sheath

Demyelination

Further information: Demyelinating disease

Demyelination is 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, chronic inflammatory demyelinating polyneuropathy, Guillain-Barré Syndrome, central pontine myelinosis, inherited demyelinating diseases such as Leukodystrophy, and Charcot Marie Tooth. Sufferers of pernicious anaemia can also suffer nerve damage if the condition is not diagnosed quickly. Sub-acute combined degeneration of the spinal 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 (TNF)^[4] or interferon.

Symptoms

Demyelination results in diverse symptoms determined by the functions of the affected neurons. It disrupts signals between the brain and other parts of the body; symptoms differ from patient to patient, and have different presentations upon clinical observation and in laboratory studies.

Typical symptoms include:

blurriness in the central visual field that affects only one eye; may be accompanied by pain upon eye movement;
double vision;
loss of vision/hearing;
odd sensation in legs, arms, chest, or face, such as tingling or numbness (neuropathy);
weakness of arms or legs;
cognitive disruption including speech impairment and 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.

Myelin repair

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 transplantation), it is still unknown whether this technique can be effective in replacing myelin loss in humans.^[5] Some researchers hypothesize that cholinergic treatments, such as acetylcholinesterase inhibitors (AChEIs), may have beneficial effects on myelination, myelin repair, and myelin integrity. It is argued that increasing cholinergic stimulation also acts through subtle trophic effects on brain developmental processes and particularly on oligodendrocytes and the lifelong myelination process they support. It is possible that by increasing oligodendrocyte cholinergic stimulation, AChEIs and other cholinergic treatments like nicotine could promote myelination during development and myelin repair in older age.^[6] Glycogen synthase kinase 3β inhibitors such as Lithium Chloride have been found to promote myelination in mice with damaged facial nerves^[7]

Dysmyelination

Dysmyelination is characterized by a defective structure and function of myelin sheaths; unlike demyelination, it does not produce lesions. Such defective sheaths often arise from genetic mutations affecting the biosynthesis and formation of myelin. The shiverer mouse represents one animal model of dysmyelination. Human diseases where dysmyelination has been implicated include leukodystrophies (Pelizaeus–Merzbacher disease, Canavan disease, phenylketonuria) and schizophrenia.^[8]^[9]^[10]

References

^ Hartline DK, Colman DR (January 2007). "Rapid conduction and the evolution of giant axons and myelinated fibers". Curr. Biol. 17 (1): R29–35. doi:10.1016/j.cub.2006.11.042. PMID 17208176.
^ Virchow R (1854). "Über das ausgebreitete Vorkommen einer dem Nervenmark analogen Substanz in den tierischen Geweben". Virchows Arch. Pathol. Anat. 6: 562–72.
^ Rinholm, Johanne; Bergersen, Linda. "Neuroscience: The wrap that feeds neurons". Nature 487: 435-436. doi:10.1038/487435a.
^ Ledeen RW, Chakraborty G (March 1998). "Cytokines, Signal Transduction, and Inflammatory Demyelination: Review and Hypothesis". Neurochem. Res. 23 (3): 277–89. doi:10.1023/A:1022493013904. PMID 9482240. http://www.ingentaconnect.com/content/klu/nere/1998/00000023/00000003/00421003.
^ [1] FuturePundit January 20, 2004
^ Bartzokis, G (2007-08-15). "Acetylcholinesterase inhibitors may improve myelin integrity.". Biological Psychiatry 62 (4): 294–301. doi:10.1016/j.biopsych.2006.08.020. PMID 17070782.
^ Makouji J, Belle M, Meffre D, Stassart R, Grenier J, Shackleford G G, Fledrich R, Fonte C, Branchu J, Goulard M, de Waele C, Charbonnier F, Sereda M W, Baulieu E, Schumacher M, Bernard S, Massad C. "Lithium enhances remyelination of peripheral nerves". Proceedings of the National Acadmey of Sciences of the United States of America. doi:10.1073/pnas.1121367109. http://www.pnas.org/content/early/2012/02/13/1121367109.abstract.
^ Krämer-Albers EM, Gehrig-Burger K, Thiele C, Trotter J, Nave KA (November 2006). "Perturbed interactions of mutant proteolipid protein/DM20 with cholesterol and lipid rafts in oligodendroglia: implications for dysmyelination in spastic paraplegia". J. Neurosci. 26 (45): 11743–52. doi:10.1523/JNEUROSCI.3581-06.2006. PMID 17093095. http://www.jneurosci.org/cgi/pmidlookup?view=long&pmid=17093095.
^ Matalon R, Michals-Matalon K, Surendran S, Tyring SK (2006). "Canavan disease: studies on the knockout mouse". Adv. Exp. Med. Biol.. Advances in Experimental Medicine and Biology 576: 77–93; discussion 361–3. doi:10.1007/0-387-30172-0_6. ISBN 978-0-387-30171-6. PMID 16802706.
^ Tkachev D, Mimmack ML, Huffaker SJ, Ryan M, Bahn S (August 2007). "Further evidence for altered myelin biosynthesis and glutamatergic dysfunction in schizophrenia". Int. J. Neuropsychopharmacol. 10 (4): 557–63. doi:10.1017/S1461145706007334. PMID 17291371. http://journals.cambridge.org/abstract_S1461145706007334.

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
Radiology and Pathology of Myelin the MedPix Medical Image Database
[2] Archives of Neurology

UpToDate Contents

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1. 小児における神経画像検査へのアプローチ approach to neuroimaging in children
2. シャルコーマリートゥース病などの一次性の遺伝性運動感覚性ニューロパチー hereditary primary motor sensory neuropathies including charcot marie tooth disease
3. シェーグレン・ラルソン症候群 sjogren larsson syndrome
4. 点頭てんかんの臨床的特徴および診断 clinical features and diagnosis of infantile spasms
5. ペリツェウス・メルツバッハー病 pelizaeus merzbacher disease

English Journal

Alterations of p75 neurotrophin receptor and Myelin transcription factor 1 in the hippocampus of perinatal phencyclidine treated rats.

Andrews JL1, Newell KA2, Matosin N3, Huang XF4, Fernandez-Enright F5.
Progress in neuro-psychopharmacology & biological psychiatry.Prog Neuropsychopharmacol Biol Psychiatry.2015 Dec 3;63:91-7. doi: 10.1016/j.pnpbp.2015.06.003. Epub 2015 Jun 10.
Postnatal administration of phencyclidine (PCP) in rodents causes major disturbances to neurological processes resulting in severe modifications to normal behavioral traits into adulthood. It is routinely used to model psychiatric disorders such as schizophrenia, producing many of the dysfunctional
PMID 26071990

Deviant white matter structure in adults with attention-deficit/hyperactivity disorder points to aberrant myelination and affects neuropsychological performance.

Onnink AM1, Zwiers MP2, Hoogman M3, Mostert JC4, Dammers J1, Kan CC5, Vasquez AA6, Schene AH5, Buitelaar J7, Franke B8.
Progress in neuro-psychopharmacology & biological psychiatry.Prog Neuropsychopharmacol Biol Psychiatry.2015 Dec 3;63:14-22. doi: 10.1016/j.pnpbp.2015.04.008. Epub 2015 May 5.
Attention-deficit/hyperactivity disorder (ADHD) in childhood is characterized by gray and white matter abnormalities in several brain areas. Considerably less is known about white matter microstructure in adults with ADHD and its relation with clinical symptoms and cognitive performance. In 107 adul
PMID 25956761

Topographical effects on fiber-mediated microRNA delivery to control oligodendroglial precursor cells development.

Diao HJ1, Low WC1, Lu QR2, Chew SY3.
Biomaterials.Biomaterials.2015 Nov;70:105-14. doi: 10.1016/j.biomaterials.2015.08.029. Epub 2015 Aug 18.
Effective remyelination in the central nervous system (CNS) facilitates the reversal of disability in patients with demyelinating diseases such as multiple sclerosis. Unfortunately until now, effective strategies of controlling oligodendrocyte (OL) differentiation and maturation remain limited. It i
PMID 26310106