myelinization

　

WordNet

1. the development of a myelin sheath around a nerve fiber (同)myelinisation
2. able to make fine distinctions; "a subtle mind"

PrepTutorEJDIC

1. (知覚しにくいほど)『かすかな』,ほのかな,微妙な / (頭脳・感覚などが)鋭い / ずるい,こうかつな / 難解な / 巧みな,精巧な

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2016/04/13 14:44:51」(JST)

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Structure of a typical neuron

Myelin sheath
Dendrite Soma Axon Nucleus Node of Ranvier Axon terminal Schwann cell Myelin sheath

Myelin is a fatty white substance that surrounds the axon of some nerve cells, forming an electrically insulating layer. It is essential for the proper functioning of the nervous system. It is an outgrowth of a type of glial cell.

The production of the myelin sheath is called myelination or myelinogenesis. In humans, myelination begins in the 14th week of fetal development, although little myelin exists in the brain at the time of birth. During infancy, myelination occurs quickly, leading to a child's fast development, including crawling and walking in the first year. Myelination continues through the adolescent stage of life.

Schwann cells supply the myelin for the peripheral nervous system, 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 been seen in some invertebrates, although they are quite different from vertebrate myelin at the molecular level. Myelin was discovered in 1854 by Rudolf Virchow.

Composition

Transmission electron micrograph of a cross-section of a myelinated axon, generated at the Electron Microscopy Facility at Trinity College, Hartford, CT

Myelin is made by different cell types, and 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 helps to insulate the axons from electrically charged atoms and molecules. These charged particles (ions) are found in the fluid surrounding the entire nervous system. Under a microscope, myelin looks like strings of sausages.

Cholesterol is an essential constituent of myelin. Myelin is about 40% water; the dry mass is about 70–85% lipids and about 15–30% proteins. Some of the proteins are myelin basic protein (MBP, which makes up ~30% of myelin protein), myelin oligodendrocyte glycoprotein, and proteolipid protein (PLP, which makes up ~50% of myelin protein). The primary lipid of myelin is a glycolipid called galactocerebroside. The intertwining hydrocarbon chains of sphingomyelin serve to strengthen the myelin sheath.

Function

Action potential propagation in myelinated neurons is faster than in unmyelinated neurons because of Saltatory conduction.

Cross section of a myelinated axon
1. Axon
2. Nucleus of Schwann Cell
3. Schwann Cell
4. Myelin Sheath

5. Neurilemma

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 saltatory conduction. Myelin decreases capacitance and increases electrical resistance across the cell membrane (the axolemma). Thus, myelination helps prevent the electric current from leaving the axon. It has been suggested that myelin permits larger body size by maintaining agile communication between distant body parts.

Myelinated fibers lack voltage-gated ion channels (approximately 25 μm⁻²) along the myelinated internodes, exposing them only at the nodes of Ranvier. Here, they are found far more abundantly (between 2,000 and 12,000 μm⁻²). Myelinated fibers succeed in reducing sodium leakage into the extracellular fluid (ECF), maintaining a strong separation of charge between the intracellular fluid (ICF) and the ECF. This increases sodium’s ability to travel along the axon more freely. However, the sodium diffuses along the axolemma rapidly, but is decremental by nature. The sodium cannot trigger the opening of the voltage-gated sodium channels as it becomes weaker. The nodes of Ranvier, being exposed to the ECF every 1 mm or so, contain large amounts of voltage-gated sodium channels, and allow enough sodium into the axon to regenerate the action potential. Each time the action potential reaches a node of Ranvier, it is restored to its original action potential (+35 mV).

When a peripheral fiber is severed, the myelin sheath provides a track along which regrowth can occur. However, 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 die without regrowth. 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 have revealed that optic nerve fibers can be regenerated in postnatal rats. This 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. These studies may lead to further understanding of nerve fiber regeneration in the central nervous system.

Disorders

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, neuromyelitis optica, transverse myelitis, chronic inflammatory demyelinating polyneuropathy, Guillain-Barré syndrome, central pontine myelinosis, inherited demyelinating diseases such as leukodystrophy, and Charcot-Marie-Tooth disease. Sufferers of pernicious anaemia can also suffer nerve damage if the condition is not diagnosed quickly. Subacute combined degeneration of spinal cord secondary to pernicious anaemia can lead to 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. A more serious case of myelin deterioration is called Canavan Disease.

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 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 or 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
• tinnitus

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 results in mice have been encouraging (via stem cell transplantation), whether this technique can be effective in replacing myelin loss in humans is still unknown. Cholinergic treatments, such as acetylcholinesterase inhibitors (AChEIs), may have beneficial effects on myelination, myelin repair, and myelin integrity. Increasing cholinergic stimulation also may act through subtle trophic effects on brain developmental processes and particularly on oligodendrocytes and the lifelong myelination process they support. By increasing oligodendrocyte cholinergic stimulation, AChEIs, and other cholinergic treatments, such as nicotine, possibly could promote myelination during development and myelin repair in older age. Glycogen synthase kinase 3β inhibitors such as lithium chloride have been found to promote myelination in mice with damaged facial nerves. Cholesterol is a necessary nutrient for the myelin sheath.

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.

Invertebrate myelin

Functionally equivalent myelin-like sheaths are found in several invertebrate taxa including Oligochaete, Penaeid, Palaemonid, and Calanoids. These myelin-like sheaths share several structural features with the sheaths found in vertebrates including multiplicity of membranes, condensation of membrane, and nodes. However, the nodes in vertebrates are annular; i.e., they encircle the axon. In contrast, nodes found in the sheaths of invertebrates are either annular or fenestrated; i.e., they are restricted to "spots." It is notable that the fastest recorded conduction speed (across both vertebrates and invertebrates) is found in the ensheathed axons of the Kuruma shrimp, an invertebrate.

See also

• The Myelin Project, project to regenerate myelin
• Myelin Repair Foundation, a nonprofit medical research foundation for multiple sclerosis drug discovery.
• Myelin protein zero

References

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
• [1] Archives of Neurology
• American Society for Neurochemistry

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. ペリツェウス・メルツバッハー病 pelizaeus merzbacher disease
• 3. 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
• 4. シェーグレン・ラルソン症候群 sjogren larsson syndrome
• 5. ペルオキシソーム病 peroxisomal disorders

English Journal

• The effect of different application timings of hyperbaric oxygen treatment on nerve regeneration in rats.

• Ince B1, Arslan A2, Dadaci M2, Oltulu P3, Bilgen F4.
• Microsurgery.Microsurgery.2016 Jan 16. doi: 10.1002/micr.30023. [Epub ahead of print]
• BACKGROUND: The aim of the study was to determine the effect of different application timings of hyperbaric oxygen treatment (HBO) on nerve regeneration in rats.MATERIALS AND METHODS: A total of forty 12-week-old female Wistar albino rats were used. The sciatic nerve was transected. The nerve ends w
• PMID 26773276

• On the Evolution of the Mammalian Brain.

• Torday JS1, Miller WB Jr2.
• Frontiers in systems neuroscience.Front Syst Neurosci.2016 Apr 19;10:31. doi: 10.3389/fnsys.2016.00031. eCollection 2016.
• Hobson and Friston have hypothesized that the brain must actively dissipate heat in order to process information (Hobson et al., 2014). This physiologic trait is functionally homologous with the first instantation of life formed by lipids suspended in water forming micelles- allowing the reduction i
• PMID 27147985

• [Microanatomy of the cranial segment of the vestibulocochlear nerve. Possible correlations with the symptoms of neurovascular compression syndrome].

• Kunel'skaya NL1, Yatskovsky AN2, Mishchenko VV1.
• Vestnik otorinolaringologii.Vestn Otorinolaringol.2016;81(1):25-8.
• in

English, RussianЦель исследования — проанализировать топографию нервных волокон, принадлежащих к слуховому и вестибулярному анализаторам, в составе интракраниальн�

• PMID 26977563

Japanese Journal

• 脳形成異常における乳児期の脳血流SPECT所見

• 吉成 聡,浜野 晋一郎,日暮 憲道,田中 学,南谷 幹之,折津 友隆
• てんかん研究 26(3), 453-460, 2009-01-31
• 成人における皮質形成異常の病変部位における脳血流SPECT所見は、低灌流であることが多いが、逆に高灌流を呈したという報告もある。小児の皮質形成異常における脳血流に関する報告は少ない。今回我々は小児4例の皮質形成異常における脳血流SPECT検査と脳波検査を行い検討した。乳児期早期は全例で病変部位に一致して高灌流を認めるも、二回目に施行した脳血流所見では3例は低灌流に、また1例は等灌流に転じ、その判読 …
• NAID 110007046199

• 聴皮質の発達と可塑性

• 内藤 泰
• 音声言語医学 42(3), 264-271, 2001-07-20
• … The primary auditory cortex and the auditory association area are different not only in their cytoarchitecture but also in their timing of myelinization during development. …
• NAID 10010871112

Related Links

• myelinization - Medical Dictionary - The Free Dictionary

myelinization /my·elin·i·za·tion/ (mi″ĕ-lin″ĭ-za´shun) the act of adding myelin; formation of a myelin sheath. myelinization. [mī′əlin′īzā′shən]. Etymology: Gk , myelos + izein, to cause. development of the myelin sheath around a nerve fiber .

• myelinization - The Free Dictionary

my·e·li·ni·za·tion (m -l -n -z sh n) also my·e·li·na·tion (-n sh n). n. The process of forming a myelin sheath. Thesaurus Legend: Synonyms Related Words Antonyms. Noun, 1. myelinization - the development of a myelin sheath around a nerve ...

Related Pictures

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★リンクテーブル★

リンク元

「subtle」

「subtle」

　　[★]

• adj.

• 繊細な。微妙な、とらえにくい、かすかな、ほのかな

These changes may be diagnostically useful and appear as increased FLAIR and T2 signal intensity in the posterior limb of the internal capsule, descending motor tracts of the brainstem, and spinal cord, all of which are subtle and may be missed.

CT scanning is used to detect intracranial calcifications and MRI to detect subtle abnormalities in myelinization.

The signs and symptoms of spontaneous bacterial peritonitis are subtle compared with those of patients who have surgical peritonitis in the absence of ascites.

• ex.

• subtle sign

(a subtle sign)かすかな兆し/兆候/前兆

関

a fraction of, dim, faint, faintly, few, insignificant, little, marginally, modest, modestly, only, scant, slight, slightly, subtly