medullated

有髄

WordNet

1. a nerve fiber encased in a sheath of myelin (同)myelinated_nerve_fiber

Wikipedia preview

出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2017/05/09 20:23:16」(JST)

wiki en

[Wiki en表示]

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 early in the 3rd trimester, 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. In brain, the myelin sheath covers the fibers of corpus callosum which constitute the inner part of cerebral hemisphere.

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

全文を閲覧するには購読必要です。 To read the full text you will need to subscribe.

• 1. 男性型多毛症の病因と原因 pathogenesis and causes of hirsutism
• 2. 小児における甲状腺結節および癌 thyroid nodules and cancer in children

English Journal

• Multiple endocrine neoplasia type 1 (MEN1) and type 4 (MEN4).

• Thakker RV.Author information Academic Endocrine Unit, Radcliffe Department of Medicine, University of Oxford, Oxford Centre for Diabetes, Endocrinology and Metabolism (OCDEM), Churchill Hospital, Headington, Oxford OX3 7LJ, United Kingdom. Electronic address: rajesh.thakker@ndm.ox.ac.uk.AbstractMultiple endocrine neoplasia (MEN) is characterized by the occurrence of tumors involving two or more endocrine glands within a single patient. Four major forms of MEN, which are autosomal dominant disorders, are recognized and referred to as: MEN type 1 (MEN1), due to menin mutations; MEN2 (previously MEN2A) due to mutations of a tyrosine kinase receptor encoded by the rearranged during transfection (RET) protoncogene; MEN3 (previously MEN2B) due to RET mutations; and MEN4 due to cyclin-dependent kinase inhibitor (CDNK1B) mutations. Each MEN type is associated with the occurrence of specific tumors. Thus, MEN1 is characterized by the occurrence of parathyroid, pancreatic islet and anterior pituitary tumors; MEN2 is characterized by the occurrence of medullary thyroid carcinoma (MTC) in association with phaeochromocytoma and parathyroid tumors; MEN3 is characterized by the occurrence of MTC and phaeochromocytoma in association with a marfanoid habitus, mucosal neuromas, medullated corneal fibers and intestinal autonomic ganglion dysfunction, leading to megacolon; and MEN4, which is also referred to as MENX, is characterized by the occurrence of parathyroid and anterior pituitary tumors in possible association with tumors of the adrenals, kidneys, and reproductive organs. This review will focus on the clinical and molecular details of the MEN1 and MEN4 syndromes. The gene causing MEN1 is located on chromosome 11q13, and encodes a 610 amino-acid protein, menin, which has functions in cell division, genome stability, and transcription regulation. Menin, which acts as scaffold protein, may increase or decrease gene expression by epigenetic regulation of gene expression via histone methylation. Thus, menin by forming a subunit of the mixed lineage leukemia (MLL) complexes that trimethylate histone H3 at lysine 4 (H3K4), facilitates activation of transcriptional activity in target genes such as cyclin-dependent kinase (CDK) inhibitors; and by interacting with the suppressor of variegation 3-9 homolog family protein (SUV39H1) to mediate H3K methylation, thereby silencing transcriptional activity of target genes. MEN1-associated tumors harbor germline and somatic mutations, consistent with Knudson's two-hit hypothesis. Genetic diagnosis to identify individuals with germline MEN1 mutations has facilitated appropriate targeting of clinical, biochemical and radiological screening for this high risk group of patients for whom earlier implementation of treatments can then be considered. MEN4 is caused by heterozygous mutations of CDNK1B which encodes the 196 amino-acid CDK1 p27Kip1, which is activated by H3K4 methylation.
• Molecular and cellular endocrinology.Mol Cell Endocrinol.2014 Apr 5;386(1-2):2-15. doi: 10.1016/j.mce.2013.08.002. Epub 2013 Aug 8.
• Multiple endocrine neoplasia (MEN) is characterized by the occurrence of tumors involving two or more endocrine glands within a single patient. Four major forms of MEN, which are autosomal dominant disorders, are recognized and referred to as: MEN type 1 (MEN1), due to menin mutations; MEN2 (previou
• PMID 23933118

• Genetic relationships between dagginess, breech bareness, and wool traits in New Zealand dual-purpose sheep.

• Pickering NK1, Blair HT, Hickson RE, Dodds KG, Johnson PL, McEwan JC.Author information 1AgResearch, Invermay Agricultural Centre, Mosgiel, New Zealand.AbstractGenetic and phenotypic parameters were estimated for dagginess, breech, wool, and fiber traits from approximately 29,500 progeny born in 2009 and 2010 in New Zealand dual-purpose ram breeding sheep flocks. Dagginess is adherence of fecal matter to the wool, and this study investigates the genetic and phenotypic correlations between dagginess and breech and wool traits. Estimates for heritability were moderate (0.21 to 0.44) for the following traits: dag score at 3 and 8 mo (DAG3, DAG8), breech bareness, wool length, wool bulk (BULK), mean fiber diameter, mean fiber diameter SD, mean fiber diameter CV, curvature (CURV), weaning weight at 3 mo, and autumn BW. Heritability estimates for fleece weight at 12 mo and proportion of medullated fibers were high (0.49 and 0.53, respectively). Dag score at 3 mo and DAG8 had low genetic and phenotypic correlations with all traits. Breech bareness had positive genetic and phenotypic correlations with CURV and BULK and mostly negative genetic correlations with all other wool traits. In summary the quantity and attributes of wool were not primary causative factors in fecal accumulation, leaving fecal consistency and composition as the major factors.
• Journal of animal science.J Anim Sci.2013 Oct;91(10):4578-88. doi: 10.2527/jas.2013-6741. Epub 2013 Jul 26.
• Genetic and phenotypic parameters were estimated for dagginess, breech, wool, and fiber traits from approximately 29,500 progeny born in 2009 and 2010 in New Zealand dual-purpose ram breeding sheep flocks. Dagginess is adherence of fecal matter to the wool, and this study investigates the genetic an
• PMID 23893990

• Effects of electroacupuncture of different intensities on energy metabolism of mitochondria of brain cells in rats with cerebral ischemia-reperfusion injury.

• Tian WQ1, Peng YG, Cui SY, Yao FZ, Li BG.Author information 1Department of Anesthesiology, Affiliated Hospital of Nanjing Medical University of Traditional Chinese Medicine, Nanjing, 210029, China, twq1972@163.com.AbstractOBJECTIVE: To observe the effects of electroacupuncture (EA) of different intensities on lactate dehydrogernase (LDH), succinate dehydrogenase (SDH) and ATPase in brain tissue of rats with cerebral ischemia-reperfusion injury (CI/R).
• Chinese journal of integrative medicine.Chin J Integr Med.2013 Sep 3. [Epub ahead of print]
• OBJECTIVE: To observe the effects of electroacupuncture (EA) of different intensities on lactate dehydrogernase (LDH), succinate dehydrogenase (SDH) and ATPase in brain tissue of rats with cerebral ischemia-reperfusion injury (CI/R).METHODS: Forty male SD rats were uniformly randomized into sham ope
• PMID 24002710

Japanese Journal

• Huge Supratentorial Extraventricular Anaplastic Ependymoma Presenting With Massive Calcification : Case Report

• Neurologia medico-chirurgica = 神経外科 50(2), 150-153, 2010-02-15
• NAID 10025950398

• 酵素抗体法による無髄毛髪のABO式血液型検査

• 日本法科学技術学会誌 15(1), 39-48, 2010-01
• NAID 40017003499

• 酵素抗体法による無髄毛髪の　ＡＢＯ　式血液型検査

• 日本法科学技術学会誌 15(1), 39-48, 2010
• NAID 130004485347

Related Links

• Medullated | Definition of medullated by Merriam-Webster

Definition of MEDULLATED 1 : myelinated 2 : having a medulla —used of fibers other than nerve fibers Ask The Editor Videos Lay vs. Lie 'Try and' vs. 'Try to' Is It ...

• Medullated | Define Medullated at Dictionary.com

medullated in Medicine Expand medullated med·ul·lat·ed (měd'l-ā'tĭd) adj. Myelinated. Having a medulla. The American Heritage® Stedman's Medical Dictionary ...

Related Pictures

---

★リンクテーブル★

リンク元	「有髄」
拡張検索	「medullated nerve」

「有髄」

　　[★]

英

medullated, myelinated

「medullated nerve」

　　[★]

• 有髄神経

関

myelinated nerve