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Encyclopedia > Myelin
Myelin sheath
Structure of a typical neuron

Myelin is an electrically insulating phospholipid layer that surrounds the axons of many neurons. It is an outgrowth of glial cells: Schwann cells supply the myelin for peripheral neurons while 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 1878 by Louis-Antoine Ranvier. Image File history File links Neuron-no_labels. ... Dendrites (from Greek dendron, “tree”) are the branched projections of a neuron that act to conduct the electrical stimulation received from other neural cells to the cell body, or soma, of the neuron from which the dendrites project. ... The soma, or perikaryon, is the bulbous end of a neuron, containing the cell nucleus. ... An axon or nerve fiber, is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neurons cell body or soma. ... HeLa cells stained for DNA with the Blue Hoechst dye. ... Nodes of Ranvier are regularly spaced gaps in the myelin sheath around an axon or nerve fiber. ... Nodes of Ranvier are regularly spaced gaps in the myelin sheath around an axon or nerve fiber. ... Illustration of the major elements in a prototypical synapse. ... Named after the German physiologist Theodor Schwann, Schwann cells are a variety of neuroglia that mainly provide myelin insulation to axons in the peripheral nervous system of jawed vertebrates. ... Phospholipid Two schematic representations of a phospholipid. ... An axon or nerve fiber, is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neurons cell body or soma. ... Drawing by Santiago Ramón y Cajal of neurons in the pigeon cerebellum. ... Neuroglia of the brain shown by Golgis method. ... Named after the German physiologist Theodor Schwann, Schwann cells are a variety of neuroglia that mainly provide myelin insulation to axons in the peripheral nervous system of jawed vertebrates. ... Oligodendrocytes (from Greek literally meaning few tree cells), or oligodendroglia (Greek, few tree glue),[1] are a variety of neuroglia. ... A diagram showing the CNS: 1. ... Classes Placodermi Chondrichthyes Acanthodii Osteichthyes Gnathostomata is the group of vertebrates with jaws. ... Louis-Antoine Ranvier (b. ...

Contents

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 fat 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. Some common lipids. ... For other uses, see FAT. Fats consist of a wide group of compounds that are generally soluble in organic solvents and largely insoluble in water. ... A representation of the 3D structure of myoglobin, showing coloured alpha helices. ... Myelin basic protein (MBP) is a protein believed to be important in the process of myelination of nerves in the central nervous system (CNS). ... Myelin Oligodendrocyte Glycoprotein (MOG) is a glycoprotein believed to be important in the process of myelinization of nerves in the central nervous system (CNS). ... Proteolipid protein 1 is a protein associated with Pelizaeus-Merzbacher disease. ... Glycolipids are carbohydrate-attached lipids. ... Sphingomyelin is a type of sphingolipid found in animal cell membranes, especially in the membranous myelin sheath which surrounds some nerve cell axons. ...


Function of myelin layer

Transmission electron micrograph of a myelinated neuron. 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 Image File history File linksMetadata No higher resolution available. ... Image File history File linksMetadata No higher resolution available. ... A. A schematic view of an idealized action potential illustrates its various phases as the action potential passes a point on a cell membrane. ... Saltatory conduction (from the Latin saltare, to hop or leap) is a means by which action potentials are transmitted along myelinated nerve fibers. ...


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, transverse myelitis, chronic inflammatory demyelinating polyneuropathy, Guillain-Barre Syndrome. 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. Neurodegenerative disease is a condition which affects brain function. ... Autoimmune diseases arise from an overactive immune response of the body against substances and tissues normally present in the body. ... Transverse myelitis is a neurological disorder caused by a loss of the myelin encasing the spinal cord, also known as demyelination. ... Chronic inflammatory demyelinating polyneuropathy (CIDP) is an acquired immune-mediated inflammatory disorder of the peripheral nervous system but often can have central nervous system involvement. ... Guillain-Barré syndrome (GBS), is an acquired immune-mediated inflammatory disorder of the peripheral nervous system (i. ...


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)[2] or interferon. A scanning electron microscope image of a single neutrophil (yellow), engulfing anthrax bacteria (orange). ... Cytokines are small protein molecules that are the core of communication between immune system cells, and even between immune system cells and cells belonging to other tissue types. ... In medicine, tumor necrosis factor alpha (TNFα, cachexin or cachectin) is an important cytokine involved in systemic inflammation and the acute phase response. ... Interferons (IFNs) are natural proteins produced by the cells of the immune system of most vertebrates in response to challenges by foreign agents such as viruses, bacteria, parasites and tumor cells. ...


Heavy metal poisoning may also lead to demyelination. Even very small amounts of mercury have been shown to be particularly destructive to nerve sheaths.[3] A heavy metal is any of a number of higher atomic weight elements, which has the properties of a metallic substance at room temperature. ... General Name, Symbol, Number mercury, Hg, 80 Chemical series transition metals Group, Period, Block 12, 6, d Appearance silvery Standard atomic weight 200. ...


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.[4] Oligodendrocyte precursor cells in nervous tissue cells precede oligodendrocytes, and may also be able to generate neurons and astrocytes. ... Mouse embryonic stem cells with fluorescent marker. ...


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) and schizophrenia. [5] [6] [7]


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. Look up Heterogeneous in Wiktionary, the free dictionary. ... Pathophysiology is the study of the disturbance of normal mechanical, physical, and biochemical functions, either caused by a disease, or resulting from a disease or abnormal syndrome or condition that may not qualify to be called a disease. ...

  • 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

The Myelin Project is a medical project intended to re-generate the nerves myelin sheath, — a process called remyelination — destroyed in a host of diseases such as multiple sclerosis and the leukodystrophies. ... Myelinogenesis is the process of sequential myelination of the parts of the central nervous system whose order approximates the evolutionary development of the central nervous system. ...

References

  1. ^ http://www.pbrc.hawaii.edu/~danh/InvertebrateMyelin/
  2. ^ [1] Ledeen R.W., Chakraborty G., "Cytokines, Signal Transduction, and Inflammatory Demyelination: Review and Hypothesis" Neurochemical Research, Volume 23, Number 3, March 1998, pp. 277-289(13)
  3. ^ [2] University of Calgary: How Mercury Causes Brain Neuron Degeneration
  4. ^ [3] FuturePundit January 20, 2004
  5. ^ Krämer-Albers at al., 2006
  6. ^ Matalon et al., 2006
  7. ^ Tkachev et al., 2007
  • Vlassara H, Brownlee M, Cerami A. Recognition and uptake of human diabetic peripheral nerve myelin by macrophages. Diabetes. 1985 Jun;34(6):553-7. PMID: 4007282
  • Thornalley PJ. Glycation in diabetic neuropathy: characteristics, consequences, causes, and therapeutic options. Int Rev Neurobiol. 2002;50:37-57. PMID: 12198817
  • Krämer-Albers EM, Gehrig-Burger K, Thiele C, Trotter J, Nave KA. Perturbed interactions of mutant proteolipid protein/DM20 with cholesterol and lipid rafts in oligodendroglia: implications for dysmyelination in spastic paraplegia. J Neurosci. 2006 Nov 8;26(45):11743-52.PMID: 17093095
  • Matalon R, Michals-Matalon K, Surendran S, Tyring SK. Canavan disease: studies on the knockout mouse. Adv Exp Med Biol. 2006;576:77-93.PMID: 16802706
  • Tkachev D, Mimmack ML, Huffaker SJ, Ryan M, Bahn S. Further evidence for altered myelin biosynthesis and glutamatergic dysfunction in schizophrenia.Int J Neuropsychopharmacol. 2007 Aug;10(4):557-63.PMID: 17291371

External links


  Results from FactBites:
 
Myelin - Wikipedia, the free encyclopedia (402 words)
Myelin is made up primarily of a sphingolipid called sphingomyelin, and it is thought that the intertwining of the hydrocarbon chains of sphingomyelin serve to strengthen the myelin sheath.
The main consequence of a myelin layer (or sheath) is an increase in the speed at which impulses propagate along the myelinated fiber.
Demyelination is a loss of myelin and is the root cause of symptoms experienced by patients with diseases such as multiple sclerosis and transverse myelitis.
  More results at FactBites »

 
 

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