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. Dendrites (from Greek δένδρον déndron, “tree” are the branched projections of a Neuron that act to conduct the electrochemical The soma, or cyton or perikaryon, is the bulbous end of a Neuron, containing the Cell nucleus. An axon or nerve fiber is a long slender projectionof a nerve cell or Neuron, that conducts electrical impulses away from the neuron's Cell In Cell biology, the nucleus (pl nuclei; from Latin la ''nucleus'' or la ''nuculeus'' "little nut" or kernel is a membrane-enclosed Nodes of Ranvier are known as the gaps (about 1 micrometer in diameter formed between myelin sheath cells along axons or nerve fibers Nodes of Ranvier are known as the gaps (about 1 micrometer in diameter formed between myelin sheath cells along axons or nerve fibers Chemical synapses are specialized junctions through which Neurons signal to each other and to non-neuronal cells such as those in Muscles or Glands Named after the German physiologist Theodor Schwann, Schwann cells (also referred to as neurolemmocytes) are a variety of Glial cell that mainly A dielectric is a nonconducting substance ie an insulator. The term was coined by William Whewell in response to a request from Michael Faraday. Phospholipids are a class of Lipids and are a major component of all Biological membranes All phospholipids contain a Diglyceride, a Phosphate An axon or nerve fiber is a long slender projectionof a nerve cell or Neuron, that conducts electrical impulses away from the neuron's Cell Neurons (ˈnjuːɹɒn also known as neurones and nerve cells) are responsive cells in the Nervous system that process and transmit information 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. Glial cells, commonly called neuroglia or simply glia (Greek for "glue" are non- Neuronal cells that provide support and nutrition Named after the German physiologist Theodor Schwann, Schwann cells (also referred to as neurolemmocytes) are a variety of Glial cell that mainly Origin Oligodendroglia arise during development from an Oligodendrocyte precursor cell, which can be identified by its expression of a number of Antigens, including In Vertebrates the central nervous system ( CNS) is the part of the Nervous system which is enclosed in the Meninges. Myelin is considered a defining characteristic of the (gnathostome) vertebrates, but it has also arisen by parallel evolution in some invertebrates. Gnathostomata is the group of Vertebrates with Jaws The group is traditionally a superclass, including the familiar classes of Fish, ^[1] Myelin was discovered in 1878 by Louis-Antoine Ranvier. Louis-Antoine Ranvier (b Lyon, France, October 2, 1835; d Vendranges, France March 22, 1922, French

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. Lipids are broadly defined as any fat- Soluble ( lipophilic) naturally-occurring Molecule, such as fats oils waxes cholesterol sterols fat-soluble Fats consist of a wide group of compounds that are generally soluble in organic solvents and largely insoluble in water Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl Some of the proteins that make up myelin are Myelin basic protein (MBP), Myelin oligodendrocyte glycoprotein (MOG), and Proteolipid protein (PLP). Myelin basic protein ( MBP) is a Protein believed to be important in the process of myelination of Nerves in the Central nervous system Myelin Oligodendrocyte Glycoprotein (MOG is a Glycoprotein believed to be important in the process of Myelinization of Nerves in the Central Proteolipid protein 1 is a protein associated with Pelizaeus-Merzbacher disease. Myelin is made up primarily of a glycolipid called galactocerebroside. Glycolipids are Carbohydrate -attached Lipids. Their role is to provide Energy and also serve as markers for cellular recognition A galactocerebroside (or galactosylceramide) is a type of Cerebroside consisting of a Ceramide with a Galactose residue at the 1-hydroxyl moiety The intertwining of the hydrocarbon chains of sphingomyelin serve to strengthen the myelin sheath. Sphingomyelin (SPH is a type of Sphingolipid found in animal Cell membranes, especially in the membranous Myelin sheath which surrounds some Nerve

Function of myelin layer

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. In Neurophysiology, the action potential is a self-regenerating Wave of Electrochemical activity that allows Nerve cells to carry a signal Along unmyelinated fibers, impulses move continuously as waves, but, in myelinated fibers, they hop or "propagate by saltation. For Saltation definition and other use disambiguation see Saltation Saltatory conduction (from the Latin saltare, to hop or leap is " Myelin increases resistance across the cell membrane by a factor of 5,000 and decreases capacitance by a factor of 50. 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. Neurodegenerative Disease ( Greek νέυρο- néuro-, "nerval" and Latin dēgenerāre, "to decline" or "to Autoimmunity is the failure of an organism to recognize its own constituent parts as self, which results in an immune response against its own cells and tissues Multiple sclerosis (abbreviated MS also known as disseminated sclerosis or encephalomyelitis disseminata) is an autoimmune condition in which the Acute disseminated encephalomyelitis ( ADEM) is an immune mediated Disease of the brain. Transverse myelitis is a Neurological disorder caused by an Inflammatory process of the grey and white matter of the Spinal cord, Alexander disease is a slowly progressing and fatal Neurodegenerative disease. Chronic Inflammatory Demyelinating Polyneuropathy (CIDP is an acquired immune-mediated inflammatory disorder of the Peripheral nervous system but often can have Guillain-Barré syndrome ( GBS) (in French ɡilɛ̃ baˈʁe in English ˈɡiːlæn ˈbɑreɪ /ɡiːˈæn bəˈreɪ/ etc Central pontine myelinolysis or osmotic demyelination syndrome is a demyelinating lesion in the brain that occurs with rapid correction of Hyponatremia 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 (TNF)^[2] or interferon. An immune system is a collection of mechanisms within an Organism that protects against Disease by identifying and killing Pathogens and Tumor Cytokines are a category of signalling Proteins and Glycoproteins that like Hormones and Neurotransmitters, are used extensively in cellular Tumor necrosis factor ( TNF, cachexin or cachectin and formally known as Tumor necrosis factor-alpha) is a Cytokine involved in systemic Interferons ( IFN s are natural Proteins produced by the cells of the Immune system of most Vertebrates in response to challenges by foreign agents

Heavy metal poisoning may also lead to demyelination. Even very small amounts of mercury have been shown to be particularly destructive to nerve sheaths. Mercury (ˈmɜrkjʊri also called quicksilver or hydrargyrum, is a Chemical element with the symbol Hg ( Latinized hydrargyrum ^[3]

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. Oligodendrocyte precursor cells in nervous tissue cells precede Oligodendrocytes, and may also be able to generate neurons and astrocytes 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. Stem cells are cells found in most if not all multi-cellular Organisms. ^[4]

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. Leukodystrophy refers to a group of disorders characterized by progressive degeneration of the White matter of the brain Pelizaeus-Merzbacher disease (PMD is a rare Central nervous system disorder in which coordination motor abilities and intellectual function are delayed to variable extents Canavan disease, also called Canavan-Van Bogaert-Bertrand disease, aspartoacylase deficiency or aminoacylase 2 deficiency, is an autosomal Phenylketonuria ( PKU) is an Autosomal recessive Genetic disorder characterized by a deficiency in the enzyme Phenylalanine hydroxylase Schizophrenia ( from the Greek roots schizein (σχίζειν "to split" and phrēn ^[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. Heterogeneous is an adjective used to describe an object or system consisting of multiple items having a large number of structural variations Pathophysiology is the study of the disturbance of normal Mechanical, Physical, and Biochemical functions either caused by a Disease, or resulting

• 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

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. 34(6):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. For Saltation definition and other use disambiguation see Saltation Saltatory conduction (from the Latin saltare, to hop or leap is ]
• Traill, R. R. (1977/1980/2006) Toward a theoretical explanation of electro-chemical interaction in memory-use. Monograph #24, Cybernetics Department, Brunel University. Brunel University is a University situated in West London, England. [[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. 11(3), 173-181.
• Traill, R. R. (2005). Strange regularities in the geometry of myelin nerve-insulation — a possible single cause. Ondwelle: Melbourne[[6]] — 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]. The optic nerve, also called cranial nerve II, is the Nerve that transmits visual information from the Retina to the Brain.

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