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Myelin

Myelin

Overview
Myelin is a dielectric
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...

 (electrically insulating) material that forms a layer, the myelin sheath, usually around only the axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....

 of a neuron
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...

. It is essential for the proper functioning of the nervous system
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...

. Myelin is an outgrowth of a type of glial cell
Glial cell
Glial cells, sometimes called neuroglia or simply glia , are non-neuronal cells that maintain homeostasis, form myelin, and provide support and protection for neurons in the brain, and for neurons in other parts of the nervous system such as in the autonomous nervous system...

. 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.
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Encyclopedia
Myelin is a dielectric
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...

 (electrically insulating) material that forms a layer, the myelin sheath, usually around only the axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....

 of a neuron
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...

. It is essential for the proper functioning of the nervous system
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...

. Myelin is an outgrowth of a type of glial cell
Glial cell
Glial cells, sometimes called neuroglia or simply glia , are non-neuronal cells that maintain homeostasis, form myelin, and provide support and protection for neurons in the brain, and for neurons in other parts of the nervous system such as in the autonomous nervous system...

. 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 cell
Schwann cell
Schwann cells or neurolemmocytes are the principal glia of the peripheral nervous system . Glial cells function to support neurons and in the PNS, also include satellite cells, olfactory ensheathing cells, enteric glia and glia that reside at sensory nerve endings, such as the Pacinian corpuscle...

s supply the myelin for peripheral neurons, whereas oligodendrocyte
Oligodendrocyte
Oligodendrocytes , or oligodendroglia , are a type of brain cell. They are a variety of neuroglia. Their main function is the insulation of axons in the central nervous system of some vertebrates...

s, specifically of the interfascicular type, myelinate the axons of the central nervous system
Central nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...

. 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. Myelin was discovered in 1854 by Rudolf Virchow
Rudolf Virchow
Rudolph Carl Virchow was a German doctor, anthropologist, pathologist, prehistorian, biologist and politician, known for his advancement of public health...

.

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.

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
Myelin basic protein
Myelin basic protein is a protein believed to be important in the process of myelination of nerves in the central nervous system .MBP was initially sequenced in 1971 after isolation from myelin membranes...

 (MBP), myelin oligodendrocyte glycoprotein
Myelin oligodendrocyte glycoprotein
Myelin Oligodendrocyte Glycoprotein ' is a glycoprotein believed to be important in the process of myelinization of nerves in the central nervous system . In humans this protein is encoded by the MOG gene...

 (MOG), and proteolipid protein (PLP). The primary lipid of myelin is a glycolipid
Glycolipid
Glycolipids are lipids with a carbohydrate attached. Their role is to provide energy and also serve as markers for cellular recognition.-Metabolism:...

 called galactocerebroside
Galactocerebroside
A galactocerebroside is a type of cerebroside consisting of a ceramide with a galactose residue at the 1-hydroxyl moiety.The galactose is cleaved by galactosylceramidase....

. The intertwining hydrocarbon chains of sphingomyelin
Sphingomyelin
Sphingomyelin is a type of sphingolipid found in animal cell membranes, especially in the membranous myelin sheath that surrounds some nerve cell axons. It usually consists of phosphorylcholine and ceramide...

 serve to strengthen the myelin sheath.

Function of myelin layer


The main purpose of a myelin layer (or sheath) is to increase the speed at which impulses
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...

 propagate along the myelinated fiber. Along unmyelinated fibers, impulses move continuously as waves, but, in myelinated fibers, they hop or "propagate by saltation
Saltatory conduction
Saltatory conduction is the propagation of action potentials along myelinated axons from one node of Ranvier to the next node, increasing the conduction velocity of action potentials without needing to increase the diameter of an axon.-Mechanism:Because the cytoplasm of the axon is electrically...

." Myelin increases electrical resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...

 across the cell membrane by a factor of 5,000 and decreases capacitance
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...

 by a factor of 50. 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 PNS
Peripheral nervous system
The peripheral nervous system consists of the nerves and ganglia outside of the brain and spinal cord. The main function of the PNS is to connect the central nervous system to the limbs and organs. Unlike the CNS, the PNS is not protected by the bone of spine and skull, or by the blood–brain...

 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.This is mainly because the central nervous system is enclosed in bone, thus suffering less trauma than the peripheral nervous system.

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.

Demyelination



Demyelination is the loss of the myelin sheath insulating the nerves, and is the hallmark of some neurodegenerative autoimmune diseases, including multiple sclerosis
Multiple sclerosis
Multiple sclerosis is an inflammatory disease in which the fatty myelin sheaths around the axons of the brain and spinal cord are damaged, leading to demyelination and scarring as well as a broad spectrum of signs and symptoms...

, acute disseminated encephalomyelitis
Acute disseminated encephalomyelitis
Acute disseminated encephalomyelitis is an immune mediated disease of the brain. It usually occurs following a viral infection but may appear following vaccination, bacterial or parasitic infection, or even appear spontaneously. As it involves autoimmune demyelination, it is similar to multiple...

, transverse myelitis
Transverse myelitis
Transverse myelitis is a neurological disorder caused by an inflammatory process of the spinal cord, and can cause axonal demyelination. The name is derived from Greek referring to the "spinal cord", and the suffix -itis, which denotes inflammation...

, chronic inflammatory demyelinating polyneuropathy
Chronic inflammatory demyelinating polyneuropathy
Chronic inflammatory demyelinating polyneuropathy is an acquired immune-mediated inflammatory disorder of the peripheral nervous system. The disorder is sometimes called chronic relapsing polyneuropathy. CIDP is closely related to Guillain-Barré syndrome and it is considered the chronic...

, Guillain-Barré Syndrome
Guillain-Barré syndrome
Guillain–Barré syndrome , sometimes called Landry's paralysis, is an acute inflammatory demyelinating polyneuropathy , a disorder affecting the peripheral nervous system. Ascending paralysis, weakness beginning in the feet and hands and migrating towards the trunk, is the most typical symptom...

, central pontine myelinosis, inherited demyelinating diseases such as Leukodystrophy
Leukodystrophy
Leukodystrophy refers to a group of disorders characterized by dysfunction of the white matter of the brain. The leukodystrophies are caused by imperfect growth or development of the myelin sheath, the fatty covering that acts as an insulator around nerve fibers...

, and Charcot Marie Tooth
Charcot-Marie-Tooth disease
Charcot–Marie–Tooth disease- , known also as Morbus Charcot-Marie-Tooth, Charcot-Marie-Tooth neuropathy, hereditary motor and sensory neuropathy , hereditary sensorimotor neuropathy , or peroneal muscular atrophy, is an inherited disorder of nerves that takes different forms...

. 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
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...

 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) or interferon
Interferon
Interferons are proteins made and released by host cells in response to the presence of pathogens—such as viruses, bacteria, or parasites—or tumor cells. They allow communication between cells to trigger the protective defenses of the immune system that eradicate pathogens or tumors.IFNs belong to...

.

Symptoms


Demyelination
Demyelinating disease
A demyelinating disease is any disease of the nervous system in which the myelin sheath of neurons is damaged. This impairs the conduction of signals in the affected nerves, causing impairment in sensation, movement, cognition, or other functions depending on which nerves are involved.The term...

 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 cell
Oligodendrocyte precursor cell
Oligodendrocyte precursor cells in nervous tissue cells precede oligodendrocytes, and may also be able to generate neurons and astrocytes. The principal function of oligodendrocytes is to provide support to axons and to produce the Myelin sheath, which insulates and lowers the effective...

s in the central nervous system and inducing myelin repair with certain antibodies. While there have been some encouraging results in mice (via stem cell
Stem cell
This article is about the cell type. For the medical therapy, see Stem Cell TreatmentsStem cells are biological cells found in all multicellular organisms, that can divide and differentiate into diverse specialized cell types and can self-renew to produce more stem cells...

 transplantation), it is still unknown whether this technique can be effective in replacing myelin loss in humans. Some researchers hypothesize that cholinergic treatments, such as acetylcholinesterase inhibitor
Acetylcholinesterase inhibitor
An acetylcholinesterase inhibitor or anti-cholinesterase is a chemical that inhibits the cholinesterase enzyme from breaking down acetylcholine, increasing both the level and duration of action of the neurotransmitter acetylcholine.- Uses :Acetylcholinesterase inhibitors:* Occur naturally as...

s (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
Oligodendrocyte
Oligodendrocytes , or oligodendroglia , are a type of brain cell. They are a variety of neuroglia. Their main function is the insulation of axons in the central nervous system of some vertebrates...

 cholinergic stimulation, AChEIs
Acetylcholinesterase inhibitor
An acetylcholinesterase inhibitor or anti-cholinesterase is a chemical that inhibits the cholinesterase enzyme from breaking down acetylcholine, increasing both the level and duration of action of the neurotransmitter acetylcholine.- Uses :Acetylcholinesterase inhibitors:* Occur naturally as...

 and other cholinergic treatments like nicotine
Nicotine
Nicotine is an alkaloid found in the nightshade family of plants that constitutes approximately 0.6–3.0% of the dry weight of tobacco, with biosynthesis taking place in the roots and accumulation occurring in the leaves...

 could promote myelination during development and myelin repair in older age.

Dysmyelination


Dysmyelination is characterized by a defective structure and function of myelin sheaths; unlike demyelination, it does not produce lesion
Lesion
A lesion is any abnormality in the tissue of an organism , usually caused by disease or trauma. Lesion is derived from the Latin word laesio which means injury.- Types :...

s. 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
Leukodystrophy
Leukodystrophy refers to a group of disorders characterized by dysfunction of the white matter of the brain. The leukodystrophies are caused by imperfect growth or development of the myelin sheath, the fatty covering that acts as an insulator around nerve fibers...

 (Pelizaeus-Merzbacher disease
Pelizaeus-Merzbacher disease
Pelizaeus–Merzbacher disease is a rare central nervous system disorder in which coordination, motor abilities, and intellectual function are delayed to variable extents.-Classification:...

, Canavan disease
Canavan disease
Canavan disease, also called Canavan-Van Bogaert-Bertrand disease, aspartoacylase deficiency or aminoacylase 2 deficiency, is an autosomal recessive degenerative disorder that causes progressive damage to nerve cells in the brain. Canavan disease is also one of the most common degenerative...

, phenylketonuria
Phenylketonuria
Phenylketonuria is an autosomal recessive metabolic genetic disorder characterized by a mutation in the gene for the hepatic enzyme phenylalanine hydroxylase , rendering it nonfunctional. This enzyme is necessary to metabolize the amino acid phenylalanine to the amino acid tyrosine...

) and schizophrenia
Schizophrenia
Schizophrenia is a mental disorder characterized by a disintegration of thought processes and of emotional responsiveness. It most commonly manifests itself as auditory hallucinations, paranoid or bizarre delusions, or disorganized speech and thinking, and it is accompanied by significant social...

.

See also

  • The Myelin Project
    The Myelin Project
    The Myelin Project is an international scientific research organization aimed at accelerating clinical and translational research focused on myelin repair and to regenerate the nerve's myelin sheath, — a process called remyelination — destroyed in a host of diseases such as multiple...

    , project to re-generate myelin
  • Myelinogenesis
    Myelinogenesis
    Myelinogenesis is the process of sequential myelination or development of a myelin sheaths around nerve fibres of the central nervous system.- Function :...

    , order of myelination of central nervous system.
  • Myelin Repair Foundation
    Myelin Repair Foundation
    The Myelin Repair Foundation is a 5013 not-for-profit organization based in Saratoga, California. The organization applies a collaborative business model to the process of medical research with the aim of accelerating the identification and development of new patient treatments...

    , a non-profit medical research foundation accelerating drug discovery in myelin repair for multiple sclerosis.

External links