Myelin

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Myelin

Myelin is an electrically-insulating dielectric

Dielectric

A dielectric is a nonconducting substance, i.e. an Insulator . The term was coined by William Whewell in response to a request from Michael Faraday....
material that forms a layer, the myelin sheath. Usually, myelin surrounds only the axon

Axon

An axon or nerve fiber is a long, slender projectionof a nerve cell, or neuron, that conducts action potentialaway from the neuron's cell body or soma....
of a neuron

Neuron

Neurons are responsive cell in the nervous system that process and transmit information by electrochemical Signal . They are the core components of the brain, the vertebrate spinal cord, the invertebrate ventral nerve cord, and the peripheral nerves....
. It is essential for proper functioning of the nervous system

Nervous system

The nervous system is a Neural network of specialized cells that communicate information about an animal's surroundings and itself. It processes this information and causes reactions in other parts of the body....
. Myelin is an outgrowth glial cell

Glial cell

Glial cells, commonly called neuroglia or simply glia , are non-neuronal cell that provide support and nutrition, maintain homeostasis, form myelin, and participate in signal transmission in the nervous system....
: Schwann cell

Schwann cell

Named after the Germany physiologist Theodor Schwann, Schwann cells are a variety of glial cell that keep peripheral nerve fibres alive. In myelinated axons, Schwann cells form the myelin sheath ....
s supply the myelin for peripheral neurons, whereas oligodendrocyte

Oligodendrocyte

Oligodendrocytes , or oligodendroglia , are a variety of neuroglia. Their main function is the insulation of the axons exclusively in the central nervous system of the higher vertebrates, a function performed by Schwann cells in the peripheral nervous system....
s supply it to those of the central nervous system

Central nervous system

The central nervous system is the part of the nervous system that functions to coordinate the activity of all parts of the bodies of multicellular organisms....
. Myelin is considered a defining characteristic of the (gnathostome) vertebrates, but it has also arisen by parallel evolution in some invertebrates.

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Encyclopedia

Myelin is an electrically-insulating dielectric

Dielectric

Axon

An axon or nerve fiber is a long, slender projectionof a nerve cell, or neuron, that conducts action potentialaway from the neuron's cell body or soma....
of a neuron

Neuron

Nervous system

Glial cell

Schwann cell

Oligodendrocyte

Central nervous system

Rudolf Virchow

Rudolf Ludwig Karl Virchow was a Medicine, Anthropology, public health activist, Pathology, prehistorian, biologist and politician. He is referred to as the "Father of Pathology," and founded the field of Social Medicine....
.

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

Lipid

Lipids are broadly defined as any fat-soluble , naturally-occurring molecule, such as fats, oils, waxes, cholesterol, sterols, fat-soluble vitamins , monoglycerides, diglycerides, phospholipids, and others....
and about 20% protein

Protein

Proteins are organic compounds made of amino acids arranged in a linear chain and joined together by peptide bonds between the carboxyl and amino groups of adjacent amino acid Residue ....
. 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 myelin of nerves in the central nervous system .MBP was initially sequencing in 1979 after isolation from myelin cell 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 ....
(MOG), and Proteolipid protein (PLP). Myelin is made up primarily of a glycolipid

Glycolipid

Glycolipids are carbohydrate-attached lipids. Their role is to provide energy and also serve as genetic marker for Cell recognition.They occur where a carbohydrate chain is associated with phospholipids on the exoplasmic surface of the cell biological membrane....
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 of the hydrocarbon chains of sphingomyelin

Sphingomyelin

Sphingomyelin, , is a type of sphingolipid found in animal cell membranes, especially in the membranous myelin sheath which surrounds some nerve cell axons....
serve to strengthen the myelin sheath.

Function of myelin layer

The main purpose of a myelin layer (or sheath) is an increase in the speed at which impulses

Action potential

An action potential is a self-regenerating wave of electrochemical activity that allows nerve cells to carry a signal over a distance. It is the primary electrical signal generated by nerve cells, and arises from changes in the permeability of the nerve cell's axonal Cell membranes to specific ions....
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 a means by which action potentials are transmitted along myelinated axons....
." Myelin increases electrical resistance

Electrical resistance

The electrical resistance of an object is a measure of its opposition to the passage of a steady electrical current. An object of uniform cross section will have a resistance proportional to its length and inversely proportional to its cross-sectional area, and proportional to the resistivity of the material....
across the cell membrane by a factor of 5,000 and decreases capacitance

Capacitance

In electromagnetism and electronics, capacitance is the ability of a body to hold an electrical charge.Capacitance is also a measure of the amount of electric charge stored for a given electric potential....
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. Unmyelinated fibers and myelinated axons of the mammalian central nervous system do not regenerate.

Demyelination and Dysmyelination

Demyelination is the 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 autoimmune disease in which the immune system attacks the central nervous system, leading to demyelinating disease. Disease onset usually occurs in young adults, and it is more common in females....
, acute disseminated encephalomyelitis

Acute disseminated encephalomyelitis

Acute disseminated encephalomyelitis is an immune mediated disease of the human brain. It usually occurs following a virus infection but may appear following vaccination, bacterial or Parasitism infection, or even appear spontaneously....
, transverse myelitis

Transverse myelitis

Transverse myelitis is a neurological disorder caused by an inflammatory process of the grey matter and white matter matter of the spinal cord, and can cause axonal demyelination....
, Alexander's disease, chronic inflammatory demyelinating polyneuropathy

Chronic inflammatory demyelinating polyneuropathy

Chronic Inflammatory Demyelinating Polyneuropathy is an acquired autoimmune disorder of the peripheral nervous system but often can have central nervous system involvement....
, Guillain-Barré Syndrome

Guillain-Barré syndrome

Guillain-Barr? syndrome is an acute inflammatory demyelinating polyneuropathy , an Autoimmune Disease disease affecting the peripheral nervous system, usually triggered by an acute infectious process....
and central pontine myelinosis

Central pontine myelinosis

Central pontine myelinolysis or osmotic demyelination syndrome is a demyelination 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 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 collection of biological processes within an organism that protects against disease by identifying and killing pathogens and tumour cells....
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 natural proteins produced by the cells of the immune system of most vertebrates in response to challenges by foreign agents such as viruses, parasites and tumor cells....
.

Heavy metal

Heavy metals

A heavy metal is a member of an ill-defined subset of elements that exhibit metallic properties, which would mainly include the transition metals, some metalloids, lanthanides, and actinides....
poisoning may also lead to demyelination. Even very small amounts of mercury

Mercury (element)

Mercury , also called quicksilver or hydrargyrum , is a chemical element with the symbol Hg and atomic number 80. A heavy, silvery d-block metal, mercury is one of six elements that are liquid at or near room temperature and pressure....
have been shown to be particularly destructive to nerve sheaths.

Research to repair damaged myelin sheaths is ongoing. Techniques include surgically implanting oligodendrocyte precursor cell

Oligodendrocyte precursor cell

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

Stem cells are Cell found in most, if not all, multi-cellular organisms. They are characterized by the ability to renew themselves through Mitosis cell division and Cellular differentiation into a diverse range of specialized cell types....
implant), it is still unknown whether this technique can be effective in humans.

Dysmyelination is characterized by a defective structure and function of myelin sheaths; unlike demyelination, it does not produce lesion

Lesion

A lesion is any abnormal tissue found on or in an organism, usually damaged by disease or trauma. Lesion is derived from the Latin word laesio which means injury....
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 progressive degeneration 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....
, Canavan disease

Canavan disease

Canavan disease, also called Canavan-Van Bogaert-Bertrand disease, aspartoacylase deficiency or aminoacylase 2 deficiency, is an autosome dominance degenerative disorder that causes progressive damage to nerve cells in the brain....
, Phenylketonuria

Phenylketonuria

Phenylketonuria is an Dominance genetic disorder characterized by a deficiency in the enzyme phenylalanine hydroxylase . This enzyme is necessary to metabolize the amino acid phenylalanine to the amino acid tyrosine....
) and schizophrenia

Schizophrenia

Schizophrenia , from the Ancient Greek Root schizein and phren, phren- is a psychiatry diagnosis that describes a mental disorder characterized by abnormalities in the perception or expression of reality....
.

Symptoms of Demyelination

Demyelination (destruction or loss of the myelin sheath) 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;
odd sensation in legs, arms, chest, or face, such as tingling or numbness (neuropathy
Neuropathy

Neuropathy is a medical term describing disorders of the nerves of the peripheral nervous system It is usually considered equivalent to peripheral 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

Composition of myelin

Function of myelin layer

Demyelination and Dysmyelination

Symptoms of Demyelination

See also

Also see

Relating to diabetes

Relating to myelin's geometry, and its fibre-optic potentiality

External links