Myelin is a
dielectricA dielectric is a nonconducting substance, i.e. an insulator. The term was coined by William Whewell in response to a request from Michael Faraday...
(electrically insulating) material that forms a layer, the
myelin sheath, usually around only the
axonAn axon or nerve fiber is a long, slender projectionof a nerve cell, or neuron, that conducts electrical impulsesaway from the neuron's cell body or soma....
of a
neuronA neuron is an excitable cell in the nervous system that processes and transmits information by electrochemical signaling. Neurons are the core components of the brain, the vertebrate spinal cord, the invertebrate ventral nerve cord, and the peripheral nerves...
. It is essential for the proper functioning of the
nervous systemThe nervous system is a network of specialized cells that communicate information about an organism's surroundings and itself. It processes this information and causes reactions in other parts of the body. It is composed of neurons and other specialized cells called glial cells that aid in the...
. Myelin is an outgrowth
glial cellGlial cells, commonly called neuroglia or simply glia , are non-neuronal cells that provide support and nutrition, maintain homeostasis, form myelin, and participate in signal transmission in the nervous system...
:
Schwann cellNamed after the German 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 . The sheath is not continuous. Individual myelinating Schwann cells cover about 100 micrometre of an...
s supply the myelin for peripheral neurons, whereas
oligodendrocyteOligodendrocytes , 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, specifically of the interfascicular type, supply it to those of the
central nervous systemThe central nervous system is the part of the nervous system that functions to coordinate the activity of all parts of the bodies of bilaterian animals—that is, all animals more advanced than sponges or jellyfish. In vertebrates, the central nervous system is enclosed in the meninges. It contains...
. Myelin is considered a defining characteristic of the (gnathostome) vertebrates, but it has also arisen by parallel evolution in some invertebrates. Myelin was discovered in 1854 by
Rudolf VirchowRudolf Ludwig Karl Virchow was a German doctor, anthropologist, public health activist, pathologist, prehistorian, biologist and politician...
.
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%
lipidLipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, phospholipids, and others...
and about 20%
proteinProteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
. Some of the proteins that make up myelin are
myelin basic proteinMyelin 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 1979 after isolation from myelin membranes...
(MBP),
myelin oligodendrocyte glycoproteinMyelin 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). Myelin is made up primarily of a
glycolipidGlycolipids are carbohydrate-attached lipids. Their role is to provide energy and also serve as markers for cellular recognition.They occur where a carbohydrate chain is associated with phospholipids on the exoplasmic surface of the cell membrane. The carbohydrates are found on the outer surface...
called
galactocerebrosideA 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.-External links:*...
. The intertwining of the hydrocarbon chains of
sphingomyelinSphingomyelin , , is a type of sphingolipid found in animal cell membranes, especially in the membranous myelin sheath which 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 an increase in the speed at which
impulsesAn action potential is a transient alteration of the transmembrane voltage across an excitable membrane generated by the activity of voltage-gated ion channels embedded in the membrane. Action potentials play multiple roles in several types of excitable cells such as neurons, myocytes, and...
propagate along the
myelinated fiber. Along
unmyelinated fibers, impulses move continuously as waves, but, in myelinated fibers, they hop or "propagate by
saltationSaltatory 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 resistanceThe electrical resistance of an object is a measure of its opposition to the passage of a steady electric 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...
across the cell membrane by a factor of 5,000 and decreases
capacitanceIn 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. A common form of charge 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. Unmyelinated fibers and myelinated axons of the mammalian central nervous system do not regenerate.
Demyelination and dysmyelination
Demyelination is the loss of the myelin sheath insulating the nerves, and is the hallmark of some neurodegenerative autoimmune diseases, including
multiple sclerosisMultiple sclerosis is an idiopathic disease of suspected autoimmune cause, in which the body's immune response attacks a person's central nervous system , leading to demyelination. Disease onset usually occurs in young adults, and it is more common in females...
,
acute disseminated encephalomyelitisAcute 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 myelitisTransverse myelitis is a neurological disorder caused by an inflammatory process of the grey and white matter of the spinal cord, and can cause axonal demyelination.-Presentation:...
, Alexander's disease,
chronic inflammatory demyelinating polyneuropathyChronic Inflammatory Demyelinating Polyneuropathy is an acquired immune-mediated inflammatory disorder of the peripheral nervous system but often can have central nervous system involvement. The disorder is sometimes called chronic relapsing polyneuropathy. CIDP is closely related to...
,
Guillain-Barré SyndromeGuillain–Barré syndrome is an acute inflammatory demyelinating polyneuropathy , an autoimmune disorder affecting the peripheral nervous system, usually triggered by an acute infectious process. It is included in the wider group of peripheral neuropathies...
,
central pontine myelinosisCentral pontine myelinolysis or osmotic demyelination syndrome is a demyelinating lesion in the brain that occurs with rapid correction of hyponatremia...
and
Charcot Marie ToothCharcot-Marie-Tooth disease , known also as Hereditary Motor and Sensory Neuropathy , Hereditary Sensorimotor Neuropathy , or Peroneal Muscular Atrophy, is a heterogeneous inherited disorder of nerves that is characterized by loss of muscle tissue and touch sensation, predominantly in the feet and...
. 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 systemAn immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumour 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
interferonInterferons are natural cell-signaling proteins produced by the cells of the immune system of most vertebrates in response to challenges such as viruses, parasites and tumor cells. They belong to the large class of glycoproteins known as cytokines and are produced by a wide variety of cells in...
.
Research to repair damaged myelin sheaths is ongoing. Techniques include surgically implanting
oligodendrocyte precursor cellOligodendrocyte precursor cells 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...
s in the central nervous system and inducing myelin repair with certain antibodies. While there have been some encouraging results in mice (via
stem cellStem cells are cells found in most, if not all, multi-cellular organisms. They are characterized by the ability to renew themselves through mitotic cell division and differentiating into a diverse range of specialized cell types. Research in the stem cell field grew out of findings by Canadian...
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
lesionA 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.-Causes of lesions:...
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
leukodystrophiesLeukodystrophy 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 diseasePelizaeus-Merzbacher disease is a rare central nervous system disorder in which coordination, motor abilities, and intellectual function are delayed to variable extents.-Overview:...
,
Canavan diseaseCanavan 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...
,
phenylketonuriaPhenylketonuria is an autosomal recessive genetic disorder characterized by a deficiency in the hepatic enzyme phenylalanine hydroxylase . This enzyme is necessary to metabolize the amino acid phenylalanine to the amino acid tyrosine...
) and
schizophreniaSchizophrenia , from the Greek roots skhizein and phrēn, phren- is a psychiatric diagnosis that describes a mental disorder characterized by abnormalities in the perception or expression of reality...
.
Symptoms of demyelination
Demyelination (i.e., the 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 is a medical term referring to disorders of the nerves of the peripheral nervous system It is usually considered equivalent to peripheral neuropathy, which is defined as deranged function and structure of peripheral motor, sensory, and autonomic neurons, involving either...
);
- 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.
See also
- 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 is the process of sequential myelination or development of a myelin sheaths around a nerve fibres of the parts of the central nervous system. The order of development approximates the evolutionary development of the central nervous system....
, order of myelination of central nervous system.
- 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.
Relating to diabetes
- Vlassara H, Brownlee M, Cerami A.
-Biography:Anthony Cerami is a member of the National Academy of Sciences and the Institute of Medicine. He received a Ph.D. in 1967 from Rockefeller University, New York, completed postdoctoral fellowships at Harvard Medical School, Massachusetts, and at the Jackson Laboratory, Maine, and served...
(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
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...
.]
- 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 higher education institution situated in West London, England. In the latest Government Research Assessment Exercise, 82% of research submitted was rated as of international standing...
.http://www.ondwelle.com/MolecMemIR.pdf, or as Part B of Thesis.http://hdl.handle.net/2438/729 — [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.
- optic nerve
The optic nerve, also called cranial nerve II, transmits visual information from the retina to the brain.-Anatomy:The optic nerve is the second of twelve paired cranial nerves but is considered to be part of the central nervous system as it is derived from an outpouching of the diencephalon during...
, physiology subsection; — [applies some of this theory].
External links