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Information about Myelination

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.

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.

Function of myelin layer

Enlarge picture
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. 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, transverse myelitis, chronic inflammatory demyelinating polyneuropathy, Guillain-Barre Syndrome. 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.

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]

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]

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.
  • 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

References

1. ^ [1]
2. ^ [2] 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. ^ [3] University of Calgary: How Mercury Causes Brain Neuron Degeneration
4. ^ [4] 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

 •  • [ e]
Histology: nervous tissue
Neurons (gray matter)soma, axon (axon hillock, axoplasm, axolemma, neurofibril/neurofilament), dendrite (Nissl body, dendritic spine)
types (bipolar, pseudounipolar, multipolar, pyramidal, Purkinje, granule)
Afferent nerve/Sensory nerve/Sensory neuronGSA, GVA, SSA, SVA, fibers Ia, Ib or Golgi, II or Aβ, III or Aδ or fast pain, IV or C or slow pain
Efferent nerve/Motor nerve/Motor neuronGSE, GVE, SVE, Upper motor neuron, Lower motor neuron (Aα motorneuron, Aγ motorneuron)
Synapsesneuropil, synaptic vesicle, neuromuscular junction, electrical synapse - Interneuron (Renshaw)
Sensory receptorsFree nerve ending, Meissner's corpuscle, Merkel nerve ending, Muscle spindle, Pacinian corpuscle, Ruffini ending, Olfactory receptor neuron, Photoreceptor cell, Hair cell, Taste bud
Glial cellsastrocyte, oligodendrocyte, ependymal cells, microglia, radial glia
Myelination (white matter)Schwann cell, oligodendrocyte, nodes of Ranvier, internode, Schmidt-Lanterman incisures, neurolemma
Related connective tissuesepineurium, perineurium, endoneurium, nerve fascicle, meninges
 •  • [ e]
Structures of the cell membrane
Caveolae/Coated pits - Cell junctions - Glycocalyx - Lipid raft/microdomains - Myelin sheath - Nodes of Ranvier - Nuclear envelope - Phycobilisomes
For the dendritic crystal structure, see .


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
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The soma, or perikaryon, is the bulbous end of a neuron, containing the cell nucleus. It is also known as the cell body. The word soma is Greek, meaning "body"; the soma of a neuron is often called the "cell body".
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axon or nerve fiber, is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma.

Anatomy


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nucleus (3) ribosome (4) vesicle (5) rough endoplasmic reticulum (ER) (6) Golgi apparatus (7) Cytoskeleton (8) smooth ER (9) mitochondria (10) vacuole (11) cytoplasm (12) lysosome (13) centrioles]]

In cell biology, the nucleus (pl.
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Nodes of Ranvier, also known as neurofibril nodes, are regularly spaced gaps in the myelin sheath around an axon or nerve fiber. About one micrometer in length, these gaps expose the axonal membrane to the extracellular fluid.
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Nodes of Ranvier, also known as neurofibril nodes, are regularly spaced gaps in the myelin sheath around an axon or nerve fiber. About one micrometer in length, these gaps expose the axonal membrane to the extracellular fluid.
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synapse. Synapses allow nerve cells to communicate with one another through axons and dendrites, converting electrical impulses into chemical signals.]]

Chemical synapses
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Named after the German physiologist Theodor Schwann, Schwann cells (also referred to as neurolemmocytes) are a variety of glial cell that mainly provide myelin insulation to axons in the peripheral nervous system of jawed vertebrates.
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U indicates the uncharged hydrophobic portion of the molecule, highlighted in blue.]]

Phospholipids are a class of lipids, and a major component of all biological membranes, along with glycolipids, cholesterol and proteins.
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axon or nerve fiber, is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma.

Anatomy


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Neurons (also known as neurones and nerve cells) are electrically excitable cells in the nervous system that process and transmit information. In vertebrate animals, neurons are the core components of the brain, spinal cord and peripheral nerves.
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Glial cells, commonly called neuroglia or simply glia (greek for "glue"), are non-neuronal cells that provide support and nutrition, maintain homeostasis, form myelin, and participate in signal transmission in the nervous system.
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Named after the German physiologist Theodor Schwann, Schwann cells (also referred to as neurolemmocytes) are a variety of glial cell that mainly provide myelin insulation to axons in the peripheral nervous system of jawed vertebrates.
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Oligodendrocyte

General Information
Tissue type Nervous
Cell type Neuroglia
Location Central nervous system
Role Myelination
Identification Robertson, 1899
Ultrastructure
Soma size
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The central nervous system (CNS) represents the largest part of the nervous system, including the brain and the spinal cord. Together with the peripheral nervous system, it has a fundamental role in the control of behavior.
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Gnathostomata

Subgroups
  • Class Placodermi
  • Superclass Chondrichthyes
  • Microphylum Teleostomi
  • Class Acanthodii
  • Class Actinopterygii

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Louis-Antoine Ranvier (b. Lyon, France, October 2, 1835; d. Vendranges, France, March 22, 1922, French physician, pathologist, anatomist and histologist, discoverer of the myelin sheath and the nodes of Ranvier, subcellular structure which covers the axons of neurons.
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Lipids can be broadly defined as any fat-soluble (hydrophobic), naturally-occurring molecules. The term is more-specifically used to refer to fatty-acids and their derivatives (including tri-, di-, and monoglycerides and phospholipids) as well as other fat-soluble sterol-containing
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Fat

Fat may refer to:
  • Fat, a group of compounds that are generally soluble in organic solvents and largely insoluble in water
  • Adipose tissue, an anatomical term for loose connective tissue composed of adipocytes

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Proteins are large 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 residues.
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Myelin basic protein (MBP) is a protein believed to be important in the process of myelination of nerves in the central nervous system (CNS).

MBP was initially sequenced in 1979 after isolation from myelin membranes
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Myelin Oligodendrocyte Glycoprotein (MOG) is a glycoprotein believed to be important in the process of myelinization of nerves in the central nervous system (CNS).

The gene for MOG, found on chromosome 6, was first sequenced in 1995 [1].
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Proteolipid protein 1 is a protein associated with Pelizaeus-Merzbacher disease. It is a 4 transmembrane domain protein which binds strongly to other copies of itself on the extracellular side of the membrane.
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Glycolipids 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.
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Sphingomyelin 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.
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An action potential is a "spike" of electrical discharge that travels along the membrane of a cell. Action potentials are an essential feature of animal life, rapidly carrying information within and between tissues. They also occur in some plants.
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Saltatory conduction (from the Latin saltare, to hop or leap) is a means by which action potentials are transmitted along myelinated nerve fibers.

Because the cytoplasm of the axon is electrically conductive, and because the myelin inhibits charge leakage through
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Neurodegenerative disease (Greek νέυρο-, néuro-, "nerval" and Latin dēgenerāre, "to decline" or "to worsen") is a condition in which cells of the brain and spinal cord are lost.
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MeSH D001327 Autoimmunity is the failure of an organism to recognize its own constituent parts (down to the sub-molecular levels) as "self", which results in an immune response against its own cells and tissues.
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Multiple sclerosis
Classification & external resources

MRI FLAIR sequence showing four bright spots (plaques) where multiple sclerosis has damaged myelin in the
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