(-lĭn, -lēn')
n.
A white fatty material, composed chiefly of lipids and lipoproteins, that encloses certain axons and nerve fibers. Also called medulla.
myelinic my'e·lin'ic adj.
myelin
Dictionary:
my·e·lin (mī'ə-lĭn)  also my·e·line
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| World of the Body: myelin |
Myelin is the fatty insulating layer that surrounds many axons (nerve fibres) in both the central and peripheral nervous systems (CNS and PNS respectively). Nerve cells, with their axons, and glia are the two major cell types of the nervous systems. Myelin is formed from membranous sheets that are elaborated by glial cells: Schwann cells in the PNS and oligodendrocytes in the CNS. A major difference between these two is that Schwann cells each myelinate part of a single axon, whereas oligodendrocytes can myelinate as many as thirty axons in the CNS. Also, whereas Schwann cells are directly associated with their myelin sheath (Fig. 1), oligodendrocyte cell bodies connect by thin, tenuous processes to their multiple myelin sheets, each of which may be some distance from the cell body (Fig 3a).
Fig. 1 (a) A myelinated axon in the peripheral nervous system and (b) its development. Each Schwann cell myelinates a single axon, to which it is directly apposed. During development (anticlockwise) Schwann cells loosely ensheath axons and the myelin sheath grows around the axon to form concentric layers, which become tightly apposed
Along the axons, myelin sheaths are arranged in segments that are separated by narrow regions of naked axolemma (the cell membrane of the axon) called nodes of Ranvier; these are the sites of action potential generation in myelinated axons (Fig. 1a and 3a). Nodes occur at regular intervals ranging from 0.3-2.0 mm, according to axon size. The myelin acts as a layer of high electrical resistance and low capacitance, facilitating the rapid saltatory (jumping) conduction of electrical impulses from node to node for long distances along axons that may be up to 1 m in length. Perhaps the most striking evidence of the importance of myelin comes from human demyelinating diseases such as multiple sclerosis, which specifically attacks oligodendrocytes. The subsequent loss of myelin causes the conduction block that underlies the crippling clinical symptoms of the disease.
Myelin is a complex structure that shows in cross-section as spirals around the axon to form a sheath made up of concentric layers (lamellae Fig.1b, 3b). The number of lamellae (N) determines the insulating properties of the sheath, whereas the intersegmental length (L) — the distance between nodes — determines the speed of conduction: since both N and L are directly and positively related to axon diameter, larger axons conduct faster than smaller ones. If unwrapped, a single myelin sheath would be seen to be a spade-like or trapezoid sheet of membrane, extraordinarily large relative to the axon it surrounded: in the order of 7 mm2 (for a 15 μm diameter axon) or 0.4 mm2 (for a 5 μm diameter axon) ; as though, say, a large handkerchief were wound around a length of thin string (Fig 3b). Thus, each myelinating cell maintains a myelin volume of 50 000-150 000 μm3 — according to axon diameter — an order of magnitude greater than its own volume. It is evident that the support of such a large volume of myelin places a considerable metabolic load on the myelinating cell, so, bearing in mind that each oligodendrocyte supports multiple sheaths, it is perhaps not surprising that oligodendrocytes have this exclusive function in the CNS. Moreover, myelin is continuously turned over and replaced by the myelinating cell.
It is difficult to imagine how the complex structure of myelin is formed. However, if the myelin sheet is envisaged as a flat layer of cytoplasm bounded by plasma membranes and then if the cytoplasm is extruded so that the two plasma membranes are directly opposed to one another, then this is essentially a single lamella of compacted myelin (Fig. 1b). Since each plasma membrane consists of a double phospholipid layer, the myelin sheet comprises two double fatty layers wrapped concentrically round the axon and it is these that give myelin its excellent insulatory properties. Antibodies to one of the lipids in myelin have been used to study oligodendrocytes.
Fig. 2 Development of myelinated axons in the central nervous system, as seen by electron microscopy in transverse section. (a) newborn. Occasional axons are loosely ensheathed by primitive, undifferentiated glial cells, g, but myelin is not yet present. × 40, 000. (b) Adult. The axons are fully myelinated. × 20, 000.
Fig. 3 Myelination in the central nervous system. A single oligodendrocyte myelinates numerous axons (a) and, in section, concentric layers of myelin are seen to spiral around the axon (b). Myelin sheaths are arranged along axons in segments 1 mm long separated by short nodes, and would appear as large sheets if they were unwrapped from around the axon
In addition to the lipids, there are a number of proteins that are enriched in myelin and are specific to it. Some proteins are believed to be important in communication between the axon and the inner lamella of the myelin sheath; others fuse and stabilize the layers. Their importance is made clear when mutations of the genes that determine their formation cause loss of one or more of these proteins, resulting in the unravelling of myelin; demyelination; hypomyelination; loss of axonal function; and ensuing clinical symptoms, such as tremor and paraplegia.
— Arthur M. Butt
Bibliography
- Ransom, B. R. and Kettenham, H. (ed.) (1995). Neuroglia. Oxford University Press
See also action potential; glia; nerves; white matter.
| Dental Dictionary: myelin |
(mī′ əlin)
n
n
A fatlike substance forming a sheath around certain nerve fibers. It is associated with volitional nervous system fibers and is believed to be related to the capacity of nerve structures for rapid transmission of nerve impulses. Various diseases, such as multiple sclerosis, can destroy these myelin wrappings.
| Biology Q&A: What is myelin? |
Myelin forms an insulating wrapping around
large nerve axons. In the peripheral nervous system myelin is formed by Schwann
cells (a type of supporting cell) that wrap repeatedly around the axon. In the
central nervous system myelin is formed by repeated wrappings of processes of
oligodendrocytes (a different type of supporting cell). The process of each
cell forms part of the myelin sheath. The space between the myelin from
individual Schwann cells or oligodendrocyte processes is a bare region of the
axon called the node of Ranvier. Nerve conduction is faster in myelinated
fibers because it jumps from one node of Ranvier to the next. For this reason
it is called saltatory (jumping) conduction.
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| Veterinary Dictionary: myelin |
1. the lipid substance forming a sheath around the axons of certain nerve fibers; these nerve fibers are spoken of as myelinated or medullated fibers.
2. lipoid substance found in various normal and pathological tissues, which differs from fats in being doubly refractive.
Myelinated nerve fibers occur predominantly in the cranial and spinal nerves and compose the white matter of the brain and spinal cord. It is the myelin sheath that gives the whitish color to the areas of white matter. Unmyelinated fibers are abundant in the autonomic nervous system.
| Wikipedia: Myelin |
| Myelin sheath |
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Myelin is a dielectric (electrically insulating) material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system. Myelin is an outgrowth of a glial cell: Schwann cells supply the myelin for peripheral neurons, whereas oligodendrocytes, specifically of the interfascicular type, myelinate the axons 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 1854 by Rudolf Virchow [2].
Contents |
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 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
The main purpose of a myelin layer (or sheath) is to 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 electrical resistance across the cell membrane by a factor of 5,000 and decreases capacitance by a factor of 50.[citation needed] 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
Further information: Demyelinating disease
Demyelination is 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, chronic inflammatory demyelinating polyneuropathy, Guillain-Barré Syndrome, central pontine myelinosis, inherited demyelinating diseases such as Leukodystrophy, and Charcot Marie Tooth. 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 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)[3] or interferon.
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 transplantation), it is still unknown whether this technique can be effective in replacing myelin loss in humans.[4]
Dysmyelination is characterized by a defective structure and function of myelin sheaths; unlike demyelination, it does not produce lesions. 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 (Pelizaeus-Merzbacher disease, Canavan disease, phenylketonuria) and schizophrenia.[5][6][7]
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);
- 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, project to re-generate myelin
- Myelinogenesis, order of myelination of central nervous system.
- Myelin Repair Foundation, a non-profit medical research foundation accelerating drug discovery in myelin repair for multiple sclerosis.
References
- ^ Invertebrate Myelin
- ^ [Virchow R (1854) Über das ausgebreitete Vorkommen einer dem Nervenmark analogen Substanz in den tierischen Geweben. Virchows Arch. Pathol. Anat. 6:562-572.]
- ^ [1] 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)
- ^ [2] FuturePundit January 20, 2004
- ^ Krämer-Albers at al., 2006
- ^ Matalon et al., 2006
- ^ Tkachev et al., 2007
- Krämer-Albers EM, Gehrig-Burger K, Thiele C, Trotter J, Nave KA. (2006 Nov 8). "Perturbed interactions of mutant proteolipid protein/DM20 with cholesterol and lipid rafts in oligodendroglia: implications for dysmyelination in spastic paraplegia". J Neurosci. 26(45):11743-52.PMID: 17093095
- Matalon R, Michals-Matalon K, Surendran S, Tyring SK. (2006). "Canavan disease: studies on the knockout mouse". Adv Exp Med Biol.; 576:77-93.PMID: 16802706
- Tkachev D, Mimmack ML, Huffaker SJ, Ryan M, Bahn S. (2007 Aug). "Further evidence for altered myelin biosynthesis and glutamatergic dysfunction in schizophrenia". Int J Neuropsychopharmacol. 10(4):557-63.PMID: 17291371
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.]
- Traill, R.R. (1977/1980/2006) Toward a theoretical explanation of electro-chemical interaction in memory-use. Monograph #24, Cybernetics Department, Brunel University.[3], or as Part B of Thesis.[4] — [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, physiology subsection; — [applies some of this theory].
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
- Radiology and Pathology of Myelin the MedPix Medical Image Database
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