Myelin

Structure of a typical neuron

Myelin sheath
Dendrite Soma Axon Nucleus Node of Ranvier Axon terminal Schwann cell Myelin sheath

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]

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 hydrocarbon chains of sphingomyelin serve to strengthen the myelin sheath.

Function of myelin layer

Transmission electron micrograph of a myelinated axon. Generated at the Electron Microscopy Facility at Trinity College, Hartford, CT.

The main purpose of a myelin layer (or sheath) is to increase 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. 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 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

Also see

Relating to diabetes

• Vlassara H, Brownlee M, Cerami A (June 1985). "Recognition and uptake of human diabetic peripheral nerve myelin by macrophages". Diabetes 34 (6): 553–7. doi:10.2337/diabetes.34.6.553. PMID 4007282. 
• Thornalley PJ (2002). "Glycation in diabetic neuropathy: characteristics, consequences, causes, and therapeutic options". Int. Rev. Neurobiol. 50: 37–57. doi:10.1016/S0074-7742(02)50072-6. PMID 12198817. 

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

• Donaldson HH, Hoke GW (1905). "The areas of the axis cylinder and medullary sheath as seen in cross sections of the spinal nerves of vertebrates". J. Comp. Neurol. 15.  — [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". J. Comp. Neurol. 60: 437–71. doi:10.1002/cne.900600305.  — [another historic paper on the myelin/axon ratio].
• Rushton WAH (1951). "A theory of the effects of fibre size in medullated nerve". J. Physiology 115: 101–22.  [Calculation of best geometry for saltatory conduction.]
• Traill RR (1977/1980/2006). Toward a theoretical explanation of electro-chemical interaction in memory-use. Monograph. 24. Cybernetics Department, Brunel University. http://www.ondwelle.com/MolecMemIR.pdf. , or as Part B of Thesis. — [showing that other extra signal-modes are possible for such "coaxials", which could make myelin even more important].
• Traill RR (1988). "The case that mammalian intelligence is based on sub-molecular memory-coding and fibre-optic capabilities of myelinated nerve axons". Speculations Sci Technol 11 (3): 173–81. 
•     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