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Structure of a typical neuron

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

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, whereas 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.

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.

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.^{[citation needed]} 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 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. Sufferers of Pernicious Anaemia can also suffer nerve damage if the condition is not diagnosed quickly. Sub-Acute Combined Degeneration of the 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)^[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]}

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

• Multiple sclerosis (MS), caused by loss of myelin
• The Myelin Project, project to re-generate myelin
• Myelinogenesis, order of myelination of central nervous system

• 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

• 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