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myelin

 

Myelin is a collection of lipid fats and proteins that sheaths the long extensions of nerve cells (neurons) called axons. Myelin considerably increases the speed that nerve signals (impulses) move down the axons. For example, a thin myelinated axon transmits impulses at anything from 5 to 30 metres per second whereas an unmyelinated one transmits them at 0.5 to 2 meters per second. It does this both by insulating and containing the nerves.

See this diagram of a neuron which shows how the myelin wraps the axon:

A nerve impulse is a wave of depolarising current called an action potential that travels along the entire neuron by allowing charged ions of sodium and potassium to flood through channels in the semi-permeable membrane around the nerve cell. At rest (resting potential), the neuron and the surrounding space act as a "capacitor" storing current which is released during the action potential.

Myelin increases the speed of the transmission by containing the current (as positively charged ions) in a small space surrounding the axon. This means that the sodium and potassium ions that contribute to the resting potential do not have far to move when the action potential occurs. Myelin also prevents current from being lost as sodium ions drift away from the neuron.

The myelin sheath is broken at intervals called the nodes of Ranvier which are rich in sodium channels. This makes the nerve impulses move in a stepwise fashion called "salutatory conduction".

Myelin is composed of about 80% lipid fats and about 20% proteins. Some of the proteins that make up myelin are Myelin Basic Protein (MBP), Myelin Oligodendrocyte Glycoprotein (MOG) and Proteolipid Protein (PLP).

Myelin occurs in both the Central Nervous System (CNS) and the Peripheral Nervous System (PNS) but the cells that produce the myelin differ. In the PNS, cells called Schwann cells produce and maintain the myelin whereas, in the CNS, glial cells called oligodendrocytes produce and maintain the myelin.

In Multiple Sclerosis, the myelin is destructively removed from around the axon which slows down nerve impulses in a process known as demyelination. In MS, axons are demyelinated in inflammatory patches called lesions. As the disease progresses, oligodendrocytes and, ultimately, the axons themselves are destroyed. There is very compelling evidence that the destruction is caused by the body's own immune system i.e. that multiple sclerosis is an autoimmune disease.

Moreover, it is likely that immune response is mediated by a division of the specific immune system called cellular immunity. Cellular immunity is driven by a type of white blood cell (leukocyte) called a T-cell. These T-cells recognise particular sub-sections of proteins called antigens. It seems likely from animal models of multiple sclerosis (experimental autoimmune encephalomyelitis) that initial relapsing-remitting phases of the disease centre on one antigen component, perhaps of myelin basic protein, but that, as the disease progresses, antigens from other proteins become involved.

Anatomy of the nervous system
myelin sheath
Myelin & Schwann Cell Components


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