Introduction that provide myelin are called Schwann cells

IntroductionThe Brain is often grouped into two different types of matter, white matter and grey matter, grey matter being made up of the neuronal cell bodies and the white matter being comprised of bundles of cells called glia. A type of glial cell, they do not carry a current through themselves, but instead cover the neuronal cells in myelin. These cells that provide myelin are oligodendrocytes in the central nervous system and can provide myelin to multiple cells at one time.

 “In humans, around 40% of the brain contains white matter comprising densely packed fibres, of which myelin is a main component (50–60% dry weight of the white matter)” (Morell and Norton, 1980), so 20% of all brain matter is entirely myelin so it must play an very important role. In the peripheral nervous system the cells that provide myelin are called Schwann cells and can only provide myelin to one cell at a time. Brief myelin histologySchwann cells and oligodendrocytes form a myelin sheath by producing a plasma membrane extension, this is imperative in the production in saltatory impulse propagation and may be important in allowing neuronal computations to work together.

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This occurs where there is a small gap in the myelin sheath where the axon is exposed, this is called a Node of Ranvier the location of depolarisation in a myelinated neurone. These nodes contain many voltage-gated sodium ion channels causing a high concentration of sodium ions to accumulate at these nodes and so facilitates rapid conductions of action potentials, this being the basis of saltatory conduction. The myelin sheath also speeds up the conduction of action potentials by providing insulation and varying the thickness of the myelin causes difference in the speed at which conductance occurs. There is optimal axonal myelination, which is caused by there being a ratio of 0.6 between the inner axonal diameter to the total outer diameter, this is the structural index or the g-ratio. (Waxman and Bennett, 1972; Chomiak and Hu, 2009)  An example of how myelination makes a difference to effectiveness of an impulse is, a myelinated axon requiring only a few ?m and 5000 times less energy than an un-myelinated neurone of a diameter of 500?m for both action potentials to travel at .(Ritchie, 1982).

Internal length also has a calculated optimum, where most sheaths have dimensions for the most efficient conduction speed. It is however also important for some parts of the brain to have fast conductor velocities but to also synchronise those velocities.As our brains have evolved, most fibres have spiralling and lengthwise myelination dimensions near the calculated optimum for the most appropriate conduction speed, Many areas of the brain require neurones to conduct quickly, but for their conduction velocities to synchronise. Axons in the brain that are of different lengths to other axons will have differences in myelin thickness and internal length in order for a more effective response.

So when they discharge at the same time, the differences in conductivity of the varying lengths of axons will allow for the action potentials to reach the relevant location. Myelin PlasticityBiogenesis of myelin is now thought to enrich brain plasticity and can be modified by the individuals surroundings and experiences. There is new evidence that shows plasticity is regulated whilst the brain is developing in early life and into adult life. This has been supported by the finding that a large amount of oligodendrocytes formed in adulthood form myelin sheaths. It has been found that all oligodendrocytes in adult mice divide every 20-40 days (Young et al., 2013; Simon et al., 2011; Kang et al., 2010), with a percentage of 30-40% going on to divide continuously which go on further to form mature oligodendrocytes and new myelin sheath.

However, the myelin formed from these mature have different properties from those formed during early development. These mature cells form shorter inter-nodal lengths as well as many internodes. It is therefore no longer accepted that myelination only occurs during early brain development but as a lifelong process.Diseases associated with myelinMyelination is also important for other processes, they also regulate the ionic environment and provide energy channels with metabolites, this allows for long term longevity of neurones, so damage to white matter, can cause ions to leak out of the axonal membrane and not carry the electrical impulse, which can slow the impulse down or can lead to neuropsychiatric and neurodegenerative disease, such as multiple sclerosis and can lead to stroke. These diseases are dangerous not only by the damage they cause but because it is also very difficult to find ways to cure or treat them.

Diseases with pathological differences associated with grey matter also exhibit changes to the white matter. This has been discovered by neural imaging techniques that myelin in the central nervous system can move so understanding how this works is crucial in understanding the mechanisms behind oligodendrocyte function and the function of the neuron.Metabolic support is received by lactate or pyruvate and is activity dependent, so it is regulated to make sure that no energy is wasted and lactic acidosis occurs less frequently, reducing the risk of some life threatening diseases and increasing the longevity of white matter and healthiness of the brain, reducing the risk of disease. This means that the brain can work under both aerobic and anaerobic conditions, though by being under anaerobic conditions, the brain is at higher risk of lactic acidosis, and it is more favourable to be respiring aerobically as to not waste energy.Myelination is key for the function of the human nervous system; it allows for a much faster, more energy efficient way of passing vital information around the body, through the presence of nodes of Ranvier; the regulation of the ionic environment in the nervous system and maintaining plasticity of the neurones and also preventing the risk of disease in the nervous system, which could prove to be fatal if myelin degeneration were to take place

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