Myelin Sheath
Myelin sheath serves as an insulator that surrounds the nerves. This includes the nerves in the brain and spinal cord. This layer is made up of fatty substances, as well as protein. Its purpose is to allow impulses to transfer quickly and efficiently along the nerve cells. If the myelin sheath layer is damages the impulses will slow down and it can cause diseases such as multiple sclerosis. In addition, Myelin Sheath is roughly 200 and 800 μm thick and diseases can also thin out the layer causing the level of protection to obviously decrease.

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Links:
http://www.nlm.nih.gov/medlineplus/ency/article/002261.htm
http://medical-dictionary.thefreedictionary.com/myelin+sheath