Myelin is a complex combination of water, fat and proteins that surrounds the arms of neurons, but not the core of the cell or the parts that transmit signals to other cells. Under a microscope, it looks like a string of sausages.  Unlike the rest of the neuron, it does not conduct electricity. It's presence is critical to the proper functioning of nerve cells.

Myelin starts to develop in the fetus 14 weeks after conception, and continues to grow quickly in infants. It grows somewhat more slowly until adulthood, when the body stops creating new myelin.  

In a properly functioning nerve cell, myelin allows an electric charge to travel the entire length of the cell. When the myelin is missing, nerve impulses jump between different neurons as they propagate, making their progress slower and more chaotic.  

Certain autoimmune diseases will attack the myelin, but not the nerve cells. The most important of these are multiple sclerosistransverse myelitisGuillain-Barre syndrome and Vitamin B-12 deficiency.  

Myelin at Wikipedia

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