Hi there. Myelin is referred to as the white matter because of its white, fatty appearance; it protects and insulates the axons. Myelin consists of a protective sheath of many different molecules that include both lipids and proteins. This is necessary for rapid transmission of electrical signals between neurons. It does this by containing the electrical molecules within the axon so that they are all properly transmitted to the next neuron. With myelin, neurons can transmit signals at speed up to 60 meters per second. when this coat is damaged as with demyelinating conditions like multiple sclerosis the maximum speed can decrease by 10 fold or more, since some signal is lost during transmission. This decrease in speed of signal transmission leads to disruption of proper functioning of the nervous system. Hope this helps. Take care.
http://en.wikipedia.org/wiki/Myelin