Myelin sheaths are multilayered membrane extensions off of the axon. Neurons with myelin on their axons are referred to as myelinated and the process of developing a myelin sheath is myelination. Myelin is formed by Schwann cells attaching to the axon. Myelin helps to speed up the propagation of an action potential along the axon by the process of saltatory conduction (“Myelin: an Overview,” 2015). Multiple sclerosis is a disease in which myelin degrades in the patient. This leads to weakness, numbness, muscle stiffness, and even issues with thinking (“Multiple Sclerosis,” 2016).
(“Conduction in a myelinated fiber”, 2012)