Demyelinated nerve. Coloured transmission electron micrograph (TEM) of a section through an axon (a structure that transmits nerve impulses to other nerve cells) that has lost its myelin sheath. The axon (red) has only its Schwann cell (brown) surrounding it. The Schwann cell would normally produce the myelin sheath. A nerve's myelin sheath increases the speed at which it conducts electrical impulses and when the myelin sheath is lost, nerve function is impaired. This is seen in nerve disorders such as multiple sclerosis (MS), where patches (lesions) of myelin sheath are destroyed.