Demyelinated nerve. Coloured transmission electron micrograph (TEM) of a section through an axon (a structure that transmits nerve impulses) that has lost its myelin sheath. The axon (red center) has only bits of its Schwann cell (yellow) surrounding it. The Schwann cell would normally produce the myelin sheath (dark brown) as seen in the two smaller axons. A nerve's myelin sheath helps it conduct 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. Magnification: x 3000 when printed at 10 centimetres wide.