Nerve demyelination. Coloured transmission electron micrograph (TEM) of a section through a Schwann cell and a nerve fibre, showing the early collapse of its myelin sheath. Myelin (red) is an insulating fatty layer that surrounds the nerve fibre (axon, orange), increasing the speed at which nerve impulses travel. It is formed when a Schwann cell (green) wraps around the fibre, depositing layers of myelin between each coil. Demyelination occurs in nerve disorders such as multiple sclerosis (MS). Patches (lesions) of myelin sheath are destroyed and nerve function is impaired. Magnification: x3800 when printed 10 centimetres wide.