Myelinated nerves. Coloured transmission electron micrograph (SEM) of a section through myelinated nerve fibres (axons). Myelin is an insulating fatty layer (red) that surrounds the nerve fibre (brown), increasing the speed at which nerve impulses travel. It is formed when Schwann cells wrap around the fibre, depositing layers of myelin between each coil. The outermost layer consists of the Schwann cell's cytoplasm and is known as the neurolemma or sheath of Schwann. Magnification: x50MB00 when printed 10 centimetres wide.