Myelinated nerves. Coloured scanning electron micrograph (SEM) of myelinated axon nerve fibres. Myelin (pink) is an insulating fatty layer that surrounds the nerve fibre, 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: x1500 when printed at 10 centimetres wide.