Peripheral nerve. Coloured transmission electron micrograph (TEM) of a section through a small peripheral nerve. Myelin is an insulating fatty layer that surrounds the myelinated nerve fibres (brown), increasing the speed at which nerve impulses travel. It is formed when Schwann cells (blue) wrap around the fibre, depositing layers of myelin between each coil. Unmyelinated nerve fibres (purple) are also present. An outer sheath or perineurium surrounds the connective tissue endoneurium that in turn surrounds the nerve fibres and Schwann cells. Magnification: x4000, when printed 10 centimetres wide.