Peripheral nerve. Black and white transmission electron micrograph (TEM) of a section through a small peripheral nerve. Myelin (dark rings) is an insulating fatty layer that surrounds the myelinated nerve fibres, 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. An outer sheath or perineurium surrounds the connective tissue endoneurium that in turn surrounds the nerve fibres and Schwann cells. Magnification: x1500, when printed 10 centimetres wide.
