Unmyelinated peripheral nerve. Coloured transmission electron micrograph (TEM) of a section through a small unmyelinated peripheral nerve. Myelin is an insulating fatty layer that surrounds the myelinated nerve fibres increasing the speed at which nerve impulses travel. Unmyelinated nerve fibres (yellow) lack this myelin. Schwann cell cytoplasm (pink) surronds these fibres. An outer sheath or perineurium surrounds the connective tissue endoneurium that in turn surrounds the nerve fibres and Schwann cells. Group C nerve fibers are one of three classes of nerve fiber in the central nervous system (CNS) and peripheral nervous system (PNS). The C group fibers are unmyelinated and have a small diameter and low conduction velocity. Magnification: x4000, when printed 10 centimetres wide.