A threadlike extension of a nerve cell and consists of an axon and myelin sheath (if it is myelinated) in the nervous system. There are nerve fibers in the central nervous system and peripheral nervous system. A nerve fiber may be myelinated and/or unmyelinated. In the central nervous system (CNS), myelin by oligodendroglia cells is formed. Schwann cells form myelin in the peripheral nervous system (PNS). Schwann cells also make a thin covering in an axon without myelin (in the PNS). A peripheral nerve fiber contains an axon, myelin sheath, schwann cells and its endoneurium. There are no endoneurium and schwann cells in the central nervous system. [ Wikipedia:Nervous_system_fiber ]