sheath, case - an enveloping structure or covering enclosing an animal or plant organ or part

axolemma - outer membrane covering an axon

medullated nerve fiber, myelinated nerve fiber - a nerve fiber encased in a sheath of myelin

nodes of Ranvier, Ranvier's nodes - small gaps in the myelin sheath of medullated axons

oligodendria, oligodendroglia - tissue consisting of glial cells with sheet-like processes that form the myelin sheath of nerve fibers

medulla, myelin, myeline - a white fatty substance that forms a medullary sheath around the axis cylinder of some nerve fibers