The unmyelinated nerve pertains to any of the nerve cells without myelin sheath (a sheath involved in the rapid conduction of action potential). Compared to myelinated nerves, unmyelinated nerve cells are slower in conducting impulses and are found in the peripheral nervous system (especially visceral nervous system) and the gray matter of the nervous system.
Word origin: un- (not) + myelinated
Compare: myelinated nerve
See also: myelin