unmyelinated fiber

n.
  1. Any of the nerve fibers that lack a fatty sheath but that are enveloped by a sheath of Schwann cells as are other nerve fibers.gray fiber nonmedullated fiber
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.