A fiber

n.
  1. Any of the myelinated nerve fibers in somatic nerves, measuring 1 to 22 microns in diameter, conducting nerve impulses at a rate of 6 to 120 meters per second.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.