reticular activating system

n.
  1. The part of the reticular formation in the brainstem that plays a central role in bodily and behavioral alertness; its ascending connections affect the function of the cerebral cortex and its descending connections affect bodily posture and reflex mechanisms.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.