sympathetic nervous system

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The part of the autonomic nervous system originating in the thoracic and lumbar regions of the spinal cord that in general inhibits or opposes the physiological effects of the parasympathetic nervous system, as in tending to reduce digestive secretions, speed up the heart, and contract blood vessels.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun neuroanatomy The part of the autonomic nervous system that under stress raises blood pressure and heart rate, constricts blood vessels and dilates the pupils.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun originates in the thoracic regions of the spinal cord; opposes physiological effects of the parasympathetic: reduces digestive secretions; speeds the heart; contracts blood vessels