Sympathetic Nervous System
A subdivision of the autonomic nervous system regulating largely involuntary internal processes focused on energy usage for real or perceived crisis (fight, fright, flight). Inhibits and opposes the parasympathetic nervous system subdivision.
Bear, M.F., Connors, B.W. & Paradiso, M.A. (2001). Neuroscience: Exploring the Brain. (2nd Edition). Baltimore, MD: Lippincott Williams & Wilkins.