The document discusses the autonomic nervous system (ANS), which is divided into the sympathetic and parasympathetic nervous systems. The sympathetic system activates the "fight or flight" response and increases heart rate and metabolism. The parasympathetic system activates the "rest and digest" response and decreases heart rate and increases digestion. The ANS innervates smooth muscle, cardiac muscle and glands, and is involved in functions like sweating, digestion, and sexual arousal.