The autonomic nervous system (ANS) is the division of the nervous system that controls the involuntary actions of muscles and glands (Fig.1). The ANS itself has two divisions: the sympathetic nervous system and the parasympathetic nervous system. The sympathetic nervous system motivates our response to stress, the so-called "fight-or-flight" response. It increases heart rate and respiration rate, stimulates the adrenal gland, and delivers more blood to skeletal muscles. The parasympathetic system returns the body to a steady state and stimulates maintenance activities, such as digestion of food. Most organs are controlled by both systems and, in general, the two systems have opposite effects on a given organ.

FIGURE 1. Autonomic nervous system (only one side is shown for each division).

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