autonomic nervous system, the part of the nervous system that regulates involuntary body functions, including the activity of the cardiac muscle, smooth muscles, and glands. It has two divisions, the sympathetic nervous system and the parasympathetic nervous system. The sympathetic component, which is often referred to as the “fight or flight” system, accelerates heart rate, constricts blood vessels, and raises blood pressure. The parasympathetic component, often called the “rest and digest” system, slows heart rate, increases intestinal peristalsis and gland activity, and relaxes sphincters.
