The autonomic nervous system regulates your involuntary bodily functions, such as heart rate, breathing, digestion and glands. It's a component of the peripheral nervous system.
The autonomic nervous system is divided into two parts with opposite functions -– the sympathetic and parasympathetic nervous systems. The two branches are often referred to the "fight-or-flight" and "rest-and-digest" systems.
The sympathetic nervous system, or fight-or-flight, takes over in times of stress or danger. It's your "adrenaline rush," speeding up your heart, constricting the blood vessels and raising your blood pressure so that you're prepared to defend yourself or get out of harm's way. In order to allocate resources in this way, it takes energy away from other functions, such as digestion.
The parasympathetic nervous system is the rest-and-digest portion and is dominant when you're not feeling threatened. Along with slowing the heart rate after a fight-or-flight reaction, it regulates intestinal function and gland activity.
An emerging theory of fibromyalgia purports that the autonomic nervous system somehow becomes stuck in the fight-or-flight mode.