The autonomic nervous system (ANS) is an involuntary system and controls the body’s internal environment. The autonomic nerves regulate the internal organs and glands to maintain homeostasis.

The ANS comprises two further subdivisions called the sympathetic and the parasympathetic divisions. They both supply the same organs in the body and are complementary, although they cause completely opposite effects.

The sympathetic nerves are composed of a network of nerves or plexuses.

The parasympathetic division consists of a double chain of ganglia called sympathetic nerve trunks. They lie from the base of the skull to the pelvis.