The peripheral nervous system (PNS) encompasses the nerves that carry information between the brain and the body. The PNS can be divided into two separate systems: the somatic system, which innervates skeletal muscles (the muscles attached to bones, such as biceps and pectorals); and the autonomic system, which innervates smooth muscles (such as the stomach and intestines), cardiac muscles (the muscles in the heart), and glands. The autonomic system, in turn, can be divided into two systems: the sympathetic and the parasympathetic.

The sympathetic division is responsible for the "fight or flight" reaction to stressful situations. In response to a perceived threat, the sympathetic system will prepare the body to either run away or engage in combat by: raising heart rate and blood pressure; activating energy stores of sugars and fats; increasing blood flow to skeletal muscles; and decreasing digestive processes (to conserve energy for skeletal muscles' actions). By contrast, the parasympathetic division attempts to conserve energy, promoting growth and regeneration within the body. The parasympathetic system generally promotes the opposite effects from those of the sympathetic system: slowing heart rate and blood pressure and stimulating activity of digestive processes.