The peripheral nervous system has two divisions: the sensory division and the motor division. The sensory division has two sets of neurons: one set (from the eyes, ears, and other external sense organs) brings in information about the outside environment; the other set supplies the central nervous system with information about the body itself, such as the acidity of the blood.

The motor division includes the somatic nervous system (which carries signals to skeletal muscles and skin, mostly in response to external stimuli, and controls voluntary actions); and the neurons of the autonomic nervous system, which are involuntary. This latter system is further divided into the sympathetic division (which prepares the body for intense activities and is responsible for the “fight-or-flight” response) and the parasympathetic division, or “housekeeper system,” which is involved in all responses associated with a relaxed state such as digestion.