The peripheral nervous system consists of peripheral nerves that link the brain and spinal cord to the rest of the body, including the muscles, the sensory organs, and the organs of the digestive, respiratory, excretory, and circulatory systems. Within the peripheral nerves are axons of sensory neurons that bring sensory information to the central nervous system from all parts of the body. Peripheral nerves also contain the axons of motor neurons that carry signals from the central nervous system to the organs and muscles.

The motor portion of the peripheral nervous system can be subdivided into two parts: the somatic nervous system and the autonomic nervous system. Motor neurons of the somatic nervous system synapse on skeletal muscles and control voluntary movement. Their cell bodies are located in the gray matter of the spinal cord, and their axons go directly to the muscles they control.

Motor neurons of the autonomic nervous system control involuntary responses. They synapse on the heart, smooth muscle, and glands. The autonomic nervous system is controlled both by the medulla and the hypothalamus of the brain. It consists of two divisions of the autonomic nervous system generally make synaptic contacts with the same organs but usually produce opposite effects.

The sympathetic nervous system acts on organs in ways that prepare the body for stressful or highly energetic activity, such as "fight-or-flight" activities, the sympathetic nervous system curtails activity of the digestive tract, redirecting some of its blood supply to be used by the muscles of arms and legs. Heart rates accelerates. The pupils of the eyes open wider, admitting more light, and the air passages in the lungs expand, accommodating more air. The parasympathetic nervous system, in contrast, dominates during maintenance activities that can be carried on at leisure, often called "rest and rumination." Under its control, the digestive tract becomes alive, heart rate slows, and air passages in the lungs constrict.

Two differences in the organization of the sympathetic and parasympathetic nervous systems are evident. First, parasympathetic axons are found in nerves that originate from two separate locations, the brain and the base of the spinal cord. In contrast, sympathetic axons are found in nerves that originate from the middle and lower portions of the spinal cord. Second, in both the sympathetic and parasympathetic divisions, there are two neurons that carry messages in sequence from the central nervous system to each target organ, but they synapse at different locations. In the sympathetic nervous system, the synapse occurs in ganglia that are near the spinal cord. In the parasympathetic nervous system, the synapse occurs in smaller ganglia located at or very near each target organ.