This is the 2007 version. Click here for the 2017 chapter 02 table of contents.

The Peripheral Nervous System

The peripheral nervous system (PNS) consists of nerves outside the brain and spinal cord: those which innervate (supply with nerves) the muscles and glands.

What are two major subdivisions of the peripheral nervous system?

There are two major subdivisions of the PNS: the somatic nervous system and the autonomic nervous system. Nerves controlling the somatic and autonomic systems often begin within the central nervous system but have most of their effect outside it.

What is controlled by the somatic nervous system?

The somatic nervous system controls skeletal muscles. These are the muscles we use for moving our limbs. Skeletal muscles are attached to the bones of the body, which is how they got their name. Soma (the root of the word "somatic") means body in Latin. So the somatic (body) nervous system controls skeletal muscles (attached to bones of the body).

What does "autonomic" mean? How are voluntary tears an exception to the general rule?

The autonomic nervous system controls smooth muscles. Smooth muscles make up the walls of the intestine, glands, and blood vessels. Autonomic means automatic or capable of acting on its own. The glands and intestines seem to act on their own, although there are some exceptions. For example, some actors can make tears flow voluntarily. Tears come from tear ducts, which are exocrine glands. Like all glands, they are innervated by the autonomic nervous system. This is the exception that proves (tests) the rule, because actors typically accomplish this feat by vivid imagination. They fool the autonomic system into reacting as if a situation is real.

Which part of the autonomic nervous system produces the "fight or flight" reaction?

The autonomic nervous system itself has two subdivisions with opposite effects. The sympathetic part of the autonomic system swings into action during fight or flight.. Your adrenaline flows, your heart rate and breathing rate go up, your palms sweat, and your skin may grow clammy as blood flows away from skin to skeletal muscles.

Sympathetic nervous system activation also leads to an excited mental state in which subtle, complex memories may become unavailable. An example is "blanking" on a test. Our nervous systems were built to take action, not tests. For a more complete description of symptoms of sympathetic nervous system activation, drawn from reports of combat pilots.

What are effects of parasympathetic activation?

The parasympathetic system takes over as your body returns to a period of rest and relaxation. It diverts your blood supply from skeletal muscles back to your intestine so you can resume digestion of food. The parasympathetic system also slows your heart rate and respiration rate, and it allows you to replenish your system with energy. Your skin warms and a rosy glow returns to your cheeks.

Many people find that digestive functions are inhibited while they are traveling or at school. Then, when they get home, they head for the bathroom. This is a sign that the nervous system is switched from sympathetic nervous system arousal, which keeps a person alert and ready to respond to outside stresses, to parasympathetic arousal, which allows rest and recovery.

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