The peripheral nervous system has two parts:
Somatic Nervous System
Autonomic Nervous System
The somatic nervous system is involved in both sensory and motor functions, serving mainly the skin and skeletal muscles. The autonomic nervous system activates internal organs, such as the lungs and intestines, and reports to the brain the current state of activity of these organs.
In the somatic nervous system, afferent neurons carry messages from sense organs to the spinal cord, as you can see the diagram. Efferent neurons carry messages to, and activate, striated (grooved) skeletal muscles, such as those in the face, arms, and legs, that we can move voluntarily. A disorder called myasthenia gravis can develop at the junction of these muscles and neurons, weakening muscle function of the head and neck. This produces characteristic symptoms— such as drooping eyelids, blurred vision, and difficulty swallowing and breathing—and can lead to paralysis and death. Although medical treatment is effective in restoring muscle function, some symptoms may recur when the person is under stress (AMA, 1989).
The diagram also shows that in the autonomic nervous system, neurons carry messages between the spinal cord and the smooth muscles of the internal organs, such as the heart, stomach, lungs, blood vessels, and glands. This system itself has two divisions, the sympathetic and parasympathetic, which often act in opposite ways, as you will see in this next diagram.
The sympathetic nervous system helps us mobilize and expend energy in responding to emergencies, expressing strong emotions, and performing strenuous activity. For instance, suppose you are crossing a street, notice a speeding car barreling toward you, and hear its brakes start to squeal. The sympathetic nervous system instantly moves into action, producing several simultaneous changes—for example, it speeds up the heart, dilates certain arteries to increase blood flow to the heart and skeletal muscles, constricts other arteries to decrease blood flow to the skin and digestive organs, decreases salivation, and increases perspiration. These changes, in general, enable you to mobilize energy, and you leap to safety out of the cars path. This system is called “sympathetic” because it acts in agreement with your current emotional state.
What does the parasympathetic division do? The prefix “Para” means “alongside of” this division acts alongside of, and often in opposition to the sympathetic division. The parasympathetic nervous system regulates “quiet” or calming processes, helping our individual organ systems conserve and store energy. One example of parasympathetic activity can be seen in the digestion of food. When you eat a meal, the parasympathetic nervous system carries messages to regulate each step in the digestive process, such as by increasing salivation and stomach contractions. Another example can be seen in the course of emotional or emergency reactions when an emergency has passed, the parasympathetic division helps restore your normal body state.
Communication within the peripheral nervous system is handled by 12 sets of cranial nerves, most of which originate in the brainstem. The Vagus nerve extends from there to muscles of most major body organs, such as the airways, lungs, heart, and intestines, and is directly involved in the regulation of sympathetic and parasympathetic activity. Efferent messages from the brain can target specific organs to increase or decrease their function. As you now realize, the nervous system is connected to and regulates all of our other body systems; and the brain is the control center.