When you have completed this section, you should be able to
• explain how the autonomic and somatic nervous systems differ in form and function; and
• explain how the two divisions of the autonomic nervous system differ in general function.
The autonomic nervous system (ANS) can be defined as a motor nervous system that controls glands, cardiac muscle, and smooth muscle. It is also called the visceral motor system to distinguish it from the somatic motor system that controls the skeletal muscles. The primary target organs of the ANS are the viscera of the thoracic and abdominal cavities and some structures of the body wall, including cutaneous blood vessels, sweat glands, and piloerector muscles.
Autonomic literally means "self-governed."1 The ANS usually carries out its actions involuntarily, without our conscious intent or awareness, in contrast to the voluntary nature of the somatic motor system. This voluntary-involuntary distinction is not, however, as clear-cut as it once seemed. Some skeletal muscle responses are quite involuntary, such as the somatic reflexes, and some skeletal muscles are difficult or impossible to control, such as the middle-ear muscles. On the other hand, therapeutic uses of biofeedback (see insight 15.1) show that some people can learn to voluntarily control such visceral functions as blood pressure.
Visceral effectors do not depend on the autonomic nervous system to function, but only to adjust their activity to the body's changing needs. The heart, for example, goes on beating even if all autonomic nerves to it are severed, but the ANS modulates (adjusts) the heart rate in conditions of rest or exercise. If the somatic nerves to a skeletal muscle are severed, the muscle exhibits flaccid paralysis—it no longer functions. But if the autonomic nerves to cardiac or smooth muscle are severed, the muscle exhibits exaggerated responses (denervation hypersensitivity).
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