Divisions of autonomic nervous system

The ANS is composed of two anatomically and functionally distinct divisions: the sympathetic system and the parasympathetic system. Two important features of these divisions include:

• Tonic activity

• Dual innervation

Both systems are tonically active. In other words, they provide some degree of nervous input to a given tissue at all times. Therefore, the frequency of discharge of neurons in both systems can increase or decrease and, as a result, tissue activity may be enhanced or inhibited. This characteristic of the ANS improves its ability to regulate a tissue's function more precisely. Without tonic activity, nervous input to a tissue could only increase.

Many tissues are innervated by both systems. Because the sympathetic and parasympathetic systems typically have opposing effects on a given tissue, increasing the activity of one system while simultaneously decreasing the activity of the other results in very rapid and precise control of a tissue's function. Several distinguishing features of these two divisions of the ANS are summarized in Table 9.2.

Each system is dominant under certain conditions. The sympathetic system predominates during emergency "fight-or-flight" reactions and during exercise. The overall effect of the sympathetic system under these conditions is to prepare the body for strenuous physical activity. More specifically, sympathetic nervous activity will increase the flow of blood that is well-oxygenated and rich in nutrients to tissues that need it, in particular, the working skeletal muscles. The parasympathetic system predominates during quiet, resting conditions. The overall effect of the parasympathetic system under these conditions is to conserve and store energy and to regulate basic body functions such as digestion and urination.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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