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  

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. 

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Wood, J. D. (1999) Neurotransmission at the interface of sympathetic and enteric divisions of autonomic nervous system. Clin J. Physiol. 42, 201-10. 

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