As mentioned in other posts, the majority of the body’s visceral organs are innervated by both divisions of the Autonomic nervous system.
We have referred to this concept as dual innervation. At any given time, both divisions are likely stimulating the target organ. That said, usually one of the divisions is stimulating the target organ at a greater rate, and therefore exerts a greater effect.
It is easy to see this concept in action when we look at two different extremes that would stimulate one of the ANS divisions. If, for instance, you are attacked by a mugger then your sympathetic division would likely stimulate its target organs at a far greater rate than the parasympathetic division. We would then expect to see the effects of this stimulation to appear in the sympathetic target organs. HR would increase, BP would increase, respiratory rate would increase, bronchiole smooth muscle relaxation would occur, and GI motility would slow.
We would expect a very different response when we are sitting down to relax and eat a meal. HR would slow, BP would decrease, respiratory rate would decrease, GI motility would increase, salivary production would increase, Pupils would constrict, etc. This very different response is due to the parasympathetic dominance that occurs during this very different situation.
The situations described above are beautiful examples of the antagonistic interaction between the two divisions of the autonomic nervous system. There are however certain target tissues that are said to demonstrate parasympathetic or sympathetic tone.
Parasympathetic and Sympathetic Tone
These terms describe tissues that are innervated by one division to a much greater extent than by the other. The vascular system is said to demonstrate sympathetic tone. Arteries constrict in response to increased sympathetic stimulation, and relax in response to decreased sympathetic stimulation (or vice versa based on receptor type). This is an example of how a tissue can be controlled primarily by only one division of the ANS. The heart for example, is said to demonstrate parasympathetic tone. Although the heart is innervated by both divisions, the parasympathetic division tends to dominate most of the time.