ABSTRACT
In the case of single-unit smooth muscle, which has an abundance of gap junction contacts between cells, the effect of ANS is to modulate a pre-existing level of activity. For example, smooth muscle cells in the wall of the gut are spontaneously active because of slow-wave activity (see Topic 16). This means that gut smooth muscle is active even in the absence of neural or hormonal influences. If one takes some smooth muscle from the gut, one finds that it is spontaneously active in vitro. However, the two branches of the ANS act to alter the pattern of smooth muscle activity on the gut. In general, parasympathetic outflow from the vagus nerve and the sacral segments of spinal cord increase the level of activity of gut smooth muscle, whereas sympathetic outflow from the thoracolumbar portions of spinal cord decrease the level of activity of gut smooth muscle.
