The descending colon functions as a conduit, with relatively rapid transit of feces to the sigmoid colon, which acts as a second reservoir. Emptying of the latter is largely under volitional control. This facilitates the opening or straightening of the rectoanal angle and expulsion of stool. Smooth muscle contraction results from fluxes of ions that alter the electrical potential of the cell membrane. These play essential roles in controlling peristalsis and the migrating motor complex.
Enteric nerves are also important in mediating sensation from the gut.
The latter nerves mediate pain as well as the autonomic and emotional responses that are commonly noted in patients with functional GI diseases. The extrinsic neural control is subdivided into the craniosacral parasympathetic outflow and the thoracolumbar sympathetic supply. The cranial outflow is predominantly through the vagus nerve, which supplies neural control from the stomach down to the right side of the colon, and through the sacral sympathetic supply to the left colon. Sympathetic fibers to the GI tract arise from T5 to L1 levels of the intermediolateral column of the spinal cord. The prevertebral sympathetic ganglia integrate afferent impulses from the gut to the central nervous system.