Innervation of Intestines

The parasympathetic supply to the intestines is derived from the vagus and pelvic splanchnic nerves. The vagal contributions pass to the celiac plexus in the larger and smaller celiac branches arising, respectively, from the posterior and anterior vagal trunks. Some fibers are distributed with branches of the celiac plexus to the stomach and duodenum (see Plate 7-14), but others descend to the superior mesenteric plexus. They comprise efferent (preganglionic) and afferent fibers, and innervate the small intestines and the colon almost to the left colic flexure. Parasympathetic fibers follow the same routes to the intestines as sympathetic postganglionic fibers but are still preganglionic and end by forming synapses in the enteric plexuses (see Plate 7-17).

The pelvic splanchnic nerves arise from the second, third, and fourth sacral nerves. They contain parasympathetic preganglionic and afferent fibers, which include those supplying the distal end of the transverse colon and left colic flexure, the descending and sigmoid colons, and the rectum. They join the inferior hypogastric (pelvic) plexuses and are distributed with their branches. The preganglionic intestinal fibers pass through the ganglia in these plexuses without relaying; like their vagal counterparts, they end by making synaptic contacts in the enteric plexuses. Some branches pass directly to the rectum and lower end of the sigmoid colon, others accompany rectal and colic vessels, and still others may ascend in the hypogastric nerves to the superior hypogastric plexus and thence to the inferior mesenteric plexus, to be distributed with its branches to the distal parts of the colon. However, the majority of the parasympathetic fibers for these parts of the colon pursue a different course: they arise by several filaments from the pelvic splanchnic nerves or the inferior hypogastric plexuses and run upward across the sigmoid and left colic vessels. They can be traced as far as the left colic flexure, and they supply offshoots to the adjacent parts of the sigmoid and descending colons and communicate with branches of the inferior mesenteric plexus.