Parasympathetic pelvic splanchnic nerves
The pelvic splanchnic nerves also known as nervi erigentes are preganglionic (presynaptic) parasympathetic nerve fibers that arise from S2, S3 and S4 nerve roots of the sacral plexus. These nerves form the parasympathetic portion of the autonomic nervous system in the pelvis.
The pelvic splanchnic are preganglionic nerves that arise from the lateral horn of sacral segments of the S2, S3 and S4 nerve roots of the sacral plexus, though the greatest contribution of these fibers is usually from the S3 nerve. The preganglionic cell bodies that contribute to parasympathetic pelvic splanchnic nerves originate from the lateral horn (lateral grey column) of the sacral segments of S2 to S4.
The pelvic splanchnic nerves anastamose with one another and ascend to join the ipsliateral inferior hypogastric plexus. The inferior hypogastric plexuses are paired structures that lie on either side of the rectum and contain both sympathetic and parasympathetic fibers. After reaching the inferior hypogastric plexus, the pelvic splanchnic nerves ramify throughout the pelvis and lower abdomen innervating ganglia embedded in walls of the descending and sigmoid colon, rectum, ureter, prostate, bladder, urethra and penis.
It should be noted that on the left some fibers rise out of the pelvis, pass to the left of the superior hypogastric plexus and join periarterial plexuses surrounding the inferior mesenteric artery to supply the large intestine distal to the splenic flexure.
The pelvic splanchnic nerves and the hypogastric plexuses merge within the pelvis. The superior and inferior hypogastric plexuses are networks of sympathetic and visceral afferent nerve fibers. The superior hypogastric plexus is a continuation of the intermesenteric plexus which sits just inferior to the bifurcation of the aorta. The superior hypogastric plexus enters the pelvis dividing in to right and left hypogastric nerves which descend (on the anterior surface of the sacrum) to the lateral aspect of the rectum within ‘hypogastric sheaths’ and merge with the pelvic splanchnic nerves to form left and right inferior hypogastric plexuses.
The pelvic splanchnic nerves have a variable course of anastomosing with one another until they merge with the hypogastric nerves on either side of the rectum.
Hirschprung’s disease (also known as aganlionic megacolon) is the congential absence of parasympathetic ganglia within the large bowel. The resulting lack of peristalsis in the affected bowel segment causes obstruction and enlargement of the bowel proximal to the aganglionic segment. The incidence of Hirschprung’s disease is approximately 1 in 5000 live births and treatment is resection and removal of the aganglionic portion of bowel.