Parasympathetic extrinsic reflex: role in defecation mechanism.

The rectum has an intrinsic nervous mechanism represented by the enteric nervous plexus (ENP) and an extrinsic one mediated by the parasympathetic nerves. Rectal distension evokes the rectoanal inhibitory reflex, which acts through the ENP and is considered the main mechanism responsible for defecation. However, the role of the parasympathetic innervation compared to the role of the intrinsic mechanism of the defecation act has so far not been sufficiently addressed in the literature. The current study investigated this point. Twelve dogs were anesthetized. The anal and rectal pressures were recorded during rectal balloon distension with normal saline in 10 ml increments until the balloon was expelled to the exterior. The test was repeated after ENP block with a (phentolamine) and b (propranolol) adrenoceptor blocking agents and then after rectal denervation by bilateral pelvic ganglionectomy. The rectal balloon was expelled to the exterior at rectal balloon distension with 30 to 40 ml. After separate administration of phentolamine and propranolol, it was dispelled at a distending volume of 50 to 60 ml, and after rectal denervation at a volume of 80 to 90 ml. The results were reproducible. The balloon expulsion test suggests that the intrinsic defecation reflex is weaker than the extrinsic one for inducing defecation, although the two reflexes appear to be complementary. The difference between them in inducing defecation might be significant to our understanding the defecation act in the neurogenic rectum, a point that needs further study.