Role of the sigmoid colon in the defecation mechanism with evidence of sigmoido-anal inhibitory and ano-sigmoid excitatory reflex.

In spite of voluminous literature that has been written on defecation, the exact mechanism has not yet been fully cleared up. The current study investigated the effect of sigmoid colon (SC) distension on anal motile activity and of anal distension on SC motility. Sixteen healthy volunteers (age 36.2 +/- 11.6 SD years, 10 men) were studied. The SC was distended by a balloon in 10 ml increments of CO2, and the anal, rectal and SC pressure response was recorded before and after their individual anesthetization. The anal, rectal and SC pressure response to anal distension in increments of 2 ml of CO2 was also registered. SC distension with big volumes (mean 86.2 +/- 1.9 ml) effected a SC pressure increase (p<0.05) and no rectal pressure response (p>0.05); the balloon was expelled to the exterior. Distension of the anesthetized SC caused no SC, rectal or anal pressure response (p>0.05, p>0.05, p>0.05, respectively); the response returned after the anesthetic effect had waned. SC distension while the rectum had been anesthetized, affected a significant SC pressure rise as well as an anal pressure decrease and balloon expulsion to the exterior. Anal balloon distension produced a significant pressure rise of the SC (p<0.001) and rectum (p<0.01). Distension of the anesthetized rectal neck (anal canal) caused no SC or rectal pressure response (p>0.05, p>0.05, respectively); response returned after the anesthetic effect had disappeared. SC distension appears to effect anal dilatation while anal distension causes SC contraction. This reciprocal action is suggested to be reflex and mediated through the "sigmoido-anal inhibitory reflex" and the "ano-sigmoid excitatory reflex". These 2 reflexes are believed to keep the SC contracting and the rectal neck dilated until complete SC evacuation occurs. The study seemingly negates the role of rectal distension as a prerequisite for balloon expulsion.