KnowHOW team explains: The defecation reflex is a complex procedure which begins when the pressure in the terminal part of the colon reaches between 2.5 and 6.5 per cent of atmospheric pressure. It is controlled by the tip of the spinal cord, via the so-called parasympathetic pelvic nerves, one of the two systems of the autonomic nervous system which regulates involuntary body functions.

It comprises two systems, the sympathetic and the parasympathetic, which oppose each other. The sympathetic nervous system is involved in mobilising energy and priming the body for action ? the ?fight or flight? response ? by increasing the heart rate, raising the blood pressure and suppressing the various activities of the gut, among other actions. In contrast, the parasympathetic nervous system, which is most active during sleep, favours maintenance and repair activities by lowering blood pressure, relaxing overall muscle tone, and activating the secretion of digestive enzymes, among other actions.

These two systems oppose each other, by exerting opposite effects on common targets, but there are also direct inhibitory connections between them. Thus the parasympathetic activity needed to relax the anus and stimulate muscle movement inhibiting of the sympathetic nervous system and resultant overall relaxation.

The question was sent by Sunirmal Pandey from Patna