27.39 : Feces Formation and Defecation

After 3 to 10 hours in the large intestine, chyme undergoes considerable water loss to form feces, the end product of digestion.

It comprises undigested dietary fiber like cellulose, mucus, sloughed-off epithelial cells, and microbes.

The descending and sigmoid colon stores feces, which undergoes haustral contractions for final drying, retaining enough water for a semisolid texture.

Next, mass peristalsis propels the feces into the usually empty rectum, distending the rectal walls to stimulate the stretch receptors.

As a result, the defecation reflex, an involuntary response for lower bowel movement, is triggered.

It begins with the myenteric plexus, triggering the short reflex to stimulate contractions in the sigmoid colon and rectum.

At the same time, parasympathetic motor neurons in the sacral region trigger the long reflex to enhance mass peristalsis.

The mass movements move the feces into the anal canal, triggering the defecation reflex which relaxes the internal anal sphincter.

Finally, the pudendal nerves of the somatic nervous system inhibit the skeletal muscles of the external anal sphincter, allowing relaxation for the voluntary expulsion of feces.