What is a defecation reflex?
The defecation reflex is an involuntary response of the lower bowels to various stimuli thereby promoting or even inhibiting a bowel movement. These reflexes are under the control of the autonomic system and play an integral role in the defecation process along with the somatic system that is responsible for voluntary control of defecation. The two main defecation reflexes are known as the intrinsic myenteric defecation reflex and parasympathetic defecation reflex.
Intrinsic Myenteric Defecation Reflex
The entry of feces into the rectum causes the distention of the rectal wall. This stretching triggers signals to the descending and sigmoid colon via the myenteric plexus to increase peristalsis. The myenteric plexus is part of the enteric nervous system which is the gut’s own internal neural network as discussed under stomach nerves.
The peristaltic waves extend all the way to the rectum an anus. In this manner, fecal matter is moved closer to the anus. When the wave reaches the anus, it causes the internal anal sphincter, which is always constricted, to relax. This is achieved by inhibitory signals via the myenteric plexus to reduce sphincter constriction.
Defecation may occur at this point if the external anal sphincter also relaxes. However, without the parasympathetic defecation reflex, defecation solely dependent on the intrinsic reflex would be weak.
Parasympathetic Defecation Reflex
The parasympathetic defecation reflex works in essentially the same way as the intrinsic myenteric defecation reflex but involves parasympathetic nerve fibers in the pelvic nerves. It triggers peristaltic waves in the descending and sigmoid colon as well as the rectum. It also causes relaxation of the external anal sphincter. The difference is that the parasympathetic defecation reflex enhances this process and makes the intrinsic reflex much more powerful. If sufficiently stimulated, it may even cause the sigmoid colon to completely empty all of its contents in the rectum rapidly.
The force triggered by the parasympathetic defecation reflex can be powerful enough to result in defecation, despite conscious efforts to keep the external anal sphincter constricted.
Other Defecation Reflexes
Apart from the two main defecation reflexes mentioned above, other reflexes can also influence the defecation process.
- Gastrocolic reflex – distention of the stomach while eating or immediately after a meal triggers mass movements in the colon.
- Gastroileal reflex – distention of the stomach while eating or immediately after eating triggers the relaxation of the ileocecal sphincter and speeds up peristalsis in the ileum (end portion of the small intestine). This causes the contents of the ileum to rapidly empty into the colon.
- Enterogastric reflex – distention and/or acidic chyme in the duodenum slows stomach emptying and reduces peristalsis.
- Duodenocolic reflex – distention of the duodenum a short while after eating triggers mass movements in the colon.
Irritation within the stomach or duodenum can stimulate or even inhibit the defecation reflexes. In addition to these gastrointestinal reflexes, there are other reflexes involving the peritoneum, kidney and bladder that can affect the defecation process. This includes the :
- Peritoneointestinal reflex involving the peritoneum and intestines.
- Renointestinal reflex involving the kidney and intestines.
- Vesicointestinal reflex involving the bladder and intestines.
When these organs are irritated and the reflexes are triggered, it inhibits intestinal activity.