Defecation Reflex and Other Reflexes that Affect Bowel Activity

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.

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  • Ashwin Patel

    what triggers the parasympathetic defecation reflex?

    • Feces in the rectum that causes stretching of the rectal walls can stimulate the parasympathetic reflex.

      • ??????

        when my son poops, he does not strain at all. He just relaxes the sphincter and the process starts by itself with no effort from his side. In fact he often is talking and joking while empying his bowels and the process seems to go on completely automatic. So the rectum can empty completely without straining, cant it? And why other people need to hold their breath and push…

        Also, there is something more I want to ask.After the initial defecation (where people consider they are done and get up) my son ALWAYS waits for another 7-10 minutes. During this time he usually passes out 2-3 small portions of stool. Is this the content of the sigmoid colon?

  • Gloria Najhawan

    Hi, I have been diagnosed with a sliding hiatal hernia and IBS. I am wondering if the end of my intestines, because of the difficulty I have to get anything out poop-wise, without straining and turning about, would the bottom of my intestines become weak and fall in some way. I feel like its sitting really low when I sit down, and can feel something even almost on the chair. Its so uncomfortable. I go into the bathroom, and use vaseline to push it back up. What am I experiencing?

    • Hi Gloria. A hiatal hernia is a problem with the stomach. This is high up and most of the stomach lies under the lower left ribcage. In a hiatal hernia a portion of the stomach moves into and out of the chest cavity. You can read more about it here
      What you may be experiencing at the other end seems to be hemorrhoids, also known as piles. Here blood vessels in the lower part of your large intestine becomes inflamed and can weaken to protrude through the anus. However, it could also be some other issue that needs to be investigated by a doctor.

  • Melanie

    My 4yr old son has never done a solid poo. .and not only does he have a small phobia of doing poos on the toilet it’s like he sometimes has no control over it and it comes out in his pull ups/jocks as a paste. He starts kindy next yr and I’m also on the waiting list to see a pediatrician as doc think she nerves in the colon aren’t being stimulated.
    Is there anything I can do or try with him at home? ?

    • Hi Melanie. Without finding out the exact reason, especially if there is a medical problem like nerve dysfunction, it is difficult to say with any certainty what can be done at home. You will have to await the pediatrician’s feedback. Some conditions cannot be simply treated or managed with conservative measures at home and may require medical or even surgical intervention. It all depends on the final diagnosis. Your doctor will advise you accordingly.