What is defecation?
Defecation is the process of passing out stool (feces) through the anus. This eliminates waste material from the rectum and colon. The process of defecation should be painless, regular and to a certain degree, it is under voluntary control.
The rectum usually does not contain feces so as not to trigger local reflexes that can initiate defecation. The structure of the lower bowel is such that the rectum is able to remain empty of feces. This is achieved by a weak sphincter at the at the juncture between the sigmoid colon and rectum. In addition, the sharp angle between the sigmoid colon and rectum also provides resistance to feces entering the rectum without nervous control.
Two anal sphincters and its nerve supply ensures that feces do not pass out in a continuous manner. The internal anal sphincter is composed of smooth muscle arranged in a circular manner. It is not under voluntary control. The external anal sphincter is composed of striated muscle, which surrounds the internal sphincter and extends beyond it. This is under voluntary control. Both these sphincters remain constricted all of the time and is supported by the pelvic floor muscles.
Usually, the first part of defecation is involuntary and triggered by the defecation reflexes. This may occur for a number of reasons higher up the gut but to a large degree it is influenced by the fecal content contained within the colon. Refer to the Defecation Reflex article for more information on all the autonomic reflexes that affect bowel movements.These reflexes are isolated within the enteric nervous system and spinal reflexes traveling between the lower bowel and conus medullaris of the spinal cord.
Once the defecation reflexes are triggered, peristaltic activity of the descending and sigmoid colon increases, feces enter the rectum and the internal anal sphincter relaxes. It results in the urge to defecate. Conscious control of the external sphincter will then complete the defecation process.
The peristaltic waves of the intrinsic reflex coupled with the distention of the rectum due to the entry of feces results in the desire to defecate. This will not occur without the relaxation of the external anal sphincter that is controlled by nerve fibers in the pudendal nerve which is part of the somatic nervous system. This means that it is under voluntary control and can be influenced by the conscious mind and to some extent, even by the subconscious mind. It partly explains the reason for diarrhea or constipation due to emotional stress – fear, anger, depression and anxiety. Furthermore, impulses from the brain that cannot reach the conus medullaris of the spinal cord will also affect the voluntary component of defecation.
The defecation reflexes also have other effects outside of the gastrointestinal tract. It triggers a deep breath and closure of the glottis. While holding the breath, the abdominal wall muscle contract increasing the pressure within the abdominal activity. Simultaneously, the pelvic floor muscles relax and reduces the pressure on the anus thereby allowing the feces to pass through. This is known as the Valsalva maneuver.
Conversely, pushing the diaphragm downwards by taking a deep breath and contracting the abdominal wall muscles can increase intra-abdominal pressure and trigger the defecation reflexes. This allows most people to defecate at will even without the urge to have a bowel movement. This ‘unnatural’ stimulation of the defecation reflex requires a degree of straining and is not as effective as when the reflexes occur naturally. Repeatedly ignoring the natural urge to have a bowel movement can diminish the effectiveness of the defecation reflexes and result in constipation.
Understanding the normal process of defecation is crucial in attempting to identify the process behind functional bowel disorders like inflammatory bowel syndrome (IBS) whether it is diarrhea or constipation-predominant.
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.