At the end of the rectum, the gastrointestinal tract turns at an 80 to 90 degree angle before it becomes the anal canal. This angle, called the anorectal angle, separates the rectum from the anus. The angle is maintained by the tone of a muscle on the pubic floor called the puborectalis muscle, which is part of the leavtor ani muscle. Although the true function of this angle is not clear, the angle does disappear when we need to have a bowel movement. Also, the ability of the levator ani to contract and relax can affect whether we can control a bowel movement on our own accord.

The levator ani muscle is a large, wide muscle that wraps around the rectum in various places. The levator ani muscle consists of 3 different muscles called the puborectalis muscle, the pubococcygeus muscle, and the ileococcygeus muscle. The puborectalis muscle starts at the front of the lower pelvis and wraps around the rectum like a “U,” which pulls the rectum forward. The pubococcygenus muscle sits below the puborectalis muscle. This muscle begins at the coccyx, or tail bone, and wraps around the rectum in a “U” shape from the rear. The ileococcygeus muscle wraps around the rectum at the lowest point and attaches to the skin of the perineum. Of these 3 muscles, the chief muscle for determining whether we can control our bowels on our own is the puborectalis muscle.

The anus itself consists of 2 muscles called the internal anal sphincter and the external anal sphincter.  As its name would suggest, the internal anal sphincter lies where the anal canal and the anus meet. The internal anal sphincter is an involuntary muscle innervated by the autonomic nervous system. Except when feces enters the rectum, the internal anal sphincter is almost always contracted.

The external anal sphincter lies at the outer edge of the anus. Unlike the internal anal sphincter, the external anal sphincter can be voluntarily controlled. This muscle is innervated by the pudendal nerves and can contract voluntarily so that we can wait to release the feces if the time is not right. This muscle is quite useful if we do not have access to a toilet or latrine.

A number of mechanisms allow us to release feces at the right time.  For instance, even if there is no urge to have a bowel movement, the rectum can have feces in it. However, when a certain amount of feces arrives in the rectum, the feces will place pressure on the rectum. This pressure will send a nerve impulse to the muscularis externa layer of the tissues in the descending colon, the sigmoid colon, and the rectum. This reflex is called the intrinsic defecation reflex. The muscles will then contract in a peristaltic wave that drives the feces downward. This will cause the internal anal sphincter, which is not under voluntary control, to relax. When internal anal sphincter relaxes, we may feel the urge to have a bowel movement. However, defecation will not occur unless the external anal sphincter, which is under voluntary control, relaxes at the same time.

Usually the intrinsic defecation reflex is not strong enough to cause a bowel movement in and of itself. It usually works together with a stronger reflex called the parasympathetic defecation reflex. In this reflex, stretching in the rectum caused by feces sends an impulse to the nerves in the sacral portion of the spinal cord (the part of the spinal cord that is contained within the tail bone). The spinal nerves then send a signal that makes the peristalsis in the muscles of the rectum stronger. At the same time, the nerve fibers send a signal that tells the internal anal sphincter to relax.

If the time is not suitable for having a bowel movement, the cerebrum portion of our brain sends a signal to the external anal sphincter so that we can suppress the urge to have a bowel movement. When we suppress the urge, the peristaltic contractions stop and the rectum relaxes for the time being. However, the reflexes will recur in a few hours or if a mass movement pushes more feces into the rectum.

When both anal sphincters relax, we push the feces out through a series of voluntary and involuntary contractions. The muscles of the rectum contract involuntarily, pushing the feces toward the anus.  At the same time, the levator ani contracts voluntarily, forcing the anal canal upward. We also use a maneuver called the Valsalva maneuver. To perform the Valsalva maneuver, we take a deep breath and close the glottis.  This increases the pressure in the thorax and depresses the diaphragm. We then contract the muscles in our abdomen, which increases the pressure within our abdomen. At the same time, the puborectalis muscle relaxes, the rectum narrows, and the anorectal angle disappears. These motions combined, allow the feces to be released. After the feces are released, the anal sphincter contracts and the anal canal returns to its original state.

Some factors that can impact whether a bowel movement is normal include how much fluid we excrete in feces and how much fluid the large intestines absorb. For instance, diarrhea results if too much fluid is secreted or not enough fluid is absorbed by the large intestine. It can also occur if food moves through the colon too quickly. If more than 200 mL of fluid is excreted in the feces per day, the stool is not considered to be normal. A number of factors such as bacterial infections and autoimmune disorders such as celiac disease can contribute to whether the stool is watery.

Just as the stool may be too watery if it is allowed to move through the digestive tract too fast, the stool may not be moist enough if it moves through the large intestine too slowly.  Because the feces stays in the large intestine for too long, the large intestine may absorb too much fluid from it, making it hard and difficult to pass. This can result in constipation. A number of factors can influence whether we become constipated. Although we cannot control some, such as spasms of the sigmoid colon, we can control others.   One way we can decrease constipation is by eating fiber, which contains cellulose. Even though we cannot digest cellulose, cellulose helps the food to move through the digestive tract by adding bulk to it. This bulk provides the food with mass so that the muscles of the digestive tract can push it along more easily. We can also avoid using laxatives too often, being too stressed out, and putting off having a bowel movement for too long.

Last Updated: Saturday, July 16, 2011