Normal motility and function
The primary functions of the rectum and pelvic floor muscles are to prevent incontinence (loss of control) or accidental leakage and to allow defecation to occur.
The rectum is very elastic, which allows it to store food residues prior to a bowel movement. But it must also be stiff enough to funnel food residues toward the anus during a bowel movement.
The pelvic floor, located below the rectum, is made up of many different muscles including the puborectalis muscle and the external and internal anal sphincter muscles.
The rectum is surrounded by sensory nerves that detect the filling of the rectum with food residues. This sensation of rectal filling enables us to consciously or unconsciously squeeze the external anal sphincter to prevent incontinence until we can reach a toilet. These sensory nerves are also involved in reflexes that let the sphincter muscles relax during a bowel movement.
Bowel incontinence or accidental leakage
Bowel incontinence means involuntary passage of stool in someone over the age of 4 years. The most common causes are:
- weakness of the anal sphincter muscles that allow us to voluntarily hold back a bowel movement;
- loss of sensation for rectal fullness so we can't tell when to squeeze the sphincter muscles;
- constipation, in which the rectum fills up and overflows; and
- stiff rectum, in which the stool is forced through the rectum so quickly that there is no time to prevent incontinence by squeezing the sphincter muscles.
Diarrhea can also lead to bowel incontinence.
There are actually two anal sphincter muscles: an internal anal sphincter that is part of the intestines, and an external anal sphincter that is part of the pelvic floor muscles.
The internal anal sphincter normally stays closed to prevent the leakage of gas or liquid from the rectum. When the rectum fills up with gas or stool, a reflex causes it to open to allow the bowel movement to pass through.
The nerves that this reflex depends on are sometimes missing at birth. The result is that the internal anal sphincter stays tightly closed and bowel movements cannot occur. This congenital (birth) defect is called Hirschsprung's disease. The disease can sometimes involve the entire colon or even part of the small intestine.
Outlet obstruction type constipation (pelvic floor dyssynergia)
The external anal sphincter, which is part of the pelvic floor, normally stays tightly closed to prevent leakage. When you try to have a bowel movement, however, this sphincter has to open to allow the stool to come out. Some people have trouble relaxing the sphincter muscle when they are straining to have a bowel movement, or they may actually squeeze the sphincter more tightly shut when straining. This produces symptoms of constipation.
Adapted from IFFGD Publiation: Gastrointestinal Motility Disorders of the Small Intestine, Large Intestine, Rectum, and Pelvic Floor by William E. Whitehead, PhD, Co-Director, Center for Functional GI & Motility Disorders Center; Professor of Medicine, Division of Digestive Diseases; Professor of Psychology, University of North Carolina, Chapel Hill, NC, and "Common Causes of Incontinence" by IFFGD.