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OVERVIEW OF CONTINENCE AND DEFECATION
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The anorectum has similar functions as the bladder and urethra: storage and emptying. To maintain continence, the anorectum must deal with solid, liquid, and gaseous contents. Continence is also dependent on proximal gut motility, the distensibility of the rectum, and the function of the anal sphincter complex (Figure 11-1). In addition, continence relies on intact sensory and motor including somatic and visceral systems, located in the anatomically correct position. Finally, the process of defecation not only depends on these systems to function independently, but requires appropriate interactions between them in order to provide socially appropriate, coordinated, and complete expulsion of fecal contents.
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Once food that has been ingested is deposited through the ileocecal valve into the colon, transit through the colon is approximately 35 hours; stool consistency and colonic motility are correlated. Propagation of fecal matter into the distal rectum produces a sensation of rectal fullness once a certain volume has been achieved. An intact and tonically contracting levator ani muscle, particularly the puborectalis portion, angulates the rectum with respect to the anal canal. This anorectal angle helps transmit elevated intra-abdominal pressure (cough, sneeze, etc) across the rectum closing the lumen, rather than directly into the anal canal.
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Rectal distention from a propagated stool bolus leads to a reflex, temporary relaxation of the anal canal to allow the high density of nerve endings and sensory cells in the anal epithelium to “sample” the stool contents. This information is transmitted via sensory nerves to the brain, and the decision to defecate or not is made. If it is not time to defecate, the individual can enhance the resisting pressure in the anal canal and pelvic floor by contracting the striated muscles of the external anal sphincter (EAS) and the puborectalis muscles (PRM).
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If defecation is chosen, squatting helps to open the anorectal angle, thereby straightening the rectum-to-anus axis. As in the urethra/bladder system, stool will be expelled when the pressure in the rectum exceeds the pressure in the anal canal. A Valsalva maneuver, potentially accompanied by rectal contraction, increases rectal pressure. A coordinated inhibition of the tonic contraction of the striated EAS and smooth muscle internal anal sphincter (IAS), as well as relaxation and descent of the pelvic floor musculature, decreases the pressure at the outlet. Stool is then propelled from the body. Once defecation has been completed, a closing reflex increases the contractility of the muscles.
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Investigations into the function or dysfunction of the anorectal system should consider all of the above aspects.
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INVESTIGATING ANATOMY AND STRUCTURE
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