The smooth and striated muscle layers in the rectum and anus, as well as the surrounding structures such as the pelvic floor muscles, work together in a coordinated manner during the process of defecation. High-amplitude propagating contractions lead to the movement of feces, filling up the rectum. The internal and external anal sphincters control the expulsion of fecal material from the body, with the internal anal sphincter contributing about 70% to 80% of anal tone at rest, and the external anal sphincter contributing about 20% to 30%. The filling of the rectum triggers the relaxation of the internal anal sphincter, which is facilitated by the release of vasoactive intestinal polypeptide and the generation of nitric oxide. This relaxation allows the anal sampling mechanism to determine the nature of the rectal contents. After toilet training, sensory nerve endings in the anal mucosa generate reflexes that activate the external sphincter to either retain the rectal contents or allow voluntary expulsion. When defecation is not convenient, the external sphincter contracts to prevent the loss of stool. Over time, the rectum adjusts to its new volume, the internal anal sphincter contracts again, and the external anal sphincter relaxes. 

On the other hand, when defecation is desired, assuming a sitting or squatting position changes the relative orientation of the intestine and the surrounding muscular structures, making the path for the exit of solid or liquid feces more direct. The relaxation of the puborectalis muscle also increases the rectoanal angle. After voluntarily relaxing the external anal sphincter, rectal contractions push the fecal material out of the body, sometimes followed by additional movements of feces from more proximal segments of the colon. Abdominal pressure is increased through simultaneous contractions of muscles, such as the diaphragm, which assists in evacuation. The voluntary expulsion of flatus follows a similar sequence of events, except that there is no relaxation of the puborectalis muscle, allowing flatus to be squeezed past the acute rectoanal angle while retaining fecal material.

The cooperative activity of the external anal sphincter, puborectalis muscle, and sensory nerve endings in the anal canal is necessary to delay defecation until appropriate, even in cases where the rectum is distended with stool or intraabdominal pressure rises sharply. This is why incontinence can develop in individuals with compromised structures, such as those who have experienced trauma, surgical or obstetrical injuries, rectal prolapse, or neuropathic diseases like long-standing diabetes. Surgical intervention may be required to correct muscle abnormalities in patients with fecal incontinence, although many can benefit from biofeedback exercises to increase external anal sphincter tone.
The process of defecation requires coordinated action of the smooth and striated muscle layers in the rectum and anus, as well as surrounding structures such as the pelvic floor muscles. During the mass movement of feces produced by high-amplitude propagating contractions, the rectum fills with fecal material. Expulsion of this material from the body is controlled by the internal and external anal sphincters, which contribute approximately 70% to 80% and 20% to 30% of anal tone at rest, respectively. Filling of the rectum causes relaxation of the internal anal sphincter via release of vasoactive intestinal polypeptide and generation of nitric oxide. Relaxation of the inner sphincter permits the anal sampling mechanism, which can distinguish whether the rectal contents are solid, liquid, or gaseous in nature. After toilet training, sensory nerve endings in the anal mucosa then generate reflexes that initiate appropriate activity of the external sphincter to either retain the rectal contents or permit voluntary expulsion (e.g., of flatus). If defecation is not convenient, the external sphincter contracts to prevent the loss of stool. Then with time the rectum accommodates to its new volume, the internal anal sphincter contracts again, and the external anal sphincter relaxes (
Fig. 31.6
When defecation is desired, on the other hand, adoption of a sitting or squatting position alters the relative orientation of the intestine and surrounding muscular structures by straightening the path for exit of either solid or liquid feces. Relaxation of the puborectalis muscle likewise increases the rectoanal angle. After voluntary relaxation of the external anal sphincter, rectal contractions move the fecal material out of the body, sometimes followed by additional mass movements of feces from more proximal segments of the colon (
Fig. 31.7 ). Evacuation is assisted by simultaneous contraction of muscles that increase abdominal pressure, such as the diaphragm. Voluntary expulsion of flatus, on the other hand, involves a similar sequence of events, except that there is no relaxation of the puborectalis muscle. This permits flatus to be squeezed past the acute rectoanal angle while retaining fecal material.
Cooperative activity of the external anal sphincter, puborectalis muscle, and sensory nerve endings in the anal canal is required to delay defecation until it is appropriate, even if the rectum is acutely distended with stool or intraabdominal pressure rises sharply. This explains why incontinence can develop in individuals in whom the integrity of such structures has been compromised, such as after trauma, surgical or obstetrical injuries, prolapse of the rectum, or neuropathic diseases such as long-standing diabetes. Surgical intervention may be necessary to correct muscle abnormalities in patients with the distressing condition of fecal incontinence, although many can be helped to increase external anal sphincter tone with the use of biofeedback exercises.
•Fig. 31.6 Responsesoftheinternalandexternalanalsphincterstoprolongeddistentionoftherectum.Notethattheresponsesofthesphinctersaretransientbecauseofaccommodation.(RedrawnfromShusterMMetal.Bull Johns Hopkins Hosp 1965;116:79.) •Fig. 31.7 Motilityoftherectumandanalsphinctersinresponsetorectalfillingandduringdefecation.Notethatfillingoftherectumcausesaninitialdecreaseininternalsphinctertonethatiscounterbalancedbycontractionoftheexternalsphincter.Theinternalsphincterthenaccommodatestothenewrectalvolume,therebyallowingtheexternalsphinctertorelax.Finally,defecationoccurswhentheexternalanalsphincterisrelaxedvoluntarily.(DatafromChangEBetal.Gastrointestinal, Hepatobiliary and Nutritional Physiology. Philadelphia:Lippincott-Raven;1996.) 1.