To keep the anal canal closed the pressure within the anal canal has to be higher than in the rectum. The resting pressure in the canal is maintained mainly by tonic activity of the internal sphincter, with sudden increases of intrarectal pressure, as with coughing or exertion, compensated for by rapid contraction of the external sphincter and puborectalis. Angulation of the anorectal junction and the 'flap valve' theory is no longer considered important in continence. The internal sphincter does not close the canal completely, and the (c.7 mm) gap is sealed by the vascular subepithelial tissues.


Defecation is a conscious physiological act in response to feeling the need to pass stool in the rectum, requiring coordinated relaxation of the pelvic floor muscles and anal sphincter. The dynamics have been studied using pressure measurements in the colon, rectum and anus (Herbst et al 1997) and by various imaging techniques including fluoroscopy, ultrasonography and magnetic resonance imaging (Kruyt et al 1991).
The process is initiated by mass colonic contractions driving faeces into the rectum. Rectal distension lowers internal sphincter tone in preparation for defecation. Defecation may be deferred by conscious contraction of the external anal sphincter until contractions cease and retrograde rectal peristalsis moves stool out of the distal rectum, so that the sensation to defecate passes off. Initiation of defecation involves relaxation of the pelvic floor muscles and external sphincter, so that the pelvic floor descends and the anal canal opens. Abdominal contraction will aid expulsion from the rectum, but continuing mass colonic contractions push more faeces down into the rectum, so that the entire left colon may be emptied.
Integrating the sensory input from the anal canal in order to control the activity of the anal musculature occurs at many levels in the nervous system, including the spinal cord, brain stem, thalamus and cortex. Neural activity monitors and regulates defecation, and other more subtle behaviours within the rectum and anal canal, such as the separation of faeces from rectal gas; local adjustments to faecal consistency and quantities; self-cleansing movements in the rectum and anal canal; and coordination with other actions of the perineal and abdominal muscles.
UPDATE Date Added: 13 December 2005
Abstract: Mechanisms controlling normal defecation and the potential effects of spinal cord injury.
Click on the following link to view the abstract: Mechanisms controlling normal defecation and the potential effects of spinal cord injury. Brading AF, Ramalingam T. Prog Brain Res. 152:345-58, 2005.


Anal incontinence is common and may occur for a variety of reasons. Abnormal high rectal pressures, as in severe diarrhoea, may overcome a normal sphincter, and autonomic disorders may produce abnormal motor activity or loss of normal sensation, so that there is no awareness of stool. There may also be damage to the anal sphincter, mostly from vaginal delivery, or atrophy of the external sphincter from pudendal damage, also of obstetric origin, except in the elderly. A damaged sphincter will not be able to overcome normal fluctuations of intrarectal pressures, or there may be a combination of damage and rectal dysfunction resulting in anal incontinence.

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