Defecation Reflex and Continence

The defecation reflex and continence describe the coordinated sequence by which the rectum signals filling, the anal sphincters and pelvic floor relax or contract, and stool is either retained or expelled under voluntary control. Continence between events and the orderly act of defecation depend on the integrated action of the rectum, the internal and external anal sphincters, the puborectalis muscle, and spinal and cortical neural pathways.

Definition

The defecation reflex is the sensorimotor sequence triggered by rectal distension that, when defecation is permitted, produces sphincter relaxation, pelvic floor descent, and expulsion of stool; continence is the complementary maintenance of stool retention between such events.

Scope

This topic covers rectal filling and sensation, the recto-anal inhibitory reflex, the roles of the internal and external anal sphincters and the puborectalis in maintaining continence, and the sequence of relaxation and straining that accomplishes defecation. It is reference material on normal physiology.

Core questions

• How does the rectum sense filling and generate the urge to defecate?
• What is the recto-anal inhibitory reflex and what does it accomplish?
• How do the anal sphincters and puborectalis maintain continence?
• What sequence of events accomplishes voluntary defecation?

Key concepts

• Rectal filling and the urge to defecate
• Recto-anal inhibitory reflex (RAIR)
• Internal anal sphincter (tonic, involuntary)
• External anal sphincter (voluntary)
• Puborectalis sling and the anorectal angle
• Sampling reflex and discrimination of contents
• Valsalva straining and pelvic floor descent
• Deferral of defecation by voluntary contraction

Mechanisms

As the rectum fills, distension is detected by mechanosensitive afferents, producing awareness of rectal content and an urge to defecate. Rectal distension elicits the recto-anal inhibitory reflex, a transient relaxation of the tonically contracted internal anal sphincter that allows rectal contents to be sampled by the sensitive anal canal, while the external anal sphincter contracts to preserve continence. At rest, continence is maintained by the tonic internal anal sphincter, the voluntarily controllable external anal sphincter, and the puborectalis muscle, whose sling around the anorectum keeps the anorectal angle acute. When defecation is appropriate, the puborectalis and external sphincter relax, the anorectal angle straightens, the pelvic floor descends, and an increase in intra-abdominal pressure (a Valsalva manoeuvre) together with rectal contraction expels the stool. When it is not appropriate, voluntary contraction of the external sphincter and puborectalis defers the event until rectal accommodation reduces the urge (Palit et al., 2012; Bharucha, 2006).

Clinical relevance

The physiology of rectal sensation, sphincter coordination, and pelvic floor function underlies the interpretation of disorders such as faecal incontinence and disorders of evacuation, and it informs the consensus framework for anorectal disorders. This entry describes normal function and is not a basis for individual diagnosis or treatment.

Evidence & guidelines

The sequence described here is drawn from physiological reviews of human defecation and pelvic floor function (Palit et al., 2012; Bharucha, 2006); the Rome consensus on anorectal disorders (Rao et al., 2016) provides a widely used classification framework for the related clinical entities.

History

Understanding of defecation advanced from early descriptions of the recto-anal inhibitory reflex and the role of the puborectalis in the anorectal angle to integrated accounts combining manometry, imaging, and neurophysiology, synthesised in modern reviews of human defecation physiology (Palit et al., 2012).

Key figures

• S. Mark Scott
• Peter J. Lunniss
• Adil E. Bharucha

Seminal works

• palit-2012
• bharucha-2006

Frequently asked questions

What is the recto-anal inhibitory reflex?

It is the reflex relaxation of the internal anal sphincter when the rectum is distended; it lets the sensitive anal canal sample the rectal contents while the external sphincter contracts to keep continence.

How can defecation be postponed?

Voluntarily contracting the external anal sphincter and the puborectalis muscle keeps the anal canal closed and the anorectal angle acute; the rectum then accommodates the contents and the urge subsides until a more convenient time.