Neural control of anorectal function

Faeces stored in the colon are transported past the rectosigmoid ‘physiological sphincter’ into the normally empty rectum, which can store up to 300 mL of contents. Rectal distension causes regular contractions of the rectal wall, which is effected by the intrinsic nervous (myenteric) plexus, and prompts the desire to defecate.

Stool entering the rectum is also detected by stretch receptors in the rectal wall and PFM; their discharge leads to the urge to defecate. It starts as an intermittent sensation, which becomes more and more constant. Contraction of the PFM may interrupt the process, probably by concomitant inhibitory infl uences to the defecatory neural ‘pattern generator’, but also by ‘mechanical’ insistence on sphincter contraction and the propelling of faeces back to the sigmoid colon.

The PFM are intimately involved in anorectal function. Apart from the ‘sensory’ role of the PFM and the external anal sphincter function, the puborectalis muscle is thought to maintain the ‘anorectal’ angle, which facilitates continence, and has to be relaxed to allow defecation. Current concepts suggest that defecation requires increased rectal pressure coordinated with relaxation of the anal sphincters and PFM.

Pelvic floor relaxation allows opening of the anorectal angle and perineal descent, facilitating fecal expulsion. Puborectalis and external anal sphincter activity during evacuation is generally inhibited. However, observations by EMG and defecography suggest that the puborectalis may not always relax during defecation in healthy subjects.