Review

The role of the external anal sphincter in the physiology of the pelvic floor

  • MICHAEL D. LEVIN

Pelviperineology 2016;35(4):108-112

Objective:

To clarify the physiology of the fecal retention and defecation.

Material & Method:

The comparison of the results of my own X-ray studies with published data on the anal manometry, anatomical dissection and 3D topography was produced.

Results:

Long-term retention of feces was due to tonic contraction of the internal anal sphincter (IAS) and the striated muscles of the pelvic floor including levator ani muscle (LAM), puborectalis muscle (PRM) and the external anal sphincter (EAS). The entrance into the rectum of the next fecal portion increases the rectal pressure, which leads to relaxation of the IAS and to mechanical contraction of the PRM and EAS. At this time, liquid feces and gas penetrate into the upper part of the anal canal, where they are identified. Following this, the rectum relaxes, adapting to this volume. The rectal pressure decreases, which causes contraction of the IAS and relaxation of the PRM and EAS. This anorectal inhibitory reflex can be repeated up to several times per hour. During defecation relaxation of the IAS, PRM occur, as well as deep and subcutaneous portions of the EAS. At the same time, a strong rectal peristalsis pushes the stool through the wide open anal canal. The upper part of the anal canal is opened as a result of the contraction of the LAM. The contraction of the superficial portion of the EAS leads to the opening of the distal part of the anal canal. The wide opening of the anal canal greatly reduces resistance to the stool promotion.

Conclusion:

All portions of the EAS have different points of attachment and, therefore, have different functions. In fact, each of them is the separate sphincter.

Abbreviations:

IAS – internal anal sphincter; EAS – external anal sphincter; PRM – puborectalis muscle; IAP– intra-abdominal pressure; LAM – levator ani muscle; TP-1 – threshold pressure of the first order, provoking of the anorectal inhibitory reflex; TP-2 - the threshold pressure of the second order, provoking the need of defecation; TP-3 - the threshold pressure of the third order, causing of the defecation reflex.

Keywords: Anorectal physiology; Defecation; Fecal retention; Hypothesis; Rectum; Anal canal