From the Guidelines
The external anal sphincter is controlled by the puborectalis muscle, which is part of the levator ani muscle group in the pelvic floor. This voluntary muscle forms a sling around the rectum and is responsible for maintaining fecal continence by keeping the anal canal closed. The puborectalis muscle receives innervation from the pudendal nerve (S2-S4), allowing for conscious control of bowel movements. When relaxed, it permits defecation, and when contracted, it prevents it. This muscle works in conjunction with the internal anal sphincter (which is involuntary and controlled by the autonomic nervous system) to provide complete control over bowel function. Key points to consider when evaluating the control of the external anal sphincter include:
- The anatomy of the pelvic floor muscles, including the levator ani and puborectalis muscles
- The innervation of the puborectalis muscle by the pudendal nerve
- The role of the internal anal sphincter in maintaining fecal continence According to a recent study 1, the internal anal sphincter is responsible for most of the anal sphincter resting tone, and its repair can improve anal incontinence rates. However, this study does not directly address the control of the external anal sphincter. In clinical practice, weakness or damage to the puborectalis muscle can lead to fecal incontinence, which is why pelvic floor exercises are often recommended to strengthen this muscle in cases of mild incontinence or following childbirth or surgery.
From the Research
Muscle Control of External Anal Sphincter
The external anal sphincter is controlled by the following muscles:
- Puborectalis muscle
- Deep portion of the external sphincter, which together form the "top loop" of the external anal sphincter 2
- The external anal sphincter itself, which consists of a series of U-shaped loops, distinguishable as three main "loops": top, intermediate, and base 2
Nerve Supply
The nerve supply to the external anal sphincter comes from the pudendal nerve, which plays a crucial role in controlling the muscle:
- Stimulation of the pudendal nerve can increase anal sphincter pressure, enhancing continence control 3
- The pudendal nerve innervation may be asymmetric, with dominant responses from either the right or left pudendal nerve in different individuals 4
Mechanism of Action
The mechanism of action of the external anal sphincter in anal continence and during defecation involves:
- An air-tight occlusion of the anal canal achieved by the "triple-loop system" of the external sphincter, which compresses opposed alternating anal segments 2
- A process of "vermicular contractions" which dispels the last fecal portion from the anal canal, resulting from the loop arrangement of the muscle bundles 2