External Anal Sphincter Function

The external anal sphincter is the voluntary, striated muscle that encircles the anal canal and provides the conscious component of faecal continence. Unlike the smooth-muscle internal sphincter, which is tonically active without conscious effort, the external sphincter can be contracted at will to raise anal pressure, allowing defecation to be deferred and continence to be preserved during sudden challenges such as coughing.

Definition

The external anal sphincter is a ring of voluntary striated muscle surrounding the anal canal, innervated chiefly by the pudendal nerve, whose contraction supplements the tonic internal sphincter to maintain faecal continence and to defer defecation.

Scope

This topic covers the structure and innervation of the external anal sphincter, its resting and voluntary activity, its contribution to the anal pressure profile, its reflex responses to rectal distension and raised abdominal pressure, and its place in the overall continence mechanism alongside the internal sphincter and puborectalis. It is reference material on normal physiology.

Core questions

• How is the external anal sphincter structured and innervated?
• How does it contribute to resting and squeeze anal pressure?
• How does it respond reflexly to rectal filling and raised abdominal pressure?
• How does it act together with the internal sphincter and puborectalis?

Key concepts

• Striated (voluntary) muscle of the anal canal
• Pudendal nerve innervation
• Tonic resting activity at low amplitude
• Voluntary squeeze pressure
• Contribution to the anal canal pressure profile
• Reflex contraction during the recto-anal inhibitory reflex
• Cough (recto-anal) reflex contraction
• Fatigability relative to the internal sphincter

Mechanisms

The external anal sphincter is composed of striated muscle and is innervated mainly by the pudendal nerve. It maintains a low level of tonic activity even at rest, contributing modestly to resting anal pressure, the larger part of which is generated by the smooth-muscle internal sphincter. On voluntary effort it contracts strongly to produce the squeeze pressure that closes the anal canal and can briefly defer defecation. It also contracts reflexly: during the recto-anal inhibitory reflex, when rectal distension relaxes the internal sphincter, the external sphincter contracts to preserve continence while contents are sampled; and during sudden rises in intra-abdominal pressure, such as coughing, a reflex contraction guards against leakage. Being striated muscle, it is more fatigable than the tonically active internal sphincter, so sustained voluntary closure cannot be maintained indefinitely; effective continence therefore relies on the combined action of the external sphincter, the internal sphincter, and the puborectalis (Bharucha, 2006; Palit et al., 2012). Studies of its mechanical properties indicate that it operates at a short sarcomere length in humans (Mittal et al., 2011).

Clinical relevance

Because the external anal sphincter supplies the voluntary component of continence, its structure, innervation, and contractile behaviour are central to understanding faecal incontinence and to the interpretation of anorectal manometry. This entry describes normal function and is not a basis for individual diagnosis or treatment.

Evidence & guidelines

The physiology summarised here is drawn from reviews of pelvic floor and defecation function (Bharucha, 2006; Palit et al., 2012) and from studies of external sphincter muscle mechanics (Mittal et al., 2011); no clinical practice guideline is invoked for this normal-physiology topic.

History

The external anal sphincter has long been recognised as the voluntary partner of the internal sphincter in continence; manometric and electromyographic study clarified its resting tone, voluntary squeeze, and reflex responses, while later biomechanical work examined its sarcomere mechanics (Mittal et al., 2011).

Key figures

• Adil E. Bharucha
• Ravinder K. Mittal
• S. Mark Scott

Seminal works

• bharucha-2006
• mittal-2011
• palit-2012

Frequently asked questions

How is the external anal sphincter different from the internal anal sphincter?

The external sphincter is voluntary striated muscle that can be contracted at will, whereas the internal sphincter is involuntary smooth muscle that stays tonically contracted and provides most of the resting anal pressure.

Why can't the external anal sphincter hold continence indefinitely?

It is striated muscle and therefore fatigues, so it cannot stay maximally contracted for long; continence over time depends on it acting together with the tonic internal sphincter and the puborectalis muscle.