Anal Sphincter Muscle Composition
The anal sphincter complex contains both smooth and skeletal muscle components: the internal anal sphincter is composed of smooth muscle, while the external anal sphincter is composed of skeletal muscle. 1, 2
Anatomical Structure
The anal sphincter complex consists of two distinct muscular layers with different tissue types:
Internal Anal Sphincter (Smooth Muscle)
- Composed entirely of smooth muscle fibers that represent a continuation and thickening of the circular smooth muscle layer of the rectum 1, 2
- Forms flat rings of smooth muscle bundles stacked concentrically, described as resembling "slats of a Venetian blind" 3
- Each ring is covered by its own fascia
- Functions under involuntary (autonomic) control
- Maintains resting anal tone and contributes to continence at rest
External Anal Sphincter (Skeletal Muscle)
- Composed of striated skeletal muscle fibers under voluntary control 1, 2
- Traditionally described as having three parts (subcutaneous, superficial, and deep), though recent anatomic studies question this tripartite subdivision 4
- Encircles the anal canal externally
- Innervated by the pudendal nerve
- Provides voluntary squeeze pressure for continence
Important Clinical Caveats
Recent histologic evidence reveals complexity beyond the traditional model: Studies have identified bundles of smooth muscle fibers interspersed between the skeletal muscle fibers of the external anal sphincter 5, 6. This finding may explain certain physiologic phenomena, such as the external anal contraction on rectal distension or coughing, which cannot be fully explained by skeletal muscle activity alone 5.
The functional anal canal is defined by these sphincter muscles, with the superior border at the palpable upper edge of the anal sphincter and puborectalis muscles (part of the levator ani), and the inferior border at the anal verge 1, 2. This functional definition is approximately 3-5 cm in length and is primarily used in surgical treatment planning 1.
Clinical Relevance
Understanding this dual muscle composition is essential for:
- Surgical planning for anal procedures, including lateral internal sphincterotomy (which targets the smooth muscle internal sphincter) 7
- Anorectal manometry interpretation, where resting pressure reflects internal sphincter (smooth muscle) function and squeeze pressure reflects external sphincter (skeletal muscle) function 8, 9
- Treatment of anal fissures, where topical therapies target smooth muscle relaxation to decrease resting anal pressure 7
- Intersphincteric resection for very low rectal cancer, where dissection occurs between these muscle layers 6