What is the anatomy of the internal and external anal sphincters?

Anatomy of the Internal and External Anal Sphincters

I cannot provide picture-form anatomical diagrams as requested, but I can describe the detailed anatomical relationships between the internal and external anal sphincters based on the available clinical guidelines and research.

Internal Anal Sphincter (IAS)

The internal anal sphincter is an involuntary ring of smooth muscle that forms the innermost muscular layer of the anal canal and is the major contributor to resting anal pressure. 1

Structural Characteristics:

• Composed of flat rings of smooth muscle bundles stacked one on top of the other, similar to slats of a Venetian blind (approximately 26 ring-like slats per anal canal, range 20-30), with each ring covered by its own fascia 2
• Arises from the continuation of the circular smooth muscle layer of the rectum 3
• The smooth muscle fibers and fascia coalesce at three equidistant points around the anal canal to form three columns that extend distally into the lumen 2
• When viewed anteriorly, these columns are located at the 5 o'clock position (anterior right), 1 o'clock position (posterior right), and 9 o'clock position (lateral left) 2
• Contributes approximately 15-20% of resting anal pressure 3

Functional Role:

• Provides involuntary continence through sustained tonic contraction 1
• Plays a significant role in both involuntary and voluntary continence mechanisms 4
• Structural injury or functional weakness results in passive incontinence of feces and flatus 1

External Anal Sphincter (EAS)

The external anal sphincter is a voluntary skeletal muscle that encircles the anal canal and provides voluntary control of defecation. 5

Structural Characteristics:

• Composed of three ellipsoid rings of skeletal muscle (subcutaneous, superficial, and deep portions) that encircle the anal canal 2
• The muscle bundles are arranged circumferentially in one continuous circle, not in separate loops 5
• All fibers retain skeletal attachment to the anococcygeal ligament and coccyx posteriorly 5
• Some fibers cross the median plane to be continuous with the transverse perinei muscle of the opposite side 5
• The lowermost muscle bundles are completely surrounded by thick fibrous tissue septa derived from the longitudinal anal coat 5

Innervation:

• The perineal branch of S4 supplies the posterior third 5
• The inferior hemorrhoidal nerves supply the anterior two-thirds 5

Anatomical Relationships Between the Sphincters

The intersphincteric space is the critical anatomical plane located between the internal and external anal sphincters. 3

Key Spatial Relationships:

• The internal sphincter lies immediately deep to the external sphincter, separated by the intersphincteric plane 3
• A cleft separates the lower border of the levator ani muscle from the upper border of the external anal sphincter 5
• Fibers from the anterior border of the levator ani muscle extend to cover and blend with the outer surface of the external sphincter 5
• The anal subepithelial smooth muscle arises from the conjoined longitudinal muscle layer, passes through the internal anal sphincter, and inserts into the subepithelial vascular space 3

Clinical Significance

Functional Anal Canal Definition:

• The functional anal canal is defined by the sphincter muscles, extending from the palpable upper border of the anal sphincter and puborectalis muscles (anorectal ring) to the anal verge 3
• Approximately 3-5 cm in length 3
• The inferior border starts at the anal verge, the lowermost edge of the sphincter muscles corresponding to the introitus of the anal orifice 3

Imaging Assessment:

• High-resolution MRI can assess anatomical layers including the internal anal sphincter, intersphincteric space between internal and external sphincters, external anal sphincter, puborectalis muscle, and levator ani muscle 3
• The intersphincteric plane is a critical landmark for identifying fistula tracts and abscesses in perianal disease 3

Common Pitfalls:

• Failure to recognize that the external sphincter is not naturally divided into distinct layers but forms one continuous circumferential muscle may lead to surgical complications 5
• The unique "Venetian blind" structure of the internal sphincter with individual fascial coverings for each ring must be preserved during surgical procedures to maintain continence 2
• Damage to the internal sphincter can result in "stress defecation," a condition of passive incontinence that is difficult to correct 4