Anorectal Anatomy and Function

Anatomical Explanation

The puborectalis muscle sits immediately above the internal sphincter and creates a palpable muscular ring that can be felt during digital rectal examination at approximately 3-5 cm from the anal verge 1
During digital examination, the puborectalis should contract during squeeze maneuvers, creating a distinct sensation separate from the sphincter complex below it 1, 2

The examining finger first encounters the anal sphincter complex (internal and external sphincters), which provides resting tone and augments with voluntary squeeze 1
Above this, the puborectalis muscle is palpable as a separate muscular band that also contracts during squeeze efforts, forming the anorectal ring 1, 2
The functional anal canal is defined by these structures, extending from the palpable upper border of the anal sphincter and puborectalis muscles down to the anal verge 3

The intersphincteric space between the internal sphincter and puborectalis is a distinct anatomical plane that becomes clinically relevant in conditions like perianal abscesses and fistulas 4

Do not confuse normal puborectalis contraction with pathology - this is an expected finding during proper digital rectal examination 1
Acute localized tenderness along the puborectalis during palpation suggests levator ani syndrome, not normal anatomy 1, 2
Failure to assess puborectalis function during simulated defecation can miss pelvic floor dysfunction, as a normal cursory examination does not exclude this diagnosis 1, 2

The internal anal sphincter arises from the continuation of the circular smooth muscle layer of the rectum and contributes approximately 15-20% of resting anal pressure 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 5

The internal sphincter lies immediately deep to the external sphincter, separated by the intersphincteric plane 6, 7
The intersphincteric space is the critical anatomical plane located between the internal and external anal sphincters 6

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, approximately 3-5 cm in length 8, 9, 10
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 7
The intersphincteric plane is a critical landmark for identifying fistula tracts and abscesses in perianal disease 6