Which statement regarding the internal anal sphincter is correct?
- A It is a thickening of the circular smooth muscle of the gut and relaxes on distension of the rectal ampulla ✓
- B It is skeletal muscle supplied by the pudendal nerve
- C It extends further distally than the external sphincter subcutaneous part
- D It is pierced by the puborectalis sling
Correct answer: A. It is a thickening of the circular smooth muscle of the gut and relaxes on distension of the rectal ampulla
Explanation
The internal anal sphincter is an involuntary thickening of the circular smooth muscle layer, extending from the anorectal junction to just below the white line of Hilton. It relaxes reflexly when the rectal ampulla is distended, part of the defecation reflex. The voluntary control muscle is the external sphincter, supplied by the pudendal nerve, which makes option B describe the wrong sphincter entirely.
Reference: Last's Anatomy: Regional and Applied, 13th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.