Internal anal sphincter is a apa of:: (A) Puborectalis muscle, (B) Deep perineal muscles, (C) Internal longitudinal fibers, (D) Internal circular fibers
Answer is D. Involuantry internal anal sphincter is formed by thickening of circular muscle layer (i.e. circular layer of muscularis externa, a smooth muscle) of upper end (2/3 or 3/4) of anal canal. This sphincter remains in the state of tonic contraction most of the time to maintain resting tone or pressures (-90 cm H20) & to prevent leakage of fluid or flatus. Its contraction (tonus) is maintained by sympathetic fibers from superior rectal (periaerial) and hypogastric plexuses; and inhibited (i.e. sphincter relaxed) by parasympathetic pelvic NERVES. Ref: Gray&;s anatomy text book of anatomy BD chaurasia 21st Ed.