<prompt> Internal anal sphincter is a apa of:Puborectalis muscleDeep perineal musclesInternal longitudinal fibersInternal circular fibers  <response>: 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 23 or 34 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. 