Internal anal sphincter is a apart of

Internal anal sphincter is a apart of:
a) Puborectalis muscle
b) Deep perineal muscles
c) Internal longitudinal fibers
d) Internal circular fibers
Correct Answer - D
D. i.e. Internal circular fibres
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 (periarterial) and hypogastric plexuses; and
inhibited (i.e. sphincter relaxed) by parasympathetic pelvic
splanchnic nerves.