Tone of the anal sphincter in maintained by
Nervi erigentes
Inferior hypogastric plexus
Inferior rectal nerve
Pelvic splanchnic nerves
The anal sphincters and puborectalis are the primary muscles responsible for continence. There are two sphincters: the internal anal sphincter, and the external anal sphincter. The internal sphincter is responsible for 85% of the resting muscle tone and is involuntary. ref - BDC 6e vol2 pg413-416
Abdomen and pelvis