Tone of the anal sphincter in maintained by
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
Nervi erigentes
Inferior hypogastric plexus
Inferior rectal nerve
Pelvic splanchnic nerves
Abdomen and pelvis