Why does stool sit in rectum without urge to defecate?

Hope this helps: When enough stool moves to rectum, the amount of stool causes the rectum to stretch/distend. w “stretch” receptors signal the brain when stretched.The defecation reflex triggers the two sphincters at anal canal. The first-internal anal sphincter- can’t be controlled voluntarily. The second-external anal sphincter,you have some control over. https://www.healthline.com/health/defecation-reflex