Defecation is a physiological act of freeing the intestines from the feces. Defecation is regulated by the central nervous system and occurs as a result of irritation of the sensitive nerve endings of the rectal mucosa . Excitation from nerve endings is transmitted to the spinal cord , where the center of the bowel movement is located at the level of the lumbar part. From this center, the excitement spreads to the cerebral cortex, causing a specific sensation and urge to defecate. But even in the presence of the urge to defecate fecal masses are held in the rectum due to the constant tonic tension of the two anal sphincters - external and internal. The act of stool occurs reflexively. Irritation of receptors in the wall of the ampulla of the rectum causes involuntary relaxation of its internal sphincter, peristaltic contractions of the muscles of the wall of the large intestine and rectum. Then there is an arbitrary relaxation of the external sphincter of the rectum. Retention of a breath, tension of an abdominal tension, a diaphragm and muscles of a pelvic bottom promote emptying of a rectum.
With damage to the spinal cord (inflammatory processes, injuries, tumors) above the center of the bowel movement, the effects coming from the overlying brain structures, especially from the cerebral cortex, are interrupted, and the bowel movement becomes involuntary. Involuntary defecation may occur with strong emotional effects (for example, with fear). Some diseases of the gastrointestinal tract (dysentery, tumors, ulcers) can cause a violation of the act of defecation, manifested in the form of diarrhea or constipation .