Defecation is a neurological reflex movement, and the centers that cause the human body’s defecation reflex are the sacral medullary defecation center (lower center) and the higher center of the cerebral cortex, and the higher center has a control role over the lower center. In general, when there is about 100ml of stool in the rectum, the urge to defecate can be generated, and when the rectal contents reach about 300ml, a strong urge to defecate will be generated. The process of impulse generation is as follows: when feces enters the rectum, the pressure receptors at the end of the rectum are indirectly stimulated by the impulse distension of the rectum, and then the afferent impulse is transmitted along the afferent fibers of the sacral and inferior ventral nerves to the defecation center (sacral medullary center); the impulse from the center is transmitted along the parasympathetic nerve of the pelvic nerve, causing contraction of the descending colon, sigmoid colon and rectum, and at this time, the internal anal sphincter is reflexively relaxed. At the same time, the impulses from the sacral medullary center via the sacral and pubic nerves cause the external anal sphincter to relax and the anorectum to form a funnel shape, so that feces can be excreted as soon as conditions permit. Under normal circumstances, the defecation reflex is under the control of the cerebral cortex, i.e., the rectal impulse is transmitted to the sacral medullary centers and also to the higher centers of the cerebral cortex and causes the intention to defecate. The instructions from the cerebral cortex can, on the one hand, strengthen the activity of the sacral medullary defecation center and, on the other hand, consciously increase the intra-abdominal pressure to promote defecation. However, if the defecation environment does not allow or intentionally interrupt defecation, the impulses from the cerebral cortex can inhibit the activity of the sacral medullary defecation center, so that the contraction of the external sphincter is strengthened, the internal sphincter is passively compressed, the rectum is reflexively dilated, the feces is “retrograde” to the rectum, and the urge to defecate gradually disappears. This is the reason why the people say that the stool can be “held back”.