defecation reflex
emptying of the rectum and lower portion of the colon in response to movement or pressure of fecal material. As the rectum fills, receptors send impulses to the spinal cord. Motor nerve impulses are transmitted through sacral fibers of the parasympathetic nervous system, causing relaxation of the inner anal sphincter and contraction of muscles of the abdominal wall. In order for the act to be completed, the external sphincter muscle, which is controlled by voluntary, skeletal muscle nerves, must also be relaxed. The voluntary nervous system can override the reflex and, under normal conditions, prevent automatic defecation. Also called rectal reflex. See also anal stage; toilet training.