During a normal anal rectal motility study, what is expected to happen to the internal and external sphincters upon stimulation?

In a normal anal rectal motility study, the expected response upon stimulation involves the reflex action of the internal and external anal sphincters. The internal sphincter, which is comprised of smooth muscle and controlled involuntarily, typically relaxes in response to rectal distension or stimulation. This relaxation is part of the normal defecation reflex, allowing for the passage of fecal material.

Simultaneously, the external sphincter—made of striated muscle and under voluntary control—tends to contract, providing a conscious defense against inappropriate bowel movements. This contraction allows for the maintenance of continence until an appropriate time to defecate. The synchronized activity of these two sphincters reflects their roles in the regulation of bowel movements.

Thus, the answer correctly states that upon stimulation, the internal sphincter relaxes while the external sphincter contracts, facilitating the body’s ability to regulate defecation appropriately.