A patient with long standing constipation enters a clinical research study. After a complete physical examination,a small intraluminal balloon is inseed through the anus to the rectum. Transducers are also inseed to measure internal and external anal sphincter pressures. Inflation of the rectal balloon causes the external anal sphincter to contract, but the internal anal sphincter, which exhibits normal tone, fails to relax and the urge to defecate is not sensed. Which of the following structures is most likely damaged?