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?

The defecation reflex that is evoked when the rectum is distended involves three responses: 1) the internal anal sphincter relaxes, 2) the external anal sphincter contracts, and 3) a conscious urge to defecate is perceived. This neural reflex involves the pelvic nerve, which provides the parasympathetic preganglionic innervation to the internal anal sphincter (composed of smooth muscle) and also carries the sensory afferent information from the rectum to the spinal cord, and the pudendal nerve, which carries the somatic efferent inputs to the external anal sphincter (composed of skeletal muscle). In the patient described above, the pelvic nerve is most likely damaged since neither the reflex relaxation of the internal anal sphincter nor the urge to defecate is evoked by rectal distention. Damage to the internal anal sphincter would most likely cause resting tone to be low and, if anything, lead to fecal incontinence rather than constipation. Fuhermore, damage to the internal anal sphincter could not explain the failure of the appearance of the urge to defecate. The external anal sphincter appears to function normally since distention of the rectum evokes the expected contraction. The normal contraction of the external anal sphincter also suggests that the pudendal nerve is intact. Ref: Moon D.A., Foreman K.B., Albeine K.H. (2011). Chapter 12. Pelvis and Perineum. In D.A. Moon, K.B. Foreman, K.H. Albeine (Eds), The Big Picture: Gross Anatomy.