Distension of the rectum by a balloon causes relaxation of the internal anal sphincter. In a child with Hirschsprung disease, balloon manometry fails to show this response. The reflex depends on:
- A Extrinsic sympathetic fibres from the hypogastric plexus contracting the sphincter
- B Intrinsic myenteric neurons mediating receptive relaxation of the internal anal sphincter, which are absent in the aganglionic segment ✓
- C Voluntary pudendal nerve activity relaxing the external anal sphincter
- D Circulating vasoactive intestinal peptide released from the adrenal medulla
Correct answer: B. Intrinsic myenteric neurons mediating receptive relaxation of the internal anal sphincter, which are absent in the aganglionic segment
Explanation
The rectoanal inhibitory reflex is mediated entirely within the enteric nervous system: rectal distension activates intrinsic neurons that release inhibitory neurotransmitters such as nitric oxide and VIP onto internal anal sphincter smooth muscle, producing relaxation. Because the aganglionic segment of Hirschsprung disease lacks these myenteric neurons, the reflex is absent, and its demonstration by anorectal manometry is a standard diagnostic test. The external sphincter and pudendal nerve are not involved.
Reference: Sabiston Textbook of Surgery, 21st ed.
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