Which reflex is most directly responsible for the internal anal sphincter relaxation in response to rectal distention?

Which reflex is most directly responsible for the internal anal sphincter relaxation in response to rectal distention?

• A. Gastrocolic reflex
• B. Sampling reflex
• C. Rectoanal inhibitory reflex
• D. Wink reflex

Answer: (C)

The rectoanal inhibitory reflex is the automatic relaxation of the internal anal sphincter in response to rectal distention. When the rectum fills, stretch signals travel to the sacral spinal cord and trigger inhibitory neurons that act on the internal anal sphincter, a smooth muscle one. This causes the sphincter to relax, lowering the resting anal tone so stool can move toward the anal canal and sensations of opportunity to defecate can be assessed. The relaxation is mediated by inhibitory neurotransmitters like nitric oxide and VIP released onto the internal sphincter. Other reflexes described either affect different parts of the digestive tract or involve different mechanisms. The gastrocolic reflex increases colonic motility after meals, not the direct relaxation of the internal anal sphincter. The sampling reflex relates to sensing stool properties and deciding on continence or evacuation, rather than the immediate relaxation response to rectal distention. The Wink reflex is not related to anorectal control.

The rectoanal inhibitory reflex is the automatic relaxation of the internal anal sphincter in response to rectal distention. When the rectum fills, stretch signals travel to the sacral spinal cord and trigger inhibitory neurons that act on the internal anal sphincter, a smooth muscle one. This causes the sphincter to relax, lowering the resting anal tone so stool can move toward the anal canal and sensations of opportunity to defecate can be assessed. The relaxation is mediated by inhibitory neurotransmitters like nitric oxide and VIP released onto the internal sphincter.

Other reflexes described either affect different parts of the digestive tract or involve different mechanisms. The gastrocolic reflex increases colonic motility after meals, not the direct relaxation of the internal anal sphincter. The sampling reflex relates to sensing stool properties and deciding on continence or evacuation, rather than the immediate relaxation response to rectal distention. The Wink reflex is not related to anorectal control.