External Anal Sphincter Response to Rectal Distention: Learned Response or Reflex

Three experiments were conducted to determine whether the external anal sphincter contraction which typically follows rectal distention is a reflex or a voluntary response. Experiment I compared 15 chronically constipated patients to 10 normal subjects. A reflex should be reliably elicited by its unconditional stimulus; but if this response is voluntary, chronically constipated patients would be less likely to show it because they have had fewer opportunities to practice it. Only half of chronically constipated patients showed the response compared to 100% of normals. Experiment II investigated whether the response is elicited by rectal distention during sleep in 10 healthy subjects. The response was significantly less likely to occur during sleep. Experiment III in 6 normal subjects revealed that this response can be voluntarily omitted. These experiments indicate that external anal sphincter contraction following rectal distention is a voluntary response, not a reflex.