What triggers relaxation of the lower esophageal sphincter (LES)?

• A. During coughing.
• B. During swallowing.
• C. In response to a meal.
• D. During sleeping.

Answer: (B)

During swallowing, a reflex triggered by the arriving bolus causes the lower esophageal sphincter to relax briefly so the food can enter the stomach. This deglutitive relaxation is coordinated with the esophageal peristaltic wave and is mediated by the vagus nerve releasing inhibitory signals (like nitric oxide and VIP) to the LES smooth muscle. Once the bolus passes into the stomach, the LES tonicity returns to its closed state to prevent reflux.

The other scenarios don’t initiate this normal opening. Coughing can raise chest pressure and may provoke reflux rather than trigger the opening, sleeping changes reflexes and can increase reflux risk, and while a meal can lead to transient relaxations, the primary trigger for LES relaxation in the act of swallowing is the swallow itself.