Question:medium
What is the effect of acetylcholine on the lower oesophageal sphincter (LES)?
What is the effect of acetylcholine on the lower oesophageal sphincter (LES)?
Show Hint
Acetylcholine excites gut smooth muscle and raises LES tone.
Updated On: Jun 24, 2026
- Contraction
- Relaxation
- No effect
- Contraction followed by relaxation
Show Solution
The Correct Option is A
Solution and Explanation
Step 1: Frame the LES as a pressure barrier. Anything that increases its resting pressure tightens the gate against acid reflux; anything that lowers it loosens the gate.
Step 2: Acetylcholine is the dominant parasympathetic excitatory transmitter for gut smooth muscle. At the LES it binds muscarinic receptors and increases sphincter tone, that is, it contracts the muscle. So the correct effect is contraction, option (a).
Step 3: Remember the contractor list: acetylcholine, gastrin, motilin, substance P, histamine, alpha adrenergic agonists, prostaglandin F2 alpha, and protein-rich food all raise LES pressure. By contrast nitric oxide and vasoactive intestinal peptide relax it.
Step 4: Therefore relaxation, no effect, and a biphasic contraction then relaxation are all inconsistent with the simple tonic contraction acetylcholine produces.
\[\boxed{\text{Contraction}}\]
Step 2: Acetylcholine is the dominant parasympathetic excitatory transmitter for gut smooth muscle. At the LES it binds muscarinic receptors and increases sphincter tone, that is, it contracts the muscle. So the correct effect is contraction, option (a).
Step 3: Remember the contractor list: acetylcholine, gastrin, motilin, substance P, histamine, alpha adrenergic agonists, prostaglandin F2 alpha, and protein-rich food all raise LES pressure. By contrast nitric oxide and vasoactive intestinal peptide relax it.
Step 4: Therefore relaxation, no effect, and a biphasic contraction then relaxation are all inconsistent with the simple tonic contraction acetylcholine produces.
\[\boxed{\text{Contraction}}\]
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