Question

What is the effect of acetylcholine on the lower oesophageal sphincter (LES)?

• A. Contraction
• B. Relaxation
• C. No effect
• D. Contraction followed by relaxation

Hint:

Acetylcholine excites gut smooth muscle and raises LES tone.

Answer 1

The Correct Option is A

Step 1: The lower oesophageal sphincter is a zone of tonically contracted smooth muscle that acts as a barrier against reflux of gastric juice (pepsin and hydrochloric acid) into the oesophagus. A high resting sphincter pressure protects the oesophageal mucosa.

Step 2: Acetylcholine is the main excitatory neurotransmitter at the smooth muscle of the LES. Acting on muscarinic receptors it raises sphincter tone, so it causes contraction. Hence option (a) is correct.

Step 3: Other agents that raise LES pressure include gastrin, motilin, substance P, alpha adrenergic stimulation, and protein-rich meals. Agents that relax the LES include nitric oxide, vasoactive intestinal peptide, secretin, and progesterone.

Step 4: Relaxation (option b) and no effect (option c) are therefore wrong. A contraction followed by relaxation (option d) does not describe the steady tonic increase produced by acetylcholine, so it is also incorrect.