A 30-year-old man shows increased contractility in the gastrointestinal tract. What neurotransmitter is most likely responsible for this response?

• A. Acetylcholine
• B. Motilin
• C. Norepinephrine
• D. Serotonin

Answer: (B)

Increased contractility in the gastrointestinal tract is primarily associated with the action of motilin. Motilin is a hormone that plays a significant role in regulating gastric motility and is secreted by the M cells in the small intestine. It enhances gastrointestinal motility by stimulating the motility of the smooth muscle in the digestive tract and is particularly important during the fasting state.

The role of motilin is especially pronounced during the migratory motor complex (MMC), which occurs during periods without food and helps to clear residual undigested material from the stomach and small intestine. This increased motility facilitates digestion and prepares the gut for the next meal, thereby ensuring efficient gastrointestinal function.

Acetylcholine, on the other hand, is a neurotransmitter that also promotes gastrointestinal motility but is primarily linked to parasympathetic nervous system activity and mediates various muscle contractions in the GI tract. Norepinephrine generally inhibits gastrointestinal motility by acting on adrenergic receptors, while serotonin plays a role in signaling within the enteric nervous system but is not as directly linked to increased gastrointestinal contractility as motilin.

Thus, motilin is the specific neurohormonal regulator responsible for the increased contractility observed in this scenario.