GI physiology (digestion, absorption, motility) — MCQs

Q: A 50-year-old woman with long-standing diabetes presents with severe, watery diarrhea that wakes her at night. Stool studies show normal osmotic gap and negative stool cultures. Colonoscopy is normal. Trial of fasting does not improve diarrhea. Gastric emptying study shows delayed emptying. What neurotransmitter deficiency in the enteric nervous system best explains both her gastric and colonic dysmotility?

Nitric oxide. ***Nitric oxide*** - **Nitric oxide (NO)** is the primary **inhibitory neurotransmitter** in the enteric nervous system responsible for mediating **receptive relaxation** of the stomach and smooth muscle inhibition during peristalsis. - In **diabetic autonomic neuropathy**, loss of nitrergic neurons leads to **gastroparesis** (failed pyloric relaxation) and **diabetic diarrhea** due to uncoordinated colonic motility and loss of descending inhibition. *Acetylcholine* - **Acetylcholine** is the primary **excitatory neurotransmitter** responsible for muscle contraction; a deficiency would likely lead to paralytic ileus rather than secretory-pattern diarrhea. - Though diabetic neuropathy affects cholinergic fibers, the hallmark of the inhibitory dysmotility seen in **gastroparesis** is specifically linked to the NO pathway. *Serotonin* - **Serotonin (5-HT)** is predominantly involved in the initiation of the **peristaltic reflex** and intestinal secretion via enterochromaffin cells. - While 5-HT levels can be altered in IBS, it is not the classic neurotransmitter deficiency cited for the specific combination of **delayed gastric emptying** and autonomic diarrhea in diabetics. *Substance P* - **Substance P** acts as a **co-transmitter** with acetylcholine to promote smooth muscle contraction and pain transmission. - Deficiency of Substance P would impair motor activity but does not explain the loss of **inhibitory control** and relaxation necessary for normal gastric emptying. *Vasoactive intestinal peptide* - **Vasoactive intestinal peptide (VIP)** is an inhibitory neurotransmitter that promotes intestinal secretion and smooth muscle relaxation. - While VIP is involved in relaxation, **Nitric oxide** is considered the more critical mediator for the **pyloric and fundal relaxation** that is specifically impaired in diabetic gastropathy.

GI physiology (digestion, absorption, motility) — MCQs

A 50-year-old woman with long-standing diabetes presents with severe, watery diarrhea that wakes her at night. Stool studies show normal osmotic gap and negative stool cultures. Colonoscopy is normal. Trial of fasting does not improve diarrhea. Gastric emptying study shows delayed emptying. What neurotransmitter deficiency in the enteric nervous system best explains both her gastric and colonic dysmotility?

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