What substance's presence in the duodenum leads to the release of ACh?

The presence of fatty acids in the duodenum plays a critical role in stimulating the release of acetylcholine (ACh). When fatty acids enter the duodenum, they trigger the secretion of hormones like cholecystokinin (CCK). CCK then acts on the vagus nerve, leading to an increase in the parasympathetic output that promotes ACh release. This process is part of the digestive system's complex feedback mechanism to enhance digestion and absorption of nutrients.

In contrast, protein and carbohydrates, like maltose, also influence digestion but not through the same direct stimulation of ACh release in response to their presence. Pepsin, a digestive enzyme activated in the stomach, is not involved in triggering ACh release upon entering the duodenum; its activity primarily takes place in the acidic environment of the stomach. This distinction highlights the specific role of fatty acids in eliciting ACh release as part of the digestive process.