Final answer:
Acetylcholine induces HCl production directly via parietal cells and indirectly via enteroendocrine G cells, which are crucial for acidifying the stomach for protein digestion.
Step-by-step explanation:
Acetylcholine stimulates secretion of HCl directly when it binds to receptors on parietal cells and indirectly when it binds to receptors on enteroendocrine G cells. When the sushi you ate reaches the stomach, it causes distention that activates the stretch receptors, leading to the stimulation of parasympathetic neurons to release acetylcholine. This release provokes increased secretion of gastric juice. As gastrin is released from enteroendocrine G cells in response to certain stimuli like partially digested proteins, caffeine, and rising pH, parietal cells are then stimulated to produce more HCl. The HCl serves a crucial function by creating an acidic environment for the conversion of pepsinogen to pepsin, thus facilitating protein digestion.