Final answer:
Parasympathetic impulses increase gastric juice secretion by releasing acetylcholine, which stimulates gastric glands, and by prompting the secretion of hormones like gastrin that lead to the production of HCl, crucial for digestion.
Step-by-step explanation:
The effect of parasympathetic impulses on gastric secretion is significant. When food enters the stomach, it causes distention and activates stretch receptors. This activation stimulates parasympathetic neurons to release acetylcholine, which results in increased secretion of gastric juice. For example, components like partially digested proteins and caffeine, as well as a rising pH, can trigger the release of gastrin from enteroendocrine G cells. This gastrin further stimulates parietal cells to produce more hydrochloric acid (HCl), which is essential for converting pepsinogen to pepsin and for digesting proteins.
Moreover, the activation of parasympathetic impulses also leads to the increase in motility within the GI tract through smooth muscle contractions. Nevertheless, the stomach has innate regulatory mechanisms to prevent over-secretion of acid, such as blocking HCl secretion and increasing mucous productions when the pH becomes too low.