Final answer:
The parasympathetic nervous system, through the vagus nerve, increases gastric acid secretion and motility in the stomach. It stimulates the release of gastrin and acetylcholine, which boost the production of HCl and promote digestion.
Step-by-step explanation:
The effects of the parasympathetic nervous system, specifically through the vagus nerve, on the stomach include stimulating gastric juice secretion and increasing motility. When food enters the stomach and distends it, this activates stretch receptors, which in turn activate parasympathetic neurons to release acetylcholine. This neurotransmitter then provokes the increased secretion of gastric juice from the gastric glands. Additionally, partially digested proteins, caffeine, and rising pH in the stomach stimulate the release of gastrin from enteroendocrine G cells. This gastrin then prompts parietal cells to produce more hydrochloric acid (HCl), creating an acidic environment necessary for the conversion of pepsinogen to pepsin, which is vital for protein digestion. Moreover, gastrin also induces vigorous smooth muscle contractions. Answering the student's question, the parasympathetic nervous system, via the vagus nerve, increases gastric acid secretion (answer option a) and induces muscle contractions rather than relaxation.