Final answer:
The control of gastric secretions involves the nervous and endocrine systems, triggering through various phases and utilizing messengers like acetylcholine, histamine, gastrin, and somatostatin, the latter of which inhibits acid secretion and completes the list of four major chemical messengers.
Step-by-step explanation:
The control of gastric secretions is a complex process involving both the nervous and endocrine systems, which ensures efficient digestion. This process incorporates various phases: the cephalic phase, triggered by the idea, sight, or smell of food; the gastric phase, stimulated by the actual presence of food in the stomach; and the intestinal phase, which starts as partially digested food enters the small intestine.
Different chemical messengers are involved in regulating gastric juice production. Acetylcholine, released by parasympathetic neurons, increases secretion of gastric juice. Histamine stimulates acid release from parietal cells. Gastrin, produced by G cells in response to proteins, prompts secretion of hydrochloric acid vital for protein digestion. The fourth major chemical messenger, responsible for inhibiting acid secretion when the stomach empties, is somatostatin.
Other hormones like secretin and cholecystokinin (CCK) also play roles in the digestive process, with secretin promoting a bicarbonate-rich pancreatic secretion and CCK stimulating the release of pancreatic enzymes and bile. However, the specifically mentioned hormonal controls for gastric acid environment include gastrin and somatostatin, pointing to the correct answer being 'c. Somatostatin' for the fourth major chemical messenger involved in gastric secretion.