Gastric Acid Secretion – Mechanism & Phases

B i.e. Parasympathetic

B i.e. Hormonal

Ans. C i.e. Nervous mechanism

The cephalic phase of gastric juice secretion

• Thirty percent of the total gastric acid secretions to be produced is stimulated by anticipation of eating and the smell or taste of food.
• This signalling occurs from higher centres in the brain through the Vagus Nerve.
• It activates parietal cells to release acid and ECL cells to release histamine.
• The Vagus nerve also releases Gastrin Releasing Peptide onto G cells.
• Finally, it also inhibits somatostatin release from D cells

Ans. D: Low gastric pH

Food in the stomach increase gastric acid secretion by stretching the receptors in the wall of the stomach (mechanical stimulus) The fibers from the receptor enter Meissner’s plexus (site of ganglion cells of vagus nerve)

Products of digestion (specially amino acids) in the stomach also stimulates gastric mucosa (chemical stimulus)

Gastric secretion

Gastric acid is produced by parietal cells (also called oxyntic cells) in the stomach. There are three phases in the secretion of gastric acid:

• The cephalic phase: Thirty percent of the total gastric acid secretions to be produced is stimulated by anticipation of eating and the smell or taste of food

The gastric phase: Sixty percent of the acid secreted is stimulated by the distention of the stomach with food. Plus, digestion produces proteins, which causes even more gastrin production

The intestinal phase: The remaining 10% of acid is secreted when chyme enters the small intestine, and is stimulated by small intestine distention.

Regulation of secretion

• Gastric acid production is regulated by both the autonomic nervous system and several hormones.
• The parasympathetic nervous system, via the vagus nerve, and the hormone gastrin stimulate the parietal cell to produce gastric acid, both directly acting on parietal cells and indirectly, through the stimulation of the secretion of the hormone histamine from enterochromaffine-like cells (ECL).
• Vasoactive intestinal peptide, cholecystokinin, and secretin all inhibit production.
• The production of gastric acid in the stomach is tightly regulated by positive regulators and negative feedback mechanisms.
• Four types of cells are involved in this process: parietal cells, G cells, D cells and enterochromaffine-like cells.
• Besides this, the endings of the vagus nerve (CN X) and the intramural nervous plexus in the digestive tract influence the secretion significantly.
• Nerve endings in the stomach secrete two stimulatory neurotransmitters: acetylcholine and gastrin-releasing peptide. Their action is both direct on parietal cells and mediated through the secretion of gastrin from G cells and histamine from enterochromaffine-like cells.
• Gastrin acts on parietal cells directly and indirectly too, by stimulating the release of histamine.
• The release of histamine is the most important positive regulation mechanism of the secretion of gastric acid in the stomach.

• Its release is stimulated by gastrin and acetylcholine and inhibited by somatostatin.