• PHASES OF ACID SECRETION — The physiologic stimulation of acid secretion has classically been divided into three interrelated phases: cephalic, gastric, and intestinal .• The cephalic phase is activated by the thought, taste, smell and site of food, and swallowing. It is mediated mostly by cholinergic/vagal mechanisms.• The gastric phase is due to the chemical effects of food and distension of the stomach. Gastrin appears to be the major mediator since the response to food is largely inhibited by immunoneutralizing or blocking gastrin action at its receptors.
• 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.
• They directly act on parietal cells and indirectly, through the stimulation of the secretion of the hormone histamine from enterochromaffine-like cells (ECL).
• 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• 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• Nerve endings in the stomach secrete two stimulatory neurotransmitters• 1/ acetylcholine• 2/ 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• Gastrin acts on parietal cells directly and indirectly too, by stimulating the release of histamine.
positive regulation mechanism• The release of histamine is the most important positive regulation mechanism of the secretion of gastric acid in the stomach
• Release of histamine by gastrin from enterochrommafin-like cells (ECL, in green) appears to be the major physiologic mechanism by which gastrin stimulates acid secretion, although parietal cells (in blue) also have gastrin receptors. In addition, the ECL cells integrate stimulatory messages from cholinergic nerves and inhibition by locally released somatostatin.