- The cephalic phase of gastric acid secretion is the initial phase that occurs even before food enters the stomach.
- This phase is primarily stimulated by the vagus nerve, which is part of the parasympathetic nervous system.
- When you see, smell, taste, or even think about food, the brain sends signals via the vagus nerve to the stomach, triggering the release of gastric acid and digestive enzymes in anticipation of food digestion.

- During this phase, the vagus nerve stimulates the parietal cells to secrete hydrochloric acid (HCl) indirectly by releasing acetylcholine, as well as promoting the release of gastrin and histamine, which further enhance acid secretion.

- In contrast, the gastric phase is stimulated by the presence of food in the stomach itself, and the intestinal phase involves the response to chyme entering the small intestine. The postprandial phase generally refers to the time after eating but is not specific to vagal stimulation.

Therefore, the primary stimulation via the vagus nerve occurs during the cephalic phase of gastric acid secretion.