Seeing, smelling and anticipating food in perceived in the brain and the brain informs the stomach that it should prepare for receipt of a meal. This communication is composed of parasympathetic stimuli transmitted thought the vagus nerve to the enteric nervous system, resulting in release of acetylcholine in the vicinity of G cells and parietal cells. Binding of acetylcholine to its receptor on G cells induces secretion of the hormone gastrin, which, in concert with acetylcholine and histamine, stimulates parietal cells to secrete small amounts of acid. Additionally, a low level of gastric motility is induced. In essense, the gastric motor is turned on and begins to idle.
Distension excites stretch receptors and irritation activates chemoreceptors in the mucosa.
Additionally, activation of the enteric nervous system and release of gastrin cause vigorous smooth muscle contractions. The net result is that secretory and motor functions of the stomach are fully turned on - lots of acid and pepsinogen are secreted, pepsinogen is converted into pepsin and vigorous grinding and mixing contractions take place. Its seems to be important for the small intestine to be able to slow down gastric emptying, probably to allow it time to neutralize the acid and efficiently absorb incoming nutrients. Hence, this phase of gastric function is dominated by the small intestine sending inhibitory signals to the stomach to slow secretion and motility. Secondly, enteric hormones such as cholecystokinin and secretin are released from cells in the small intestine and contribute to suppression of gastric activity.