This is initiated by the sight, thought or smell of food. Which can either increase the stimulation of stomach secretions or inhibit them. As you have probably experienced before, the thought of a nice meal will increase the saliva in your mouth and give you the sensation that your mouth is watering. If you haven’t ever noticed this phenomenon, try it now. Imagine your most favourite meal, notice an increase in saliva? As you can imagine this can happen, the smell of rotten food can decrease appetite and therefore, inhibit stomach secretions.
The image below is useful in describing the mechanisms which govern the inhibition or stimulation of stomach secretions.
This phase occurs when the food reaches the stomach, causing it to distend. This distension then increases the stomach secretions, this can also be completed by an increase in PH. A decrease in PH has an opposite effect, this inhibits stomach secretions.
As you may know, stomach secretions are acidic and therefore, the addition of chyme (partially digested food) decreases the PH and therefore, inhibits stomach secretions. If this process did not occur then there would be an increase in peptic (stomach) ulcers caused by over acidification. Over acidification is known as Zollinger-Ellison syndrome and is caused by over secretion of gastrin which promotes and stimulates the release of hydrochloric acid. The extra hydrochloric acid decreases the PH further which causes damage to the mucosa lining of the stomach which normally protects the stomach from corrosion. This results in peptic ulcers which are extremely painful during eating.
The chyme has now moved from the stomach into the duodenum (first section of the small intestine) via the pyloric sphincter. This causes a distention of the duodenum, distention plus the presence of fatty acidic chyme inhibits stomach secretions and prevents over acidification. A breakdown of this process would result in an increase in peptic ulcers.