Stimulation occurs by hormones and nerve cells.

Saliva is stimulated entirely by the nervous system in response to food, real or imagined.

Gastric juice is initially stimulated by nerves in response to the presence of food in the buccal cavity (mouth!). Once food is in the stomach its wall makes the hormone gastrin. This enters the blood stream and on returning to the stomach also stimulates the production of gastric fluid, hence the second peak in this graph.

Gastrin stimulates the continued production of gastric juice.

Entrogasterone is released in response to lipid in the stomach. It decreases the flow of gastric juice and slows the churning of the stomach, delaying the release of fatty food from the stomach.

The fluid form by the action of the stomach is called chyme. When chyme enters the duodenum the fall in pH stimulates the release of secretin which stimulates the release of pancreatic juice. Colecystokinin-pancreozymin (CCK-PZ) stimulates the gall bladder to contract releasing bile onto the food. It also stimulates the release of digestive enzymes from the pancreas.