Three basic stimuli are important in causing pancreatic secretion:
1. Acetylcholine, which is released from the parasympathetic vagus nerve endings and from other cholinergic nerves in the enteric nervous system
2. Cholecystokinin, which is secreted by the duodenal and upper jejunal mucosa when food enters the small intestine
3. Secretin, which is also secreted by the duodenal and jejunal mucosa when highly acidic food enters the small intestine
The first two of these stimuli, acetylcholine and chole cystokinin, stimulate the acinar cells of the pancreas, causing production of large quantities of pancreatic digestive enzymes but relatively small quantities of water and electrolytes to go with the enzymes. Without the water, most of the enzymes remain temporarily stored in the acini and ducts until more fluid secretion comes along to wash them into the duodenum. Secretin, in contrast to the first two basic stimuli, stimulates secretion of large quantities of water solution of sodium bicarbonate by the pancreatic ductal epithelium.
Multiplicative Effects of Different Stimuli. When all the different stimuli of pancreatic secretion occur at once, the total secretion is far greater than the sum of the secretions caused by each one separately. Therefore, the various stimuli are said to “multiply,” or “potentiate,” one another. Thus, pancreatic secretion normally results from the combined effects of the multiple basic stimuli, not from one alone.
