Cholecystokinin is a duodenal hormone released by the pancreas, but also produced by neurons in the enteric nervous system. It is structurally similar to gastrin, and plays a key role in facilitating digestion within the small intestine. Cholecystokinin is secreted from mucosal epithelial cells in the duodenum by I-cells in response to fatty acids, peptides and amino acids. It stimulates delivery into the small intestine of digestive enzymes from the pancreas and bile from the gall bladder, the latter by relaxation of the sphincter of Oddi . The net effect is the metering of calories through the pylorus of the stomach.