The endocrine portion, known as the islets of Langerhans, secretes the hormones insulin and glucagon. The regulation of gastric function occurs in three phases. In the cephalic phase, the activity of higher brain centers, acting via the vagus nerve, stimulates gastric juice secretion. In the gastric phase, the secretion of HCl and pepsin is controlled by the gastric contents and by the hormone gastrin, secreted by the gastric mucosa. In the intestinal phase, the activity of the stomach is inhibited by neural reflexes and hormonal secretion from the duodenum.
Intestinal function is regulated, in part, by local short reflexes.
The submucosal and myenteric plexuses contain autonomic motor neurons, sensory neurons, and interneurons. Gastrointestinal hormones may be needed for the maintenance of the GI tract and accessory digestive organs. The digestion of starch begins in the mouth through the action of salivary amylase. Protein digestion begins in the stomach through the action of pepsin. The brush border contains digestive enzymes that help to complete the digestion of proteins into amino acids.