<P> The cephalic phase occurs at the sight, thought and smell of food, which stimulate the cerebral cortex . Taste and smell stimuli are sent to the hypothalamus and medulla oblongata . After this it is routed through the vagus nerve and release of acetylcholine . Gastric secretion at this phase rises to 40% of maximum rate . Acidity in the stomach is not buffered by food at this point and thus acts to inhibit parietal (secretes acid) and G cell (secretes gastrin) activity via D cell secretion of somatostatin . </P> <P> The gastric phase takes 3 to 4 hours . It is stimulated by distension of the stomach, presence of food in stomach and decrease in pH . Distention activates long and myenteric reflexes . This activates the release of acetylcholine, which stimulates the release of more gastric juices . As protein enters the stomach, it binds to hydrogen ions, which raises the pH of the stomach . Inhibition of gastrin and gastric acid secretion is lifted . This triggers G cells to release gastrin, which in turn stimulates parietal cells to secrete gastric acid . Gastric acid is about 0.5% hydrochloric acid (HCl), which lowers the pH to the desired pH of 1 - 3 . Acid release is also triggered by acetylcholine and histamine . </P> <P> The intestinal phase has two parts, the excitatory and the inhibitory . Partially digested food fills the duodenum . This triggers intestinal gastrin to be released . Enterogastric reflex inhibits vagal nuclei, activating sympathetic fibers causing the pyloric sphincter to tighten to prevent more food from entering, and inhibits local reflexes . </P> <Table> <Tr> <Td> </Td> <Td> This section needs expansion with: digestion of other substances . You can help by adding to it . (August 2011) </Td> </Tr> </Table>

Where does digestion take place in the digestive system