Editing Digestion (section)

=== Neural and biochemical control mechanisms ===
Different [[phases of digestion]] take place including: the [[cephalic phase]], [[gastric phase]], and [[intestinal phase]].

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 (biology)|D cell]] secretion of [[somatostatin]].

The gastric phase takes 3 to 4 hours. It is stimulated by [[Gastric distension|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 cell]]s to secrete gastric acid.  Gastric acid is about 0.5% [[hydrochloric acid]], which lowers the pH to the desired pH of 1–3.  Acid release is also triggered by [[acetylcholine]] and [[histamine]].

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 nervous system|sympathetic fibers]] causing the [[Pylorus|pyloric sphincter]] to tighten to prevent more food from entering, and inhibits local reflexes.