Cephalic Phase
During the cephalic phase of digestion, the smell, sight, thought, or
initial taste of food activates neural centers in the cerebral cortex,
hypothalamus, and brain stem.o Neural regulation.When the stomach walls
are distended or pH increases because proteins have entered the
stomach and buff ered some of the stomach acid, the stretch receptors and chemoreceptors are activated, and a neural negative feedback loop is set in motion (Figure 24.26).From the stretch receptors
and chemoreceptors, nerve impulses propagate to the submucosal
plexus, where they activate parasympathetic and enteric neurons.The facial and
glossopharyngeal nerves stimulate the salivary glands to secrete
saliva, while the vagus nerves stimulate the gastric glands to secrete
gastric juice.Neural and hormonal mechanisms regulate the gastric phase
of digestion to promote gastric secretion and gastric motility.The
resulting nerve impulses cause waves of peristalsis and continue to
stimulate the flow of gastric juice from gastric glands.The peristaltic
waves mix the food with gastric juice; when the waves become strong
enough, a small quantity of chyme undergoes gastric emptying intoThe brain stem then activates the
facial (VII), glossopharyngeal (IX), and vagus (X) nerves.Food of any kind distends the stomach and stimulates stretch receptors in its walls.Chemoreceptors in the stomach
monitor the pH of the stomach chyme.Gastric Phase
Once food reaches the stomach, the gastric phase of digestion
begins.