Phases of Digestion

How many Phases of Digestion?

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. The brain stem then activates The facial (VII),glossopharyngeal (IX), and vagus (X) nerves.The facial and glossopharyngeal nerves stimulate the salivary glands to secrete saliva, while the vagus nerves stimulate the gastric glands to secrete gastric juice. The purpose of the cephalic phase of digestion is to prepare the mouth and stomach for food that is about to be eaten.

Gastric Phase :

Once food reaches the stomach, the gastric phase of digestion begins. Neural and hormonal mechanisms regulate the gastric phase of digestion to promote gastric secretion and gastric motility.

• Neural regulation. 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. When the stomach walls are distended or pH increases because proteins have entered the stomach and buffered some of the stomach acid, the stretch receptors and chemoreceptors are activated,and a neural negative feedback loop is set in motion  From the stretch receptors and chemoreceptors,

nerve impulses propagate to the submucosal plexus, where

they activate parasympathetic and enteric neurons. 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 into the duodenum. The pH of the stomach chyme decreases (becomes more acidic) and the distension of the stomach walls lessens because chyme has passed

into the small intestine, suppressing secretion of gastric juice.

• Hormonal regulation. Gastric secretion during the gastric phase is also regulated by the hormone gastrin. Gastrin is released from the G cells of the gastric glands in response to

several stimuli: distension of the stomach by chyme, partially digested proteins in chyme, the high pH of chyme due to the presence of food in the stomach, caffeine in gastric chyme, and

acetylcholine released from parasympathetic neurons. Once it is released, gastrin enters the bloodstream, makes a round-trip

through the body, and finally reaches its target organs in the digestive system. Gastrin stimulates gastric glands to secrete large amounts of gastric juice. It also strengthens the contraction of the lower esophageal sphincter to prevent reflux of acid chyme into the esophagus, increases motility of the stomach,and relaxes the pyloric sphincter, which promotes gastric emptying. Gastrin secretion is inhibited when the pH of gastric juice drops below 2.0 and is stimulated when the pH rises. This

negative feedback mechanism helps provide an optimal low

pH for the functioning of pepsin, the killing of microbes, and the denaturing of proteins in the stomach.

Intestinal Phase

The intestinal phase of digestion begins once food enters the small intestine. In contrast to reflexes initiated during the cephalic and gastric phases, which stimulate stomach secretory activity

and motility, those occurring during the intestinal phase have inhibitory effects that slow the exit of chyme from the stomach. This prevents the duodenum from being overloaded with more

chyme than it can handle. In addition, responses occurring during the intestinal phase promote the continued digestion of foods that have reached the small intestine. These activities of the intestinal phase of digestion are regulated by neural and hormonal mechanisms.

• Neural regulation. Distension of the duodenum by the presence

of chyme causes the enterogastric reflex .Stretch receptors in the duodenal wall send nerve impulses to The medulla oblongata, where they inhibit parasympathetic stimulation and stimulate the sympathetic nerves to the stomach. As a result, gastric motility is inhibited and there is an

increase in the contraction of the pyloric sphincter, which decreases gastric emptying.

• Hormonal regulation. The intestinal phase of digestion is

 mediated by two major hormones secreted by the small intestine: cholecystokinin and secretin. Cholecystokinin (CCK) is

secreted by the CCK cells of intestinal glands in the small intestine in response to chyme containing amino acids from

partially digested proteins and fatty acids from partially digested triglycerides. CCK stimulates secretion of pancreatic juice that is rich in digestive enzymes. It also causes contraction of the wall of the gallbladder, which squeezes stored bile

out of the gallbladder into the cystic duct and through the common bile duct.

 In addition, CCK causes relaxation of the

sphincter of the hepatopancreatic ampulla (sphincter of Oddi),which allows pancreatic juice and bile to flow into the duodenum. CCK also slows gastric emptying by promoting contrac-

tion of the pyloric sphincter, produces satiety (a feeling of fullness) by acting on the hypothalamus in the brain, promotes normal growth and maintenance of the pancreas, and enhances the effects of secretin. Acidic chyme entering the duodenum

stimulates the release of secretin from the S cells of the intestinal glands in the small intestine. In turn, secretin stimulates the flow of pancreatic juice that is rich in bicarbonate (HCO3)

ions to buffer the acidic chyme that enters the duodenum from

the stomach. In addition to this major effect, secretin inhibits secretion of gastric juice, promotes normal growth and maintenance of the pancreas, and enhances the effects of CCK. Overall, secretin causes buffering of acid in chyme that reaches the duodenum and slows production of acid in the stomach.