Phases of Digestion Lecture Notes PDF

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Digestive System Phases of Digestion:  Cephalic phase o Smell, sight, though or initial taste of food activates neural centres in cerebral cortex, hypothalamus and brain stem  Brain stem activates  Facial nerves (VII) o Stimulate salivary glands to secrete saliva  Glossopharyngeal nerves (IX) o Stimulate salivary glands to secrete saliva  Vagus nerves (X) o Stimulate gastric glands to secrete gastric juice o Prepares the mouth and stomach for food to be eaten  Gastric phase o Food reaches stomach o Neural and hormonal mechanisms regulate phase  Neural  Distension of stomach stimulates stretch receptors  Chemoreceptors monitor stomach pH  Negative feedback loop  Nerve impulses propagate to submucosal plexus o Activate parasympathetic and enteric neurons o Resulting nerve impulses  Peristalsis and stimulate flow of gastric juice  Hormonal  Gastrin o Regulates gastric juice secretion o Released from G cells of gastric glands o Stimuli  Distension of stomach by chyme  Partially digested proteins in chime  High pH of chime  Caffeine present  Acetylcholine o Enters bloodstream reaching target organs  Stimulates gastric glands to secrete gastric juice and strengthens contraction of lower oesophageal sphincter  Prevent gastric acid reflux and relaxes the pyloric sphincter o Inhibited  pH 2

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Intestinal phase

Digestive System o Begins when food enter small intestine o Slows the exit of chyme from stomach  Preventing duodenum from being overloaded o Neural regulation  Distension of duodenum  Enterogastric reflux stimulation  Stretch receptors in wall  Nerve impulses to medulla oblongata o Inhibit parasympathetic stimuli and stimulate sympathetic nerves to stomach o Gastric motility inhibited o Increased contraction of pyloric sphincter  Decreases gastric emptying o Hormonal regulation  Cholecystokinin  CCK  Secreted by CCK cells o Small intestinal crypts of Leiberkuhn o Response to chyme containing amino acids  Stimulates secretion of pancreatic juice  Causes contraction of the wall of the gallbladder o Squeezing bile into cystic duct  Causes relaxation of the sphincter of the hepatopancreatic ampulla o Allowing pancreatic juice and bile to flow into duodenum  Increases contraction of the pyloric sphincter o Slowing gastric emptying  Produces satiety o Feeling of fullness o Acting on hypothalamus  Secretin  Secreted from the S cells o Small intestinal crypts of Leiberkuhn  Stimulates flow of pancreatic juice rich in bicarbonate ions to buffer acidic chyme  Inhibits secretion of gastric juice  Promotes normal effects of CCK

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Other hormones of the digestive system

Digestive System o Motilin  Stimulate late motility in intestines o Substance P  Stimulate late motility in intestines o Bombesin  Stimulate late motility in intestines o Vasoactive intestinal polypeptide  VIP  Stimulates secretion of ions and water by intestines  Inhibits gastric acid secretion o Gastrin releasing peptide  Stimulates release of gastrin o Somatostatin  Inhibits gastrin release...