PCB3743 Vertebrate Physiology Summer 2014 Exam #4 Study Guide 1 Chapter 18: Digestive System Enzymes 1. Know amylase, pepsinogen/pepsin, trypsinogen/trypsin, enterokinase. Recognize other enzyme types by prefix name. Amylase: Enzyme that can catalyze the partial digestion of starch • Located in the salivary glands – starch digestion begins in the mouth • Amylase is inactivated in the stomach because of the strong acidity of gastric juice • Also found in the pancreas Pepsinogen: Inactive form of pepsin • Inactive enzyme secreted by the gastric mucosa • In the presence of HCl, the active enzyme pepsin is produced Pepsin: Protein-digesting enzyme that is more active under acidic conditions • pH optimum = ~2.0 • Catalyzes the hydrolysis of peptide bonds in the ingested protein • Cooperative activities of pepsin & HCl permit the partial digestion of food protein in the stomach Trypsinogen: Inactive form of trypsin • Converted to trypsin by enterokinase Trypsin: Protein-digesting enzyme • Cleaves internal peptide bonds • Activated by enterokinase within the small intestine • Found in pancreatic juice o Activates other enzymes in the pancreatic juice Enterokinase: A brush border enzyme required for activation of the protein-digesting enzyme trypsin • Located in the cell membrane of microvilliPCB3743 Vertebrate Physiology Summer 2014 Exam #4 Study Guide 2 Motility 2. Understand the difference between peristalsis and segmentation in the gut, and the circuitry which generates them (i.e. reflex responses vs. pacemaker interstitial cells of Cajal) Peristalsis: Wave of muscular relaxation & contraction in reflex response to stretching of GI wall • Moves food down the esophagus & stomach (one direction) • Circular smooth muscle contracts behind & relaxes in front of the bolus • Weaker in the intestines Segmentation: Rhythmic coordinated contraction of segments of intestine that mixes chyme, mucus, & enzymes • Faster at proximal end of intestine • Creates pressure that moves food down the intestinesPCB3743 Vertebrate Physiology Summer 2014 Exam #4 Study Guide 3 Interstitial Cells of Cajal (ICC): Unique pacemaker cells in the muscularis layer that have long processes which join them to each other and to smooth muscle cells by gap junctions • Are neither neurons nor smooth muscle cells • Generate graded depolarizations called slow waves that produce automatic contractions of intestinal smooth muscle • Depolarize the smooth muscle • ICC have muscarinic receptors o Parasympathetic acetylcholine increases amplitude & duration of slow waves Stomach 3. Know the major cell types of the gastric gland (chief, parietal, ECL, and G cells) and what chemicals they produce. Gastric Glands: Exocrine glands that line the gastric pits – the openings of the folds into the stomach lumen • Composed of several cell types Parietal Cells: Secrete hydrochloric (HCl) acid Chief Cells: Secrete pepsinogen Enterochromaffin-like (ECL) Cells: Secrete histamine and serotonin that stimulate chief cells • Act as paracrine regulators of the GI tract • Found in the stomach & intestine G Cells: Secrete the hormone gastrin into the blood that stimulates ECL cells 4. Understand the mechanism of gastric acid secretion. Secretion of gastric acid by parietal cells • Carbonic anhydrase converts CO2 to H+ & HCO3- (bicarbonate) • H+ is pumped into the lumen by the H+/K+ ATPase pump • HCO3- is exchanged for Cl- from the bloodPCB3743 Vertebrate Physiology Summer 2014 Exam #4 Study Guide 4 • Cl- moves through membrane channel into the lumen • Therefore, the parietal cells secrete Cl- & H+ into the gastric juice while they secrete HCO3- (bicarbonate) into the blood 5. Know the pathway for producing pepsin from pepsinogen. High concentration of HCl from the parietal cells makes gastric juice very acidic (pH = <2) • Under these acidic conditions, weak pepsinogen enzymes partially digest each other • Frees the fully active pepsin enzyme as small inhibitory fragments are removed • Pepsin is more active under acidic conditions o pH optimum = 2.0PCB3743 Vertebrate Physiology Summer 2014 Exam #4 Study Guide 5 6. Understand the chemical and neural factors that regulate gastric acid secretion, and the 3 phases of gastric secretion. Regulation of Gastric Acid Secretion • Vagus nerve releases acetylcholine (ACh) onto muscarinic receptors on ECL cells & G cells • ECL cells release histamine that has paracrine effect on H2 receptors (a type of histamine receptor) on nearby parietal cells to increase HCl secretion • Distension of stomach stimulates vagus nerve • Digested amino acids in the chyme stimulate chief cells to secrete pepsinogen & G cells to secrete gastrin • Gastrin stimulates ECL cells to release histamine, which stimulates parietal cells to increase HCl secretion o Gastrin release is inhibited by the presence of HCl in the stomach (negative feedback mechanism) o HCl secretion promotes amino acid digestion • Fat, amino acids, and distension of the intestine inhibit acid secretion & gastric emptying – shut the pyloric sphincter to decrease movement of food from stomach small intestine Three (3) Phases of Gastric Secretion 1. Cephalic Phase: Control by the brain via the vagus nerve o Sight, smell, & taste of food cause stimulation of vagus nuclei in the brain o Vagus stimulates acid secretion Indirect stimulation of parietal cells major effect Stimulation of gastrin secretion lesser effect 2. Gastric Phase: Stimulated by the arrival of food into the stomach o Distension of stomach stimulates vagus nerve o Vagus stimulates acid secretion o Amino acids & peptides in stomach lumen stimulate acid secretionPCB3743 Vertebrate Physiology Summer 2014 Exam #4 Study Guide 6 Direct stimulation of parietal cells lesser effect Stimulation of gastrin secretion & gastrin stimulates acid secretion major effect o Gastrin secretion inhibited when pH of gastric juice falls below 2.5 3. Intestinal Phase: Inhibition of gastric activity when chyme enters the small intestine o Neural inhibition of gastric emptying & acid secretion Arrival of chyme in duodenum causes distension & increase in osmotic pressure These stimuli activate a neural reflex that inhibits gastric activity o Duodenum secretes a hormone that inhibits gastric acid secretion in response to fat in chyme 7.
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