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Harvard-MIT Division of Health Sciences and Technology HST.121: Gastroenterology, Fall 2005 Instructors: Dr. Jonathan Glickman Overview of Gastroenterology Gastroenterology Made Really Simple! Waste Food Nutrients The Basic Structure Image removed due to copyright reasons. The Propulsion System: Driven by the Muscles Periodic phasic contractions Waste Food Nutrients Local tonic contractions (sphincters) The Neural Control System: The Brain in the Gut ACh Muscularis Mucosae Mucosa NE NE Submucosal plexus Circular muscle ACh NE Myenteric plexus ACh Longitudinal muscle Parasympathetic nerves (Vagus/Pelvic) Sympathetic ganglia The Neurocrine Effectors Peptide Actions Acetylcholine (ACh) • •Smooth muscle contraction •Sphincter relaxation • Increased salivary, gastric, and pancreatic secretions Norepinephrine (NE) Smooth muscle relaxation Sphincter contraction Increased salivary secretion Vasoactive intestinal peptice (VIP) Smooth muscle relaxaton Increased intestinal and pancreatic secretions Gastrin-releasing peptide (GRP, bombesin) Increased gastrin secretion Enkephalins Smooth muscle contraction Decreased intestinal secretions Substance P Smooth muscle contraction Increased salivary secretions Neuropeptide Y Smooth muscle relaxation Decreased intestinal secretions The hormonal control system Hormone Stimuli Site of secretion Actions Gastrin Vagal stimulation Gastric distension Peptide and amino acids G cells (stomach) Stimulation of gastric acid secretion Growth of gastric mucosa Cholecystokinin (CCK) Fatty acids Peptide and amino acids I cells (duodenum and jejunum) Pancreatic enzyme, HCO3 secretion Gallbladder contraction Inhibition of gastric emptying Secretin Fatty acids Duodenal motility S cells (duodenum) Pancreatic enzyme, HCO3 secretion Inhibition of gastrin, acid secretion Ghrelin Fasting Stomach Stimulates appetite (CNS) Glucagon-like peptide (GLP-1) Fatty acids, amino acids Oral glucose L cells (duodenum and jejunum) Satiety (CNS) Inhibition of acid secretion Stimulation of insulin secretion Gut-brain hormonal interactions Image removed due to copyright reasons. Salivary Secretion Alpha-Amylase Kallikrein Mucus Electrolytes Acinus Na H Cl Na, Cl Duct K, HCO3 Hypotonic saliva Na HCO3 H K K Cl Gastric Secretions HCl Parietal cells IF Mucous cells Pepsinogen Gastric Lipase Chief cells Mucus NE cells Hormones Biliary Secretion Water and electrolytes (CCK) Water and electrolytes Cholesterol & bile acids (Secretin) Bile acids Pancreatic Secretions Acinus Alpha-Amylase Lipases Proteases Na K Centroacinar Cells Na Cl H HCO3 Duct H2CO3 Bicarbonate Carbonic Anhydrase H2O + CO2 Basic Pancreatic Secretions Na Paracellular Carbohydrate Digestion 1:6 glucosidic link 1:4 glucosidic link Maltose Alpha-limit dextrin Maltotriose Alpha-amylase breaks 1:4 linkages except at either end of starch molecules. This results in the production of three simple sugars as shown above. Overview of Carbohydrate Digestion Starch Maltose Dextrin Maltotriose Lactose Maltase Sucrase Lactase Dextrinase Glucose Sucrose Sucrase Glucose Galactose Glucose Fructose Overview of Protein Digestion Trypsinogen Pepsinogen B. B. Enterokinase Low pH Trypsin Pepsin Trypsinogen Trypsin Trypsin Chymotryp. Trypsin Chymotrypsin Proelastase Trypsin Elastase Procarboxy. Trypsin Carboxypeptidase Overview of Protein Digestion Stomach Small Intestine Proteins Proteins Trypsin, et al. Pepsin Amino acids Oligopeptides Oligopeptides Amino acids Dipeptides Tripeptides Amino acids B. B. Peptidases Overview of Lipid Digestion Triglycerides Lipases Monoglyceride Phospholipids Phospholipase A2 Fatty Acids Lysolecithin Chol. Ester Chol. ester hydrolase Fatty Acids Cholesterol Fatty Acids
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