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Digestive.docx

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Department
Biology
Course
BLG 10A/B
Professor
Charlotte Youngson
Semester
Winter

Description
23 -The Digestive System, edited by Charlotte Youngson Pre-lecture questions 1. List the correct order of structures through which food passes as it enters the mouth and travels through the digestive tract to the rectum. 2. Draw an intestinal villus and label the parts. 3. Outline the enzymatic events involved in the digestion of carbohydrates, proteins and lipids. Digestive System • Two groups of organs 1. Alimentary canal (gastrointestinal or GI tract) • Digests and absorbs food • Mouth, pharynx, esophagus, stomach, small intestine, and large intestine 2. Accessory digestive organs • Teeth, tongue, gallbladder • Digestive glands • Salivary glands • Liver-important in maiking bile, and observing nutrients, removing toxins • Pancreas-makes juice Upper part of esophagus is voluntary, everything else is autonomic Digestive Processes • Six essential activities 1. Ingestion 2. Propulsion- muscular activity. And innervation. 3. Mechanical digestion 4. Chemical digestion-enzymes 5. Absorption 6. Defecation *NOTES • If u wanna move someth quicly-peristalsis. For example down esophagus. • In stomach-segmentation(back forth movement) GI tract regulatory mechanisms 1. Mechanoreceptors and chemoreceptors • Respond to stretch, changes in osmolarity and pH, and presence of substrate and end products of digestion • Initiate reflexes that • Activate or inhibit digestive glands to release enzymes or not • Stimulate smooth muscle to mix and move lumen contents 2. Intrinsic and extrinsic controls • Enteric nerve plexuses initiate short reflexes in response to stimuli in the GI tract (series of nerves) • Long reflexes in response to stimuli inside or outside the GI tract involve CNS centers and autonomic nerves • Hormones from cells in the stomach and small intestine stimulate target cells in the same or different organs *NOTES • Syphalis phase-when brain tells the stomach that it will get food soon. For example when u smell food. • Never fall in syphalic phase-always have some food with u. cz at 4 you atrt eating anything. • If there is non proteins in the diet you will go defecate very soon. If u eat proteins, it stays longer and u feel ful longer. *NOTES • Short reflexes don’t involve brain. Long do involve brain. Histology of the Alimentary Canal • Four basic layers (tunics) • Mucosa • Submucosa • Muscularis externa • Serosa Enteric Nervous System • Intrinsic nerve supply of the alimentary canal • Submucosal nerve plexus • Regulates glands and smooth muscle in the mucosa • Myenteric nerve plexus • Controls GI tract motility • Linked to the CNS via afferent visceral fibers • Long ANS fibers synapse with enteric plexuses • Sympathetic impulses inhibit secretion and motility (inhibitory to digestion) • Parasympathetic impulses stimulate Mouth • Oral (buccal) cavity • Bounded by lips, cheeks, palate, and tongue • Lined with stratified squamous epithelium Salivary Glands • Extrinsic salivary glands (parotid, submandibular, and sublingual) Composition of Saliva • Secreted by serous and mucous cells • 97–99.5% water, slightly acidic solution containing + + • Electrolytes—Na , K • Salivary amylase(breakdown to disaccharide) and lingual lipase(very important in newborns) • Mucin • Metabolic wastes • Lysozyme, IgA, Esophagus • Esophageal mucosa contains stratified squamous epithelium • Changes to simple columnar at the stomach • Esophageal glands in submucosa secrete mucus to aid in bolus movement • Muscularis: skeletal muscle superiorly; smooth inferiorly Digestive Processes: Mouth • Ingestion • Mechanical digestion • Mastication is partly voluntary, partly reflexive • Chemical digestion (salivary amylase and lingual lipase) • Propulsion • Deglutition (swallowing) 3 is autonomic. Parystolisis. No enzymes or anything. 2 weeks reducing portion will reduce stomach size Single cell. Chief cells produce perpsin. Gastric Glands • Cell types • Mucous neck cells (secrete thin, acidic mucus) • Parietal cells • Chief cells • Enteroendocrine cells –produce hormones. (gastrin-stimulates secretion) Positive feedback Gastric Gland Secretions • Glands in the fundus and body produce most of the gastric juice • Parietal cell secretions • HCl •  pH 1.5–3.5 denatures protein in food, activates pepsin, and kills many bacteria • Intrinsic factor • Glycoprotein required for absorption of vitamin B in s12ll intestine • Chief cell secretions • Inactive enzyme pepsinogen • Activated to pepsin by HCl and by pepsin itself (a positive feedback mechanism) • Enteroendocrine cells • Secrete chemical messengers into the lamina propria • Paracrines • Serotonin and histamine • Hormones • Somatostatin and gastrin • B12-important for metabolism Parietal Digestive Processes in the Stomach • Physical digestion • Denaturation of proteins • Enzymatic digestion of proteins by pepsin • Secretes intrinsic factor required for absorption of vitamin B 12 • Lack of intrinsic factor  pernicious anemia • Delivers chyme to the small intestine Small Intestine: Gross Anatomy • Major organ of digestion and absorption • 2–4 m long; from pyloric sphincter to ileocecal valve
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