MBBS Year 1 - Prof GE Mann L5 Motility of the gut.pdf

Prof GE Mann, Waterloo Campus (Email: [email protected]) MBBS Year 1 Gastrointestinal Lectures L1: Overview of the digestive system L2: Digestion and absorption of nutrients L3: Salivary, gastric and pancreatic secretions L4: Bile and biliary system L5: Motility of the gut Physiology, 5th...

Prof GE Mann, Waterloo Campus (Email: [email protected]) MBBS Year 1 Gastrointestinal Lectures L1: Overview of the digestive system L2: Digestion and absorption of nutrients L3: Salivary, gastric and pancreatic secretions L4: Bile and biliary system L5: Motility of the gut Physiology, 5th Edition, eds. RM Berne, MN Levy, BM Koeppen, BA Stanton (2004) (see Chapters 31-33) [see also recent Editions] Medical Physiology, eds. WF Boron & EL Boulpaep (2003) Color Atlas of Physiology, 4th Edition, eds. A. Despopoulos & S. Silbernagl, (1991) 1 MBBS Year 1 Gastrointestinal Lectures L5: Motility of the gut Learning Objectives Describe the role that gastric smooth muscle contractions in facilitating breakdown of ingested food Describe the cells that serve as pacemakers for muscle contraction in the GI tract Describe the neural and hormonal stimuli involved in regulating gastric motility Describe the mechanisms inhibiting gastric emptying 2 Neural control of gastrointestinal contractility 3  Gastrointestinal motility The functional role(s) of gastrointestinal tract smooth muscle contraction Functions of motility: Mixing Propulsion Motility is dependent on interaction between: Smooth muscle Enteric nervous system Autonomic nervous system Hormones 4  Gastrointestional tract smooth muscle contractile activity Visceral smooth muscle shows rhythmic cycles of activity – Pacemaker cells (Interstitial Cells of Cajal) Peristalsis – Waves that move a bolus Segmentation – Churn and fragment a bolus Sphincter tone and relaxation 5  Factors influencing gastrointestinal tract motility  Smooth muscle functional syncytium, electrical activity of muscle cells, slow waves (pacemaker potentials, basal electrical rhythm), spike potentials result in contraction  Neural control Autonomic + Enteric nervous systems Parasympathetic (vagus nerve) - Excitatory fibres (acetylcholine) - Inhibitory fibres (e.g. VIP, ADP) Sympathetic - Inhibitory effects (noradrenaline)  Hormonal control - Endocrine (gastrin, CCK) - Local (paracrine, neurocrine) 6 Muscle layers in the stomach 7 Response of the Stomach to Filling 8  Gastric Contractile Activity Most vigorous peristalsis and mixing occurs near the pylorus Chyme is either: – Delivered in small amounts (about 3ml) to the duodenum or – Forced backward into the stomach for further mixing 9  Regulation of Gastric Emptying Gastric emptying is regulated by: – Neural enterogastric reflex – Hormonal (enterogastrone) mechanisms These mechanisms inhibit gastric secretion and duodenal filling Carbohydrate-rich chyme quickly moves through duodenum Fat-laden chyme is digested more slowly causing food to remain in stomach longer 10 Regulation of Gastric Activity HORMONAL CONTROL Secretin – stimulate HCO3- secretion - inhibit stomach activity CCK (cholecystokinin) – stimulates gallbladder contraction and emptying - inhibit stomach activity GIP (gastric inhibitory peptide) - inhibits gastric contractions Motilin -increases gastric motility -increases intestinal motiliy 11 Regulation of Gastric Activity NEURONAL CONTROL Short reflexes operating in the enteric nervous system (ENT) inhibit gastric secretion/contraction Long reflexes operating via the CNS alter autonomic nerve activity 12 Local and central reflex pathways in gastrointestinal tract 13 Connections in the enteric nervous system 14 Neural control of gastrointestional tract function 15