Gastro-intestinal tract Sophie Young 27th June 2007

Objectives Anatomy Physiology Physiology of vomiting Motility Secretory Digestion Absorption Physiology of vomiting

Anatomy Taken from http://human. freescience Anatomy Taken from http://human.freescience.org/ htmx/parts/organ_systems/gastrointestinal_tract.php

GI Motility Basic contractile unit Na/K/ATPase pump – transmembrane potential Basic electrical rhythm – slow wave activity Rhythmical depolarisation + repolarisation Gap junctions (stomach vs colon) Frequency + amplitude of slow wave activity affected by: Autonomic nervous system Hormones Drugs

Control of GI Motility Intrinsic neuronal control Myenteric plexus (Auerbach’s) Cholinergic (+ve), adrenergic (+ve), NANC neurones (-ve) Integrates all information Extrinsic neuronal control Somatic and autonomic control Chemical control Neurocrine, paracrine and endocrine

Localised GI Motility Swallowing Motor reflex- voluntary, pharyngeal + oesophageal stages Gastric motility Fundus – prolonged tonic contractions Antrum – slow wave activity, pylorus affected by fasted and fed states (?hormonal + neuronal control), composition of chyme in duodenum Bowel Small – slow wave activity, various types of activity Large – slow propulsion + mass movements

Secretory function of GI tract Salivary glands – under Autonomic control Lubrication  Buffer Digest starch (amylase pH dep)  Antibacterial/viral Digest fat (lipase, pH indep) Gastric Oxyntic glands – chief cells (pepsinogen) (all gastric mucosa – mucus cells except lesser curve) – parietal cells (HCl + intrinsic factor) Pyloric glands – G cells (gastrin) (pylorus) – mucus cells

Control of gastric secretions Cephalic stage - gastrin Gastric stage - gastrin Intestinal stage - gastrin

Secretory function of GI tract Pancreatic Stored as proenzymes Fat/protein – stimulus for release Secretin, cholecystokinin and vagus mediated Biliary Emulsification of fat globules in SB Enterohepatic circulation of 94% bile salts

Digestion Carbohydrates Cellulose -  glucose linkage, indigestible Salivary and Pancreatic Amylases Oligo-/Disaccharides Brush border enzymes eg lactase/sucrase Monosaccharides Glucose Fructose Galactose Cellulose -  glucose linkage, indigestible

Digestion Protein Pepsin (stomach) Trypsin/chymotrypsin (SB) Polypeptides Carboxypeptidase (pancreas) Aminopeptidase (brush border) Amino acids Monopeptides Dipeptides Hydrolysis in epithelial cell after absorption Amino acids

Digestion Fat Triglycerides (90% dietary fat) Lingual + pancreatic lipases Free fatty acid Monoglycerides Combine with bile salts + cholesterol Micelles

Absorption Carbohydrates Taken from http://www.colorado.edu/kines/Class/IPHY3430-200/image/figure16c.jpg

Absorption Protein Secondary active transport across apical membrane Di/tripeptides hydrolysed to amino acids Facilitated diffusion into blood Taken from http://instruct1.cit.cornell.edu/courses/biog105/pages/demos/105/unit6/media/villus.structure.jpg

Absorption Fat Taken from http://www.colorado.edu/kines/Class/IPHY3430-200/15digest.html

Absorption Vitamins Fat Soluble – micelles Water Soluble: B1- active in jejunum, C - active + passive in SB, Folic acid – active in SB, B12 – combines with IF ?pinocytosis in terminal ileum Sodium and Chloride Na – cotransporter mechanism, Cl – colonic uptake Water SB + colon – ANS and hormone control Iron and Calcium

Physiology of Vomiting Protective physiological function against ingested toxins

3 distinct phases Nausea: unpleasant sensation that immediately proceeds vomiting.  Retching: follows nausea and comprises laboured spasmodic respiratory movements against a closed glottis with contractions of abdominal muscles, chest wall and diaphragm without any expulsion of gastric contents. Vomiting: as for retching but with the opening of gastric cardia, resulting in rapid + forceful evacuation of stomach contents up to and out of the mouth

Vomiting Reflex mechanism – sensory afferents, central integrative centre, motor efferents Sensory afferents GI – vagally mediated mechanoreceptors and chemoreceptors CTZ – area postrema 4th ventricle, outside blood-brain barrier, many receptors present: Muscarinic Serotonergic (5HT3) Opioid () Histamine (H1) Dopaminergic (D2) Adrenergic (1 + 2) Vestibular system – labyrinthine stimulation Cortex

Taken from www.frca.co.uk

Vomiting ‘Vomiting Centre’ Oversimplication, probable multiple discrete interconnecting areas within medulla Motor efferents Pre-ejection: nausea, sympathetic activity, vagal mediation proximal stomach relaxation + retrograde SB contraction Ejection: retching – perioesophageal diaphragm contracted, vomiting – perioesophageal diaphragm relaxes to expel contents with halting of respiration

Taken from http://www. nauseaandvomiting. co

References Pinnock et al (2003) Fundamentals of Anaesthesia Vander et al (1998) Human Physiology Anaesthesia UK www.frca.co.uk