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Gastrointestinal Physiology: Understanding Digestion and Absorption

Explore the concepts of digestion and absorption, smooth muscle characteristics, secretory and neural control functions, and more in gastrointestinal physiology. Learn how the body breaks down and absorbs nutrients for optimal health.

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Gastrointestinal Physiology: Understanding Digestion and Absorption

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  1. Gastrointestinal physiology Yue Limin(岳利民) Department of Physiology School of Preclinical and Forensic Medicine Sichuan University

  2. The old saying "you are what you eat" "you are what you absorb and digest".

  3. Introduction Concepts of digestion and absorption Characteristics of the smooth muscle of gastrointestinal tract Secretory function of digestive gland Neural control of gastrointestinal tract Endocrine function of gastrointestinal tract

  4. Carbohydrads, fat , proteinlarge molecules Vitamine, inorganic salt, water small molecules fiber Ⅰ. Concepts of digestion and absorption The main types of foods

  5. Digestion: The breakdown of proteins, fats and carbohydrates into absorbable molecules in the gastrointestinal tract. Protein amino acid Carbohydrate monosaccharide Fat fatty acid and glycerol Absorption: The movement of the digested products ,water and electrolytes from the lumen of digestive tract into the lymph or the blood. closed relationship between digestion and absorption

  6. How are the foodstuffs digested ?

  7. Mechanical Digestion: The motility of smooth muscle in gastrointestinal tract grinds food and mixes it with digestive juice to prepare it for chemical digestion and absorption, and then propels the food along the gastrointestinal tract. Chemical Digestion: Nutrients in the food are broken down by the enzymes in digestive juice secreted by the secretory glands.

  8. Ⅱ.Characteristics of Gastrointestinal Smooth Muscle 1. General physiological characteristics • low excitibility • slow contractility • high extensibility • tonicity • Sensitive for chemical, thermal and mechanical stretch stimulation

  9. 2. Electrophysiological characteristics Slow wave potential Action potential Resting potential

  10. Value: -50 -60mV Resting potential of smooth muscle IN GI Formation: out flow of K+; Na+ Ca2+ electrogenicNa+ -K+ pump

  11. Slow wave orbasic electric rhythm(BER) Definition: Spontaneous, slow and rhythmic fluctuation ( depolarization and repolarization) on resting potential in gastrointestinal smooth muscle cells. Size: 5-15mV (from –65 to-45mV) Frequency: 3-12/min stomach--- 3; duodenum---12; ileum---8，9 Size and frequency is influenced by neural and humoral regulation.

  12. Origin: interstitial Cajal cells, which serve as pacemaker for GI smooth muscle Mechanism: cyclic changes of the activity of Na+ -K+ pump Function: weak contraction the base of AP formation

  13. Action Potential Inducing: the depolarization of slow wave reaches TP by stimulation Characteristics: single or mutiple spikes Action: the number of spike potentialdetermine contraction

  14. Mechanism: depolarization: slow Ca2+-Na+channels Ca2+ inward flow , Na+ inward flow repolarization: K+ outward flow

  15. frequency direction speed AP RP BER Muscle Contraction strength Relationship among RP, BER and contraction

  16. 3. Basic types of movements in digestive tract mastication and swallowing: Tonic contraction: a constant level of contraction, without regular period of relaxation Peristaltic contraction: a series of coordinated sequential contraction. Migration motor complex (MMC) Massmovement Segmentation contraction: alternate contraction and relaxation of a section of intestine

  17. Receptive relaxation: stimulation of food on pharynx and esophagus produce relaxation of the lower esophageal sphincter and the orad stomach.

  18. Peristalsis contraction relaxation

  19. Segmentation contraction

  20. salivary glands saliva, 1500 ml/day gastric gland gastric juice, 2500 ml/day exocrine cells of pancreas pancreatic juice, 1500 ml/day liver bile, 500 ml/day Ⅲ. Secretory function of digestive glands Secretion is the addition of fluids, enzymes and mucus from the digestive glands to the lumen of gastrointestinal tract. Main digestive gland:

  21. Hydrolyze nutrients into absorbable units Dilute the food in favor of absorption Change the pH value in digestive cavity to meet the need of digestive enzymes Mucous, antibodies and fluid in digestive juice protect gastrointestinal mucosa from physical and chemical injuries Main Effects of Digestive Juice

  22. Major digestive enzymes in main digestive juice Source enzyme substrate products a-amylase starch maltose,dextrin maltotriose saliva Gastric juice pepsin polypeptide protein a-amylase starch maltose, dextrin maltotriose lipase triglycerides monoglyceride fatty acids Pancretic juice trypsin protein polypeptides chymotrypsin polypeptide amino acid carboxypolypeptidase bile no enzyme bile salts

  23. Carbohydrate (CHO) Digestion

  24. Digestion of Fat colipase • Emulsification by bile salts • Pancreatic lipase • Micelles are water-soluble

  25. Ⅳ. Neural control of gatrointestinal tract 1. Central Nervous System (CNS) Positions: medullaoblongata: maintain basic function of gastrointestinal tract hypothalamus: higher nervous center for gastrointestinal activities. e.g. feeding center, satiety center limbic cortex in cerebral cortex (hippocampus, amygdala): control the activities of hypothalamus

  26. enteric neuron postganglionic N. smooth muscle Preganglionic N. adrenergic N. endocrine cell Cholinergic N. secretory cell thoracic and lumbar spinal cord segments (T5—L2) celiac ganglion superior mesenteric~ inferior mesenteric~ 2. Extrinsic nervous system sympathetic innervation ACh NE Inhibition sphincters+

  27. postganglionic N. ACh preganglionic N. cholinergic N. cholinergic N. peptidergic Peptides motor nucleus of vagual nerve in medulla oblongata myenteric and submucosal plexuses in the wall of gut upper parts stomach small intestine ascending colon right transverse colon Parasympathetic innervation Vagus nerve sphincters- excitation ACh vessels relaxation

  28. postganglionic N. ACh preganglionic N. cholinergic N. cholinergic N. peptidergic Peptides lower parts sacral spinal Cord(S2--S4) myenteric and submucosal plexuses in the wall of gut left 1/3 transverse colon Pelvic nerve excitation ACh relaxation

  29. Salivary glands stomach liver pancreas intestine pelvic nerve Pelvic N

  30. 3. Intrinsic or Enteric Nervous System (ENS) The nervous networks of neurons and nerve fibers in the wall of gastrointestinal tract Components: myenteric plexus:between the longitudinal and circular muscle layers submucosal plexus:between the middle circular muscle layer and the mucosa characteristics local reflexes being controlled by extrinsic nerves

  31. parasympathetic sympathetic myenteric plexus submucosal plexus Sensory neurons epithelium Spinal cord Brain sterm

  32. conditoned stimuli eye,nose,ear ⅠⅡⅧ Center Afferent N. Efferent N. Long reflex muscle gland receptor plexus Short reflex contraction secretion Unconditioned Stimuli

  33. Intestine without nerve innervation Secretion of pancreatic juice extract HCl Mucosa of duodenum blood ∥ blood Ⅴ. Endocrine function of gastrointestinal tract the biggest endocrine organ more than 40 types of endocrine cells first discovered hormone – Starling and Bayliss , 1902

  34. Ernest Starling William Bayliss John Henderson: Ernest Starling and ‘Hormones’: an historical commentary. Journal of Endocrinology 2005，184, 5-10 

  35. 1. Gastrointestinal Hormones: biologically active polypeptides secreted by the endocrine cells in gastrointestinal tract which influence the motility and secretion of gastrointestinal tract. 2. Pathways of gastrointestinal hormones secretion • endocrine • paracrine • neurocrine • autocrine • solinocrine

  36. Regulating activities of the gastrointestinal tract Trophic act ion Regulating the secretion of other hormones Regulating immune function 3. Functions of Gastrointestinal Hormones 4. Brain-gut peptide : exists both in CNS and gastrointestinal tract more than 20 kinds: gastrin, CCK, Motilin, neurotensin

  37. . Stimulus and Response of a few Gastrointestinal Hormones hormone secretion effects Antrum,upper small intestine Gastrin (+) Gastric juice (+) pancreatic enzymes (+) Gastric motility (+) Trophic to mucosa G cell Upper small intestine (+) Pancreatic juice HCO一3 (+) Bile flow (-) Gastric acid Secretin S cell Upper small intestine CCK cholecystokinin (+) Pancreatic enzyme (+) Gallbladder contraction I cell GIP (-) gastric juice secretion (-)motility of stomach (+) insulin secretion Upper small intestine K cell Mo cell Motilin (+) Gastric and small intestinal motility

  38. Digestive function of Stomach Ⅰ. Secretion of Gastric Juice Gastric juice: the fluid secreted by the cells of the gastric mucosa Major exocrine glands in mucosa mucous gland: mucus oxyntic gland in the body: 4 cell types Parietal cells body HCl, intrinsic factor Chief cells body pepsinogen Neck mucous cell antrum mucus; pepsinogen G cell antrum gastrin pyloric gland in the antrum: G cell; mucous cell

  39. HCl action: pepsinogen pepsin kill bacteria secretion of secretin absorption of Ca2+ and Fe2+ The components and their functions of gatric juice 1.Hydrochloric acid(gastric acid) Production: parietal cell

  40. Warren and Marshall discovered Helicobacter pylori,Hp In 1983, and got Noble prize in 2005

  41. Lumen Gastric parietal cell Blood Na+ ATP K+ K+ K+ _ Proton pump Cl ATP H+ _ HCO3- HCl is secreted HCO3 H2CO3 alkaline tide - Cl Carbonic anhydrase CO2 + H2O Cellular mechanism of HCl secretion

  42. HCl Activation: pepsinogen pepsin + Action: Hydrolyze protein 2. Pepsinogen: Production: chief cell and mucous cell

  43. 3. Mucus: Action: to form mucus-bicarbonate barrier with HCO3-that protects the gastric epithelium from damage of H+and pepsin HCl H+ Layer of mucus HCO3- － Production:epithelial cell, mucous neck cell, pyloric gland Characteristics: glycoproteins; viscid; gel pH2 pH7

  44. 4. Intrinsic factor: Production: parietal cell，glycoprotein Action: the absorption of vitamine B12 in ileum pernicious anemia

  45. H+ Ca2+ Ca2+ M ? ACh Gastrin IP3 IP3 cAMP H2 Histamine G cell Cholinergic neuron Enterochromaffin- like cell Gastrin ACh Ⅱ. Regulation of gastric secretion Substances that stimulate HCl secretion in the body endocrine paracrine

  46. Gi (-)AC cAMP Substances that inhibit gastric secretion in the body somatostatin: neurotensin epidermal growth factor GIP secretin

  47. Regulation of gastric secretion during the digestive period ⅠⅡⅧ ⅤⅦ cephalic phase ⅨⅩ center Ⅹ Ⅹ Distension of fundus ,body Gastric gland plexus Distension of antrum Gastric phase G cell Chemical stimuli gastrin Intestinal phase Entero-oxyntin Conditoned stimuli unconditioned Stimuli food Mecho-chemical stimuli cephalic phase: large amount(30%); high acid intensity large amount of enzyme gastric phase: large amount(60%); less acid intensity less amount of enzyme

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