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Chapter 21b

Chapter 21b. The Digestive System. Regulation of GI Function. Long reflexes integrated in CNS Short reflexes integrated in ENS Reflexes involving GI peptides / Hormones. Regulation of GI Function. Integration of long and short reflexes in the digestive system. KEY. External stimulus.

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Chapter 21b

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  1. Chapter 21b The Digestive System

  2. Regulation of GI Function • Long reflexes integrated in CNS • Short reflexes integrated in ENS • Reflexes involving GI peptides / Hormones

  3. Regulation of GI Function • Integration of long and short reflexes in the digestive system KEY Externalstimulus Stimulus Receptor (sight, smell,etc.) Integratingcenter Efferent path Sensoryreceptors Short reflexes Tissueresponse Long reflexes Thecephalicbrain Sympathetic and parasympathetic neurons Neuronsofmyentericandsubmucosalplexuses Muscle contraction/relaxation orExocrine secretion (enzymes, mucus,acid, bicarbonate), paracrines orEndocrine secretions Smoothmusclesorsecretorycells Sensoryreceptorsandneurons Localstimulus Inter-neurons (pH, stretch,osmolarity,products ofdigestion) Enteric nervous system“The little brain” Effectors Digestive system responses Figure 21-11

  4. Regulation of GI Function KEY Stimulus Receptor Integratingcenter Efferent path Short reflexes Tissueresponse Muscle contraction/relaxation orExocrine secretion (enzymes, mucus,acid, bicarbonate), paracrines orEndocrine secretions Neuronsofmyentericandsubmucosalplexuses Smoothmusclesorsecretorycells Sensoryreceptorsandneurons Localstimulus Inter-neurons (pH, stretch,osmolarity,products ofdigestion) Enteric nervous system“The little brain” Effectors Digestive system responses Figure 21-11 (1 of 3)

  5. Regulation of GI Function KEY Externalstimulus Stimulus Receptor (sight, smell,etc.) Integratingcenter Efferent path Sensoryreceptors Tissueresponse Long reflexes Thecephalicbrain Sympathetic and parasympathetic neurons Muscle contraction/relaxation orExocrine secretion (enzymes, mucus,acid, bicarbonate), paracrines orEndocrine secretions Neuronsofmyentericandsubmucosalplexuses Smoothmusclesorsecretorycells Sensoryreceptorsandneurons Localstimulus (pH, stretch,osmolarity,products ofdigestion) Enteric nervous system“The little brain” Effectors Digestive system responses Figure 21-11 (2 of 3)

  6. Regulation of GI Function KEY Externalstimulus Stimulus Receptor (sight, smell,etc.) Integratingcenter Efferent path Sensoryreceptors Short reflexes Tissueresponse Long reflexes Thecephalicbrain Sympathetic and parasympathetic neurons Muscle contraction/relaxation orExocrine secretion (enzymes, mucus,acid, bicarbonate), paracrines orEndocrine secretions Neuronsofmyentericandsubmucosalplexuses Smoothmusclesorsecretorycells Sensoryreceptorsandneurons Localstimulus Inter-neurons (pH, stretch,osmolarity,products ofdigestion) Enteric nervous system“The little brain” Effectors Digestive system responses Figure 21-11 (3 of 3)

  7. Regulation of GI Function • Reflexes involving GI Peptides Overview of GI Reflexes Cephalic phase of digestion (feedforward)DistensionAcidPresence of food Stimuli Receptor/integratingcenter Secretory cells of thestomach and small intestine Efferentpathway GI peptides GI smooth muscleExocrine cells of stomach, pancreas, intestineOther endocrine cellsNervous system Effectors Changes in GI motilityRelease of bile and pancreatic secretionsEnzyme, acid, and bicarbonate synthesis/releaseHunger/satiety Responses Figure 21-12

  8. ENS Shares Features with CNS • Intrinsic neurons • Neurotransmitters and neuromodulators • Support cells • Diffusion barrier • Integrating center

  9. Digestive Hormones • Gastrin family • Gastrinand CCK • Secretin family • Secretin, VIP, GIP, and GLP-1 • Others • Motilin

  10. Secretin

  11. CCK • Fats and Proteins in SI • Bile • Pancreatic enzymes • Trypsin • Chymotrypsin • Procarboxypepsidase • Bicarbonate ions

  12. Control of the Digestive System PLAY Interactive Physiology® Animation:Digestive System: Control of the Digestive System

  13. Digestion and Absorption • A villus and a crypt in the small intestine Brush border Microvilli Enterocyte Enterocytes Capillaries Goblet cells Lacteals Cryptlumen Lamina propria Endocrine cells Muscularis mucosae Figure 21-13

  14. Digestion and Absorption • Carbohydrates break down into monosaccharides Glucose polymers digest to Starch, glycogen Disaccharides Amylase Lactose Maltose Sucrose Maltase Sucrase Lactase 1 glucose +1 fructose 1 glucose +1 galactose 2 glucose Monosaccharides Figure 21-14

  15. Digestion and Absorption • Carbohydrate absorption in the small intestine Glucose orgalactose Lumen ofintestine Fructose Na+ Na+ K+ Intestinalmucosa Glucose enters the cell with Na+ on theSGLT symporter and exits on GLUT2.Fructose enters on GLUT5 and exitson GLUT2. KEY SGLT GLUT2 GLUT5 Figure 21-15

  16. Digestion and Absorption Aminoacids Amino-terminal end Peptidebonds Carboxy-terminal end • Enzymes for protein digestion H2N COOH (a) Peptide structure Endopeptidasedigests internalpeptide bonds. +H2O COOH H2N H2N H2N COOH COOH 2 smaller peptides (b) Exopeptidase digests terminal peptidebonds to release amino acids. Aminopeptidase Carboxypeptidase +H2O +H2O COOH H2N COOH COOH H2N H2N Amino acid Amino acid H2N COOH (c) Peptide Figure 21-16

  17. Digestion and Absorption • Peptide absorption Proteins Peptides Di- and tripeptidescotransport with H+. Small peptidesare carried intactacross the cell bytranscytosis. Amino acidscotransportwith Na+. H+ H+ Na+ Na+ Peptidases K+ ATP H+ Na+ Na+ Blood To the liver Figure 21-17

  18. Digestion and Absorption • Triglycerides digest into monoglycerides and free fatty acids Triglyceride Lipase, colipase + Monoglyceride Free fatty acids Figure 21-18

  19. Digestion and Absorption • Bile salts emulsify fats Bile salt-coatedlipid droplet Hydrophobic sideassociateswith lipids. Polar side chains(hydrophilic sideassociates withwater) Water (a) Bilesalt Free fatty acids Cholesterol Bilesalt Monoglyceride Phospholipids Diglyceride (b) Figure 21-19

  20. Digestion and Absorption of Fats Bile saltsfrom liver 1 Bile salts from livercoat fat droplets. 1 Large fatdroplets fromstomach 2 Pancreatic lipase and coli-pase break down fats into monoglycerides and fatty acids stored in micelles. Emulsion Bile saltsrecycle Lipaseand colipase 3a Lumen ofsmall intestine Monoglycerides and fattyacids move out of micellesand enter cells by diffusion. 2 2 Micelles 3b Cholesterol is transportedinto cells by a membranetransporter. 3b 3a 4 Absorbed fats combine withcholesterol and proteins inthe intestinal cells to formchylomicrons. SmoothER 4 Triglycerides + cholesterol + protein 5 Cells ofsmallintestine Chylomicrons are releasedinto the lymphatic system. Chylomicron Golgiapparatus 5 Interstitialfluid Capillary Lacteal Lymphtovenacava Figure 21-20

  21. Digestion and Absorption of Fats Bile saltsfrom liver 1 Bile salts from livercoat fat droplets. 1 Large fatdroplets fromstomach Emulsion Lumen ofsmall intestine Cells ofsmallintestine Interstitialfluid Capillary Lacteal Figure 21-20, step 1

  22. Digestion and Absorption of Fats Bile saltsfrom liver 1 Bile salts from livercoat fat droplets. 1 Large fatdroplets fromstomach 2 Pancreatic lipase and coli-pase break down fats into monoglycerides and fatty acids stored in micelles. Emulsion Lipaseand colipase Lumen ofsmall intestine 2 2 Micelles Cells ofsmallintestine Interstitialfluid Capillary Lacteal Figure 21-20, steps 1–2

  23. Digestion and Absorption of Fats Bile saltsfrom liver 1 Bile salts from livercoat fat droplets. 1 Large fatdroplets fromstomach 2 Pancreatic lipase and coli-pase break down fats into monoglycerides and fatty acids stored in micelles. Emulsion Bile saltsrecycle Lipaseand colipase 3a Lumen ofsmall intestine Monoglycerides and fattyacids move out of micellesand enter cells by diffusion. 2 2 Micelles 3a Cells ofsmallintestine Interstitialfluid Capillary Lacteal Figure 21-20, steps 1–3a

  24. Digestion and Absorption of Fats Bile saltsfrom liver 1 Bile salts from livercoat fat droplets. 1 Large fatdroplets fromstomach 2 Pancreatic lipase and coli-pase break down fats into monoglycerides and fatty acids stored in micelles. Emulsion Bile saltsrecycle Lipaseand colipase 3a Lumen ofsmall intestine Monoglycerides and fattyacids move out of micellesand enter cells by diffusion. 2 2 Micelles 3b Cholesterol is transportedinto cells by a membranetransporter. 3b 3a Cells ofsmallintestine Interstitialfluid Capillary Lacteal Figure 21-20, steps 1–3b

  25. Digestion and Absorption of Fats Bile saltsfrom liver 1 Bile salts from livercoat fat droplets. 1 Large fatdroplets fromstomach 2 Pancreatic lipase and coli-pase break down fats into monoglycerides and fatty acids stored in micelles. Emulsion Bile saltsrecycle Lipaseand colipase 3a Lumen ofsmall intestine Monoglycerides and fattyacids move out of micellesand enter cells by diffusion. 2 2 Micelles 3b Cholesterol is transportedinto cells by a membranetransporter. 3b 3a 4 Absorbed fats combine withcholesterol and proteins inthe intestinal cells to formchylomicrons. SmoothER 4 Triglycerides + cholesterol + protein Cells ofsmallintestine Chylomicron Interstitialfluid Capillary Lacteal Figure 21-20, steps 1–4

  26. Digestion and Absorption of Fats Bile saltsfrom liver 1 Bile salts from livercoat fat droplets. 1 Large fatdroplets fromstomach 2 Pancreatic lipase and coli-pase break down fats into monoglycerides and fatty acids stored in micelles. Emulsion Bile saltsrecycle Lipaseand colipase 3a Lumen ofsmall intestine Monoglycerides and fattyacids move out of micellesand enter cells by diffusion. 2 2 Micelles 3b Cholesterol is transportedinto cells by a membranetransporter. 3b 3a 4 Absorbed fats combine withcholesterol and proteins inthe intestinal cells to formchylomicrons. SmoothER 4 Triglycerides + cholesterol + protein 5 Cells ofsmallintestine Chylomicrons are releasedinto the lymphatic system. Chylomicron Golgiapparatus 5 Interstitialfluid Capillary Lacteal Lymphtovenacava Figure 21-20, steps 1–5

  27. Digestion and Absorption • Nucleic acids are digested into nitrogenous bases and monosaccharides • Intestine absorbs vitamins and minerals • Fat-soluble absorbed with fats • Water-soluble by mediated transport • Vitamin B12when complexed to intrinsic factor and absorbed in ileum • Mineral by active transport

  28. Digestion and Absorption • NaCl reabsorption in the small intestine and colon Lumen ofsmall intestineor colon Intestinal cell Interstitialfluid K+ Na+reabsorbed. Na+ 1 2 Na+ Na+ ATP Cl– K+ Na+ H+ HCO3– Cl– Cl– 1 2 Na+ enters cells bymultiple pathways. The Na+-K+-ATPase pumpsNa+ into the ECF. Figure 21-21

  29. Digestion and Absorption • Summary of motility, secretion, digestion, and absorption in different regions of the digestive system KEY M: motilityS:secretionD:digestionA:absorption ORAL CAVITY AND ESOPHAGUS M: swallowing, chewing S:saliva (salivary glands), lipase D:carbohydrates, fats (minimal) A:none Salivary gland Upper esophagealsphincter STOMACH M:peristaltic mixing and propulsion S:HCl (parietal cells); pepsinogen and gastric lipase (chief cells); mucus and HCO3– (surface mucous cells); gastrin (G cells); histamine (EC cells)D:proteins, fatsA:lipid-soluble substances such as alcohol and aspirin Esophagus Lower esophagealsphincter Liver SMALL INTESTINE Gallbladder M:mixing and propulsion primarily by segmentation S:enzymes; HCO3– and enzymes (pancreas); bile (liver);mucus (goblet cells); hormones: CCK, secretin, GIP,and other hormonesD:carbohydrates, fats, polypeptides, nucleic acids A:peptides by active transport; amino acids, glucose,and fructose by secondary active transport; fats by simple diffusion; water by osmosis ions,minerals, and vitamins by active transport Pylorus Pancreas Ileocecal valve LARGE INTESTINE M:segmental mixing; mass movement for propulsion S:mucus (goblet cells) D:none (except by bacteria) A:ions, water, minerals, vitamins, and small organicmolecles produced by bacteria Rectum Anal sphincters Figure 21-22

  30. Digestion and Absorption PLAY Interactive Physiology® Animation:Digestive System: Digestion and Absorption

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