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AHS 2013 Digestion and Metabolism 3 Sarah Harney 3. Regulation of Digestive Function • How does the nervous system influence digestion • What hormones regulate digestive function? • Regulation of motility • Regulation of secretion • Reading: Stanfield pp 592-594 Allied Health Sciences 1 Peripheral Nervous System Afferent Efferent Information from periphery to CNS Information from CNS to periphery Sensory neurons Somatic Nervous System Autonomic Nervous System Skeletal muscle Smooth muscle – gut, blood vessels, bladder Sympathetic Nervous System Allied Health Sciences Parasympathetic Nervous System 2 1 AHS 2013 Digestion and Metabolism 3 Sarah Harney Autonomic Control of GI Activity Autonomic Nervous System Involuntary activity Smooth muscle – gut, blood vessels, bladder Glands Parasympathetic Nervous System Sympathetic Nervous System ‘Fight or Flight’ ‘Rest and Digest’ Inhibition of GI tract contraction and secretion Increases GI motility and secretion of digestive enzymes and hormones (via vagus nerve) 3 Signalling Pathways Allied Health Sciences 4 2 AHS 2013 Digestion and Metabolism 3 Sarah Harney Regulation Phases Mechanisms • Cephalic • Enteric Nervous System • Gastric • Central Nervous System • Intestinal • Gastric hormones • Intestinal hormones Allied Health Sciences 5 Cephalic phase • ‘Anticipatory’ occurs before food reaches stomach • Sight, smell, hunger, thirst • Stimulation of saliva secretion • Increases gastric secretion – Activity of vagus nerve increases secretion of HCl and pepsinogen by secretory cells – Vagal stimulation of G cells increases gastrin secretion - gastrin also increases HCl and pepsinogen secretion Allied Health Sciences Famously described in Pavlov’s dog experiments 6 3 AHS 2013 Digestion and Metabolism 3 Sarah Harney Gastric phase • Begins when food reaches the stomach • Stimuli in stomach act to increase gastric secretion – Protein, distension, caffeine, alcohol • Mechanism (as for cephalic) – Activation of vagal nerve and intrinsic nerves of enteric nervous system → Increased gastrin secretion → increased HCl and pepsinogen secretion Allied Health Sciences 7 Intestinal phase • Occurs when chyme enters small intestine • Acts to shut off gastric secretion as stomach empties – therefore, is an inhibitory phase - Reduced protein in stomach removes stimulus for gastrin secretion so gastric secretion is reduced -Lower pH in stomach with less food also reduces secretion • Stimuli - Fat, acid or distension of duodenum inhibits gastric emptying • Mechanism Cholecystokinin (CCK) release (from small intestine mucosa) inhibits gastric emptying and secretion Allied Health Sciences 8 4 AHS 2013 Digestion and Metabolism 3 Sarah Harney Example of Phases Gastric Motility Cephalic Gastric • Stimulated by - Anticipation, smell - Anger, aggression Intestinal • Stimulated by • Stimulated by - Protein, distension - Protein, fat, H+, of stomach distension in duodenum Mediated by • Mediated by gastrin release CCK release Mediated by gastrin release Increased secretion • Inhibited by Increased force of contraction -Stomach emptying, low pH • Inhibited by - Fear, depression, pain -CCK, GIP release Allied Health Sciences Reduced Gastric motility Reduced gastric secretion 9 Gut Hormones • 4 principal gut hormones: – – – – Gastrin Cholecystokinin (CCK) Secretin Glucose-dependent insulinotropic peptide (GIP) (formerly known as gastric inhibitory peptide) • Released by endocrine cells in gastric/duodenal mucosa Allied Health Sciences 10 5 AHS 2013 Digestion and Metabolism 3 Sarah Harney Gut hormones — 1 GASTRIN Source: G cells in gastric glands of stomach Release: protein in stomach Functions • Stimulates gastric secretion • Increases gastric and intestinal motility • Induces colonic mass movements • Trophic to gastric and small intestinal mucosa Allied Health Sciences 11 Gut hormones — 2 SECRETIN Source: duodenal mucosa Release: acid in duodenum Functions • Inhibits gastric emptying and secretion • Increases NaHCO3 secretion by pancreas/in bile • Trophic to exocrine pancreas Allied Health Sciences 12 6 AHS 2013 Digestion and Metabolism 3 Sarah Harney Gut hormones — 3 CCK (CHOLECYSTOKININ) Source: duodenal mucosa Release: duodenal nutrients (esp. fat) Sherwood 16-13 Functions • Inhibits gastric emptying and secretion • Increases enzyme secretion by pancreas • Promotes bile release • Trophic to exocrine pancreas Allied Health Sciences 13 Gut hormones — 4 Glucose-dependent insulinotrophic peptide (GIP) (formerly known as gastric inhibitory peptide) Source: duodenal mucosa Release: duodenal fat, acid, glucose, distension Functions • Increases insulin secretion by endocrine pancreas • Inhibits gastric emptying and secretion Allied Health Sciences 14 7 AHS 2013 Digestion and Metabolism 3 Sarah Harney Regulation of Gastric Function — Outline Stomach Protein + products Distension Parasympathetic stim. Gastrin + Gastric Motility and Secretion - Small Intestine Secretin H+ GIP Glucose Fats H+ Distension CCK Fats Protein + products Allied Health Sciences 15 Regulation of gastric function • Stimulus: Protein /Amino acids, distension in stomach • Signal: gastrin, neural input • Effect: increased HCl and pepsin production Allied Health Sciences 16 8 AHS 2013 Digestion and Metabolism 3 Sarah Harney Regulation of bile function • More complex… • Two hormones – Secretin — synthesis – CCK — release Allied Health Sciences 17 Regulation of Pancreatic Function • Endocrine (secretion into the blood) – Islet cells , secretion of insulin,glucagon • Exocrine (secretion into external environment) – Acini – enzymes i.e. amylase, lipases, proteases – Ducts bicarbonate Allied Health Sciences 18 9 AHS 2013 Digestion and Metabolism 3 Sarah Harney Regulation of Pancreatic Function • Pancreatic secretions are increased by CCK and secretin CCK Enzyme secretion Secretin Bicarbonate secretion (acinar cells) (duct cells) Allied Health Sciences 19 Regulation of Pancreatic Function Allied Health Sciences 20 10 AHS 2013 Digestion and Metabolism 3 Sarah Harney Regulation of motility Gastric motility Increased by: Reduced by: • Gastrin • GIP •Secretin •CCK • Cephalic phase stimuli: anger, aggression • Cephalic phase stimuli : pain, fear, depression, anxiety Allied Health Sciences 21 11