Gastrointestinal Function Notes

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Osmosis High-Yield Notes

This Osmosis High-Yield Note provides an overview of Gastrointestinal Function essentials. All Osmosis Notes are clearly laid-out and contain striking images, tables, and diagrams to help visual learners understand complex topics quickly and efficiently. Find more information about Gastrointestinal Function:

Enteric nervous system

Gastrointestinal hormones

Hunger and satiety

NOTES NOTES GASTROINTESTINAL FUNCTION ENTERIC NERVOUS SYSTEM osms.it/enteric-nervous-system-and-slow-waves ▪ Intrinsic component of gastrointestinal (GI) innervation that can function without extrinsic innervation ▪ Communicates with sympathetic nervous system, parasympathetics ▪ Ganglia located in myenteric, submucosal plexuses ▫ Myenteric/Auerbach’s plexus: located between longitudinal, circular smooth muscle layers; GI movement function ▫ Submucosal/Meissner’s plexus: located in submucosa; function in GI secretions, blood flow ▪ Neurons of extrinsic system release neurocrines ▫ Neurochemicals consisting of neurotransmitters, neuromodulators ▪ Extrinsic branch of GI innervation ▫ Sympathetic, parasympathetic divisions PARASYMPATHETIC INNERVATION ▪ Parasympathetic ganglia located within myenteric, submucosal plexuses ▪ Parasympathetic preganglionic, postganglionic neurons either cholinergic (release acetylcholine) or peptidergic (release substance P/vasoactive intestinal peptide) ▪ Vagus nerve innervates upper GI ▫ Upper ⅓ of esophagus, stomach, small intestine, ascending, proximal transverse colon ▫ Consists of 75% afferent, 25% efferent fibers ▫ Vagovagal reflexes: reflexes in which both afferent, efferent limbs originate from vagus nerve Figure 37.1 Locations of the myenteric (Auerbach’s) and submucosal (Meissner’s) plexuses within the four layers of the gastrointestinal tract. The myenteric plexus is located between the circular and longitudinal smooth muscle layers, which produce different movements in the GI tract. 302 OSMOSIS.ORG
Chapter 37 Gastrointestinal Physiology: Gastrointestinal Function ▪ Pelvic nerves innervate lower GI ▫ Distal transverse colon, descending, sigmoid colon Figure 37.3 Sympathetic preganglionic neurons synapse outside the GI tract, in the four ganglia shown above. Figure 37.2 The vagus nerve provides parasympathetic innervation from the upper esophagus to the proximal transverse colon. The pelvic nerve provides innervation from the distal transverse colon to the rectum. SYMPATHETIC INNERVATION ▪ Sympathetic preganglionic neurons synapse in ganglia outside GI tract ▫ Celiac, superior mesenteric, inferior mesenteric, hypogastric ganglia ▪ Sympathetic postganglionic neurons either synapse on ganglia in myenteric/ submucosal plexuses or directly innervate target organs ▪ Sympathetic preganglionic neurons are cholinergic; sympathetic postganglionic neurons are adrenergic (release norepinephrine) MECHANISMS ▪ Slow waves ▫ Duodenum: 12 waves/minute ▫ Ileum: 9 waves/minute ▪ Migrating myoelectric complexes every 90 minutes clears any remaining chyme ▪ Sympathetic via fibers from celiac/superior mesenteric ganglia → ↓ contractions ▪ Two forms of contractions; coordinated by enteric nervous system Innervation ▪ Parasympathetic via vagus nerve (CN X) ▫ ↑ contractions; most nerves cholinergic, some release neurocrines (e.g. peptidergic) ▪ Motilin ▫ Secreted by endocrinocytes in proximal small intestine, regulate contractions ▪ Vasoactive intestinal peptide (VIP) ▫ Induces smooth muscle relaxation (e.g. sphincters); induces water secretion into pancreatic juice, bile; inhibits gastric acid secretion ▪ Enkephalins ▫ Inhibitory modulators in myenteric, submucosal plexuses Segmental contractions (↓ diameter) ▪ Mix, expose chyme to secretions, enzymes ▪ Contraction → splits chyme → both orad, caudad directions → relaxation → merging of chyme → repeated OSMOSIS.ORG 303
▪ No forward/propulsive movement along small intestine Peristaltic/longitudinal contractions (↓ length) ▪ Move chyme down GI tract ▪ Contraction behind bolus → proximal portion of intestine relaxes simultaneously → chyme propelled in caudad direction ▪ Longitudinal, circular muscles reciprocally innervated → do not contract together; if circular muscle contracts → longitudinal muscle in same segment relaxes simultaneously Peristalsis reflex ▪ Enterochromaffin-like (ECL) cells in intestinal mucosa sense food bolus → secrete serotonin (5-HT) → binds to intrinsic primary afferent neuron receptors → activates peristalsis reflex → excitatory neurotransmitters (acetylcholine, substance P, neuropeptide Y) released behind bolus → ↑ circular muscle contraction → ↓ longitudinal muscle activation → segment narrows, lengthens → in front of bolus, circular muscle inhibitory mechanisms (VIP, nitric oxide) activate, excitatory pathways in longitudinal segment activate → segment shortens, widens → chyme propelled forward in caudad direction 304 OSMOSIS.ORG
Chapter 37 Gastrointestinal Physiology: Gastrointestinal Function GASTROINTESTINAL HORMONES osms.it/gastrointestinal-hormones SOMATOSTATIN ▪ Members of G protein coupled-receptor superfamily Function ▪ ↓ secretion of many other hormones (e.g. gastrin, bicarbonate, digestive enzymes) ▪ ↓ nutrient absorption from gut by prolonging gastric emptying time ▪ ↓ pancreatic secretions ▪ ↓ visceral blood flow Secretion and activation ▪ Central nervous system, pancreatic delta cells, enteroendocrine delta cells ▪ Somatostatin binds receptors → activates inhibitory G protein → inactivate adenylate cyclase → ↓ cAMP production → protein kinase not activated → ↓ Ca2+ → inhibitory effect ▪ Site of action ▫ Stomach, pancreas, small intestine, gallbladder, liver ▪ Secretory stimulants ▫ Glucose, arginine, leucine, glucagon, vasoactive intestinal peptide (VIP), cholecystokinin (CCK) GASTRIN Function ▪ Induces gastric acid secretion Secretion and stimulation/inhibition ▪ Secreted by enteroendocrine G cells of stomach, duodenum ▪ Secretory stimulants ▫ Presence of acidic content, partially digested food in duodenum, vagus nerve stimulation ▪ Inhibited by somatostatin MOTILIN Function ▪ Stimulates gastric, pancreatic enzyme secretion Secretion and stimulation/inhibition ▪ Secreted by enteroendocrine M cells of proximal small intestine ▪ Secretory stimulants ▫ Duodenal alkalinization, gastric distension ▪ Inhibited by duodenal nutrients PANCREATIC PEPTIDE (PP) Function ▪ ↓ gastric emptying, slows small intestine motility Secretion and stimulation/inhibition ▪ Secreted by endocrine cells in pancreatic islets ▪ Secretory stimulants: intraluminal nutrients, vagal nerve activation, hypoglycemia PEPTIDE Y (PPY) Function ▪ Inhibits gastric acid secretion, gastric motility, slows intestinal motility ▪ Inhibits pancreatic exocrine secretion Secretion and stimulation/inhibition ▪ Secreted by pancreatic islet alpha cells, enteroendocrine cells ▪ Secretory stimulants ▫ Ingestion of nutrition, bile acids, fatty acids OSMOSIS.ORG 305
SECRETIN Function ▪ ↓ acidity to improve pancreatic enzyme function ▪ ↑ pancreatic secretion, biliary bicarbonate, water ▪ Regulates pancreatic enzyme secretion ▪ ↓ gastric emptying, gastrin release, gastric acid secretion Secretion and stimulation/inhibition ▪ Secreted by enteroendocrine S cells in proximal small intestine ▪ Secretory stimulants ▫ Gastric acid, bile salts, peptides, fatty acids, ethanol ▪ Inhibited by somatostatin CHOLECYSTOKININ (CCK) Function ▪ Promotes food delivery from stomach into small intestine ▪ Regulates nutrient-stimulated enzyme secretion ▪ ↑ gallbladder contraction ▪ ↑ enzymatic pancreatic secretion output Secretion and stimulation/inhibition ▪ Secreted by enteroendocrine I cells ▪ Secretory stimulants ▫ Nutrition ingestion, fatty acids, amino acids INSULIN Function ▪ Major anabolic hormone ▪ ↓ blood glucose ▪ Promotes liver, muscle glycogen storage ▪ Fatty acids, triacylglycerol storage in adipose tissue ▪ Protein synthesis, glucagon suppression Secretion and stimulation/inhibition ▪ Secreted by beta cells of pancreatic islet cells 306 OSMOSIS.ORG ▪ Secretory stimulants ▫ High blood glucose, glucose, arginine, leucine, glucagon, VIP, CCK ▪ Inhibited by somatostatin GLUCAGON Function ▪ Counteracts insulin ▪ ↑ blood glucose, promotes glycogenolysis, gluconeogenesis, ketogenesis ▪ Works mainly on liver Secretion and stimulation/inhibition ▪ Secreted by alpha cells of islet cells, L-cells of intestine ▪ Inhibited by somatostatin INCRETINS ▪ Includes glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic peptide (GIP) Function ▪ ↑ insulin release from pancreatic beta-cells ▪ ↑ levels of cAMP in islets leading to expansion of beta-cells GASTRIC INHIBITORY PEPTIDE (GIP) ▪ AKA glucose-dependent insulinotropic peptide Function ▪ Weakly inhibits HCl production, stimulates insulin release Secretion and stimulation/inhibition ▪ Secreted by duodenal mucosa ▪ Secretory stimulant ▫ Fatty chyme HISTAMINE Function ▪ Activates parietal cells to release HCl Secretion and stimulation/inhibition ▪ Secreted by stomach mucosa ▪ Secretory stimulant ▫ Food in stomach
Chapter 37 Gastrointestinal Physiology: Gastrointestinal Function SEROTONIN GHRELIN Function ▪ Stomach muscle contraction Function ▪ Stimulate hunger ▪ Site of action ▫ Hypothalamus lateral nucleus Secretion and stimulation/inhibition ▪ Secreted by stomach, duodenal mucosa ▪ Secretory stimulant ▫ Food in stomach VASOACTIVE INTESTINAL PEPTIDE (VIP) Function ▪ Dilates intestinal capillaries ▪ ↑ secretions, ↓ acid secretion ▪ Relaxes intestinal smooth muscle ▪ Site of action ▫ Small intestine, pancreas, stomach Secretion and stimulation/inhibition ▪ Secreted by enteric neurons/ parasympathetic ganglia ▪ Secretory stimulant ▫ Chyme, parasympathetic stimulus Secretion and stimulation/inhibition ▪ Secreted by gastric cells ▪ Secretory stimulant ▫ Empty stomach ▪ Inhibited by stomach stretching when food present LEPTIN Function ▪ Stimulate satiety ▪ Site of action ▫ Ventromedial nucleus of hypothalamus Secretion and stimulation/inhibition ▪ Secreted by adipocytes ENKEPHALINS Function ▪ Smooth muscle constriction causing ↓ fluid flow into intestines (opiates acting on enkephalin receptors ↓ fluid flow to intestines, cause constipation) ▪ Site of action ▫ Intestine ▪ Secreted by ▫ GI tract neurons OSMOSIS.ORG 307
SATIETY osms.it/satiety ▪ Hypothalamus controls appetite, satiety ▫ Ventral posteromedial nucleus (VPN) of hypothalamus: activates satiety ▫ Lateral hypothalamic area: activates hunger, feeding HORMONES Leptin ▪ Stimulates satiety, decreases appetite ▪ Secreted by fat cells in proportion to fat amount in adipocytes Insulin ▪ Stimulates satiety, decreases appetite ▪ Fluctuates throughout day Peptide YY (PYY) ▪ Stimulates satiety, decreases appetite directly (via hypothalamus), indirectly (via inhibiting release of ghrelin) GLP-1 ▪ Stimulates satiety, decreases appetite ▪ Secreted by intestinal L cells Ghrelin ▪ Increases appetite, hunger ▪ Secreted by gastric cells before meal ▪ Starvation, weight loss stimulates ghrelin release Figure 37.4 Locations of two areas in the hypothalamus that control appetite and satiety. 308 OSMOSIS.ORG

Osmosis High-Yield Notes

This Osmosis High-Yield Note provides an overview of Gastrointestinal Function essentials. All Osmosis Notes are clearly laid-out and contain striking images, tables, and diagrams to help visual learners understand complex topics quickly and efficiently. Find more information about Gastrointestinal Function by visiting the associated Learn Page.