Gastrointestinal System Anatomy & General Principles of Motility PDF

Gastrointestinal System Anatomy & General Principles of Gastrointestinal Function- Motility , Nervous Control, and Blood Circulation Lecture Outline I. Gross Anatomy II. Tissue Layers (Tunics) III. General Principles of Gastrointestinal Motility IV. Neural Control of Gastrointestinal Function V. Hor...

Gastrointestinal System Anatomy & General Principles of Gastrointestinal Function- Motility , Nervous Control, and Blood Circulation Lecture Outline I. Gross Anatomy II. Tissue Layers (Tunics) III. General Principles of Gastrointestinal Motility IV. Neural Control of Gastrointestinal Function V. Hormonal Control of Gastrointestinal Function VI. Functional Movements in the GI Tract VII. GI Blood Flow- Splanchnic Circulation VIII. Pathophysiology 1 Gastrointestinal System Anatomy & General Principles of Gastrointestinal Function- Motility , Nervous Control, and Blood Circulation Objectives 1.Describe the organization of the GI System including major organs and their constituent parts 2. List the functional subdivisions of the gut (foregut, midgut, hindgut), major structures of each, and their major blood supply 3.Identify the four major layers (tunics) of tissue of the GI tract from superficial to deep layers- include their tissue type, function, and any specializations 4.Explain the two types of electrical waves that excite GI smooth muscle 5.Describe the dual innervation provided by the ENS and ANS; explain the two components of each and their interaction 6.Explain the gut- brain axis 7.Identify 5 major GI hormones, their stimulus for secretion, site of secretion, and action 8.Explain the two different types of functional movements in the GI tract 9.Recognize the implication of splanchnic circulation 10.Identify the role of a lacteal in intestinal absorption 11.List three disease states of altered motility 2 References Assigned reading from your text: Hall Chapter 63 begins slide 31 Images used from Ganong 3 I. Gross Anatomy 4 The GI Tract q The gastrointestinal tract serves as a portal for nutrient and water absorption An epithelial-lined tube that begins at the oral cavity and extends to the anal canal Includes associated glands Aka- alimentary canal, gut tube, GI tract 5 Structures of the GI System q Structures and their functions include: Oral cavity includes tongue, teeth, salivary glands Pharynx (throat) includes the oropharynx and laryngopharynx Chemical breakdown of materials by acid and enzymes Mechanical processing through muscular contractions Small intestine includes duodenum, jejunum, ileum Transports materials to stomach Stomach includes fundus, body, pylorus Muscular propulsion of materials into the esophagus Esophagus includes upper (UES) and lower esophageal sphincters (LES or cardiac sphincter); 10 inches long Mechanical processing, moistening, and mixing with salivary secretions Enzymatic digestion and absorption of nutrients Large intestine includes cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal Dehydration and compaction of indigestible material 6 Features of the Stomach qStructures of the stomach include: Cardia- Close to heart, small circular region Just below the LES- near the GE junction Fundus- blind sac on left side of the stomach Extends superiorly Functions as part of the Body Corpus or Body- largest region Incorporates most of the greater and lesser curvatures Pylorus- located below the body composed of: Antrum- designated by an imaginary line diagonally from the angular notch to the greater curvature Pyloric canal- terminal region connects to the duodenum at the GD junction Pyloric sphincter is the last structure of the pylorus 7 Regions of the Small Intestines qRegions of the small intestines: Duodenum- 25 cm long Mostly retroperitoneal 4 parts Superior Descending Inferior Ascending Jejunum Proximal 2/5 of mesenteric small intestine Most absorption takes place Ileum Distal 3/5 of mesenteric small intestine opens via the ileocecal valve into the cecum 8 Structures of the Large Intestines q Structures of the large intestine: Cecum & its vermiform appendix Ascending colon Transverse colon Descending colon Sigmoid colon Rectum Anal canal q During a colonoscopy- these structures will be encountered from superficial to deep beginning with the anus 9 Functional Subdivisions Of The GI System q Based on primary arterial supply during embryonic development Foregut- celiac trunk Midgut- superior mesenteric artery Hindgut- inferior mesenteric artery 10 Venous Drainage Of The Functional Divisions Of The GI System q Venous drainage includes: – Foregut- drained by portal and splenic veins – Midgut- drained by superior mesenteric vein – Hindgut- drained by inferior mesenteric vein 11 Foregut q Supplied primarily by the celiac artery (aka celiac trunk) This region extends from the distal end of the esophagus to the proximal half of the duodenum Includes esophagus, stomach, part of the duodenum, pancreas, liver, and gallbladder Portal and splenic veins drain foregut 12 Midgut q Supplied primarily by the superior mesenteric artery This region of the gut tube extends from the distal half of the duodenum to the splenic flexure of the colon Includes the distal half of the duodenum, jejunum, ileum, cecum, ascending colon, and proximal half of the transverse colon Superior mesenteric vein drains midgut 13 Hindgut q Supplied primarily by the inferior mesenteric artery This region of the gut tube extends from the splenic flexure of the colon to the rectum Distal half of the transverse colon, descending colon, sigmoid colon, and proximal third of rectum Inferior mesenteric vein drains hindgut 14 II. Tissue Layers (Tunics) 15 Tissue Layers q Four layers (tunics) of tissues superficial to deep from mouth to anus Typical cross section of the gut: Serosa Muscular – Longitudinal smooth muscle – Circular smooth muscle Submucosa Mucosa 16 Tissue Layers- Ganong FIGURE 25–2 Organization of the wall of the intestine into functional layers. (Adapted with permission from Yamada T: Textbook of Gastroenterology, 4th ed. New York, NY: Lippincott Williams & Wilkins; 2003.)of the gut: 17 Tunica Serosa q Tunica Serosa (serous membrane) – – – Outer covering of digestive tube Connective tissue covered by mesothelium (serous epithelium) Houses vascular and nervous supplies to GI tract – – Continuous with cavity lining Peritoneal structures are covered in t. serosa q Structures formed by tunica serosa: In stomach – forms greater and lesser omenta In intestine (except duodenum)- mesenteries attach organs to posterior abdominal wall and hold organs in place In ascending, transverse, descending, and sigmoid colon- forms mesocolon aka Tunica Adventitia - outside of peritoneal cavity (esophagus and rectum) – – White fibrous connective tissue eg connects esophagus to mediastinal structures 18 Tunica Muscularis Tunica muscularis- two smooth muscle layers provide motility q Inner circular layer-circularly aligned around the gut pushes the bolus forward Outer longitudinal layer- longitudinal alignment contracts in front of the bolus Between muscular layers- myenteric plexus of nerves 19 Variations of the Tunica Muscularis- Esophagus Esophagus is unique- contains both skeletal and smooth muscle – – – Upper 1/3 is skeletal muscle – cricopharyngeus muscle Middle 1/3 is a combination skeletal and smooth Lower 1/3 is smooth muscle Two smooth muscle layers: outer (longitudinal) inner (circular) 20 Variations of the Tunica Muscularis- Stomach q Stomach is composed of three layers of smooth muscle – Outer layer: longitudinal – Middle layer: circular – Inner layer: oblique - unique to the stomach 21 Variations of the Tunica Muscularis- Small Intestine q Small intestine is composed of two layers of muscle Outer layer – Longitudinal smooth muscle Contracts in front of the food bolus Inner layer – Circular smooth muscle Contracts behind the food bolus 22 Variations of the Tunica Muscularis- Large Intestine q Large intestine (colon) Outer layer: teniae coli- reduced to three strips longitudinal smooth muscle Inner layer of circular smooth muscle forms pouches – “haustra” – Haustra created by muscle tone within the tenaie coli 23 Tunica Submucosa q Tunica submucosa is connective tissue Contains blood vessels, nerves, lymphatics Connects T. muscularis to T. mucosa Contains a small circular layer of smooth muscle (mostly in small intestine) Submucous plexus- digestive nervous system- controls mucosa 24 Tunica Mucosa q Tunica mucosa- innermost lining of the lumen of the digestive tube – Of the four tunics- this varies the most – Specialized epithelium that accomplishes regional digestive tasks Secretion, absorption, or hormone production – Multiple types of epithelium in same area: 25 Mucosal Variations q Small intestines vary by segment Duodenum: “C”-shaped; initial part of small intestine; – – About ~10-12” long Contains several types of endocrine cells Jejunum and ileum: 8-9’ past duodenum – Submucosa and mucosa are modified in circular folds – “Plicae circulares” or folds of Kerckring Well developed in duodenum and jejunum Increases surface area of small intestine 3x – Villi from these folds increase surface area 10x – Microvilli of epithelial cells increase surface area 20x More absorption in jejunum, distal structures coincide with progressively decreased absorption (Plicae Circulares) 26 27 Features of the Colon qColon is ~5’ in length qRegions: Cecum: A blind pouch/sac The appendix extends from it posteriorly Ileum opens into the cecum at the ileocecal valve Increased ileal pressure opens valve and allows chyme to enter cecum Increased colonic pressure maintains tone to keep valve closed to prevent reflux from colon into ileum 28 Parts of the Colon q The large intestine -or colon- has distinct regions Ascending colon - extends from cecum up the right side of the abdominal cavity to the liver at the hepatic flexure where it makes a right turn to form: Transverse colon - extends from the hepatic flexure to the left side of the abdominal cavity laterally to the spleen at the splenic flexure; turns to form: Descending colon - extends from the splenic flexure to the iliac crest Sigmoid colon “S-shaped” structure extends from iliac crest into the pelvic cavity terminating at the rectum that is located in the pelvic cavity 29 III. General Principles of Gastrointestinal Motility 30 Electrical Activity Of GI Smooth Muscle q GI smooth muscle functions as syncytium Bundles of small cells made of electrically coupled gap junctions Each muscle layer functions as a syncytium- an AP anywhere in the muscle travels all directions in the muscle Connections between longitudinal and circular layers allow excitation of the other 31 GI Electrophysiology -Slow Waves q Smooth muscle is excited by 2 types of electrical waves: slow waves and spikes q Slow waves Basal Electrical Rhythm (BER) is a continual, slow variation in RMP Resting potential of smooth muscle cells between -50 to -60 mV Cause entry of sodium ions Do not cause contraction of smooth muscle- they are not action potentials Are stimulated by the interstitial cells of Cajal – electrical pacemakers for smooth muscle cells Modified smooth muscle cells with no contractile properties Communicate by gap junctions 32 GI Electrophysiology –Spike Potentials q Spike Potentials are action potentials Appear when peaks of slow waves RMP becomes greater than – 40 mV (less negative) Longer duration of AP due to channels opening/closing more slowly GI smooth muscle fibers, channel responsible for AP is Ca+2/Na+-Voltage-Gated Large amounts of Ca+2 and Lesser Na+ enters Smooth Muscle Cellsà Contraction 33 Factors That Affect Membrane Potential q Membrane potential responds to various stimuli Factors that depolarize the membrane (more excitable) ésecretion, épropulsive motility) Factors that hyperpolarization the membrane (less excitable) êRMP Norepinephrine (from sympathetic ns or adrenal medulla) Epinephrine (from adrenal medulla) Anticholinergic drugs Effectual peristalsis depressed when person treated with atropine Some stimuli produce a mixed response Distention- stretching of smooth muscle Acetylcholine from parasympathetic fibers Stimulation by specific G-I hormones e.g. Vasoactive intestinal peptide (VIP) (éElectrolyte secretion, êpropulsive motility) Entry of calcium ions causes smooth muscle contraction Slow waves only cause sodium ion entry- they do not cause calcium to enter the fibers 34 IV. Neural Control of Gastrointestinal Function 35 GI Innervation- Intrinsic and Extrinsic q Dual innervations Intrinsic innervation- Enteric nervous system (ENS) Can function autonomously Called “little brain” since it is the central control regulating GI function Two plexuses Extrinsic innervation- ANS Parasympathetic cholinergic activity increases activity of intestinal smooth muscle Sympathetic activity decreases intestinal smooth muscle activity and causes sphincters to contract 36 Interaction of the ENS and ANS q Intrinsic control by ENS – Myenteric plexus – Controls muscle activity along the length of the gut Increases tone, rate, intensity, velocity of contractions causing more rapid peristalsis Inhibits sphincters Submucosal plexus Controls local intestinal secretion, absorption, and mucosal infoldings q Extrinsic control by ANS – Parasympathetic and sympathetic efferent fibers – Sensory fibers pass from the luminal epithelium and gut wall to: The enteric plexuses Then to the prevertebral ganglia of the SC and Directly to the SC and brain stem 37 The enteric nervous system. A. Schematic illustration of the organization of the enteric nervous system within the intestinal wall. B. Flow chart demonstrating long (extrinsic) and short (intrisic) reflexes involving the enteric nervous system. 38 Intrinsic Innervation q Intrinsic control- the enteric nervous system (ENS) Composed of neurons, ganglia, fibers in a network q Two major plexuses of the ENS are interconnected from superficial to deep layers Myenteric (Auerbach’s) plexus – Located between muscle layers from proximal esophagus to rectum – Stimulates G-I motility Submucosal (Meissner’s) plexus – Located between circular smooth muscle and submucosa of small and large intestines mainly (sparsely found in the stomach) – Stimulates gland secretions, contraction of the muscularis mucosae – Controls local blood flow 39 Extrinsic Innervation Foregut Sympathetics from greater splanchnic nerves (T5-T9) Parasympathetics from vagus nerves Midgut Sympathetics from lesser splanchnic nerves (T10-T11) Parasympathetics from the vagus nerves Hindgut Sympathetics from the lumbar splanchnic nerves Parasympathetics from the pelvic splanchnics (S2-S4)- defecation 40 ANS Modifies the ENS- Parasympathetic NS q Parasympathetic system stimulates motility and secretion Cranial parasympathetic preganglionic motor neurons arise from the vagus nerve – Their axons innervate postganglionic neuron cell bodies located in the myenteric plexus (proximal esophagus to proximal large intestine) and – Myenteric neurons innervate neurons in the submucosal plexus (small intestine to proximal large intestine) Sacral parasympathetic preganglionic neurons arise from S2 to S4 spinal segments; – Their axons pass through pelvic nerves to distal half of large intestine and anus where they innervate neuron cell bodies located in the myenteric plexus – Myenteric neurons innervate neurons in the submucosal plexus (distal large intestine) q Acetylcholine is the excitatory neurotransmitter 41 ANS Modifies the ENS- Sympathetic NS q Sympathetic system: inhibits motility and secretion; maintains muscular tone of sphincters Sympathetic preganglionic neurons arise from T5 to L2 spinal cord segments; Their axons pass to collateral ganglia and terminate on cell bodies of sympathetic postganglionic neurons; Postganglionic axons terminate on neurons in the myenteric plexus, and in turn myenteric axons innervate neurons in the submucosal plexuses q The neurotransmitter norepinephrine: Inhibits intestinal tract smooth muscle Excites mucosal muscle NE inhibits the neurons of the entire ENS Epinephrine from adrenal medulla has same effects as NE 42 Gut-Brain Axis q Gut-Brain Axis coordinates reflexes and responses between the ENS and ANS ENS can function alone or with ANS Three types of ENS GI reflexes exist. There are reflexes: -That are integrated entirely within gut wall ENS -From the gut to paravertebral ganglia and back to GI tract (gastrocolic reflex) -From the gut to SC and back to GI tract GI activity linked to behavior -Efferent nerve fibers Emotions e.g. anxiety produces diarrhea Salivation and gastric secretion occur with thought/smell of food -Afferent sensory nerve fibers Fullness/satiety Nausea/pain 43 V. Hormonal Control of Gastrointestinal Function 44 Hormonal Control of Gastrointestinal Motility q Enteroendocrine cells sense gut lumen stimuli and secrete appropriate hormones 45 VI. Functional Movements in the GI Tract 46 Functional Movements of the GI Tract q Two types of functional movement in the GI tract: Propulsive Movements cause food to move forward -Peristalsis- property of syncytium -Stimulation at any point in the gut causes a contractile ring to appear in circular muscle and spread -Usual stimulus is distention -Law of the Gut- receptive relaxation allows food to be propelled toward anus Mixing movements keep the intestinal contents mixed at all times -Local intermittent constrictive contractions occur every few centimeters 47 VII. GI Blood Flow- Splanchnic Circulation 48 Splanchnic Circulation q Splanchnic circulation is the blood vessels of the GI system part of splanchnic circulation Blood flows through the gut, spleen, pancreas, and immediately into liver by the portal vein – GI blood passes through liver sinusoids then leaves by hepatic veins that empty into the vena cava – Oral medications absorbed from gut must first-pass into splanchnic circulation before going into general circulation Liver sinusoids: Removes bacteria Absorb carbohydrates and protein Fats absorbed from intestinal tract are not carried in portal blood; they are absorbed into lacteals (intestinal lymphatics) and conducted to systemic circulation by the thoracic duct 49 Two Routes Of Abdominal Venous Drainage q Portal drainage: Veins àLiverà IVCà Heart Venous blood from the gut tube return to the heart via the hepatic portal vein q Systemic (caval) drainage: Veinsà IVCà Heart Venous blood from anterior and posterior abdominal walls and retroperitoneal organs Caval drainage (darker purple) Hepatic portal drainage (lighter blue) 50 Effect of Gut Activity and Metabolic Factors on GI Blood Flow q Under normal conditions, blood flow to the GI tract is directly related to gut activity Parasympathetic stimulation increases local blood flow and glandular secretion Sympathetic stimulation causes intense arteriolar vasoconstriction and decreases blood flow – Heavy exercise and circulatory shock can cause sympathetic vasoconstriction of large-volume intestinal and mesenteric veins- provides up to 400 ml blood to sustain general circulation – Autoregulatory escape- local metabolic vasodilator mechanisms elicited by ischemia return normal blood flow to glands and muscle – Local metabolic vasodilator mechanisms elicited by decreased oxygen content and ischemia eg adenosine 51 Effect of Gut Activity and Metabolic Factors on GI Blood Flow q Countercurrent blood flow in the villi Arterial flow into the villus and venous flow out are in opposite directions Blood oxygen diffuses out of arterials directly into adjacent venules without being carried to the tips of the villi Anatomical “shortcut”- Shunting of 80% of oxygen across the villus without reaching the tips is harmful in disease conditions – Tips of villi become ischemic – Gut may be malabsorptive in shock/disease states 52 VIII. Pathophysiology 53 Altered Motility Related To Gastric Innervation q States of altered motility include: Gastroparesis- Significantly delayed gastric emptying Common complication poorly controlled DM S/S- Early satiety, delayed emptying Anesthetic concern- potential full-stomach Hirschprung’s- aka megacolon (toxic megacolon) Congenital absence of a myenteric plexus Abnormal dilation of colon with accumulated fecal matter Paralytic ileus- temporary cessation of gut motility – S/S- nausea/vomiting, abdominal distension, absent bowel sounds Related to surgery and electrolyte imbalance (hypokalemia) 54 1. Water is absorbed in the jejunum, ileum, and colon and excreted in the feces. Arrange these in order of the amount of water absorbed or excreted from the greatest to the smallest. A. B. C. D. E. Colon, jejunum, ileum, feces Feces, colon, ileum, jejunum Jejunum, ileum, colon, feces Colon, ileum, jejunum, feces Feces, jejunum, ileum, colon 2. Postprandially, several gastrointestinal hormones are released. The presence of fat, carbohydrate, or protein in the duodenum stimulates the release of which hormone from the duodenal mucosa? A. B. C. D. E. Cholecystokinin Glucose-dependent insulinotropic peptide (GLIP) Gastrin Motilin Secretin 3. The cecum is a part of which GI structure? A. B. C. D. Stomach Small Intestine Large intestine Appendix 4. What is the function of the interstital cells of Cajal? A. B. C. D. 5. A. B. C. D. Secretory cells for stomach acid Mucus-producing cells Electrical pacemakers for smooth muscle cells Electrical pacemakers for skeletal muscle cells The stomach is supplied by which artery? Portal vein Celiac trunk Superior mesenteric artery Inferior mesenteric artery 55