GI Motility: Propulsion & Mixing of Food (PDF)

GI motility. Propulsion and Mixing of Foods. Konstantinos Ekmektzoglou MD, PhD, FEBGH Assistant Professor School of Medicine European University Cyprus November 2024 GI motility Overview of GI motility Gastrointestinal Smooth Muscle Functions as a Syncytium longitudinal muscle...

GI motility. Propulsion and Mixing of Foods. Konstantinos Ekmektzoglou MD, PhD, FEBGH Assistant Professor School of Medicine European University Cyprus November 2024 GI motility Overview of GI motility Gastrointestinal Smooth Muscle Functions as a Syncytium longitudinal muscle layer -> bundles extend longitudinally down to the intestinal tract circular muscle layer -> bundles extend around the gut Within each bundle -> fibers connected through gap junctions -> electrical signals that initiate muscle contractions travel readily from one fiber to the next within each bundle more rapidly along the length of the bundle than sideways  Electrical Activity of GI Smooth Muscle Continual slow, intrinsic electrical activity along the membranes of the muscle fibers Two basic types of electrical waves: Slow Waves Spike Potentials  Electrical Activity of GI Smooth Muscle Slow Waves Rhythmical gastrointestinal contractions Low changes in the resting membrane potential Intensity: 5 - 15 millivolts Frequency 3/min body of the stomach 8-9/min terminal ileum 12/min duodenum Caused by complex interactions among the smooth muscle cells and the interstitial cells of Cajal (electrical pacemakers) Do not by themselves cause muscle contraction except in the stomach Excite the appearance of intermittent spike potentials excite the muscle contraction  Electrical Activity of GI Smooth Muscle Spike Potentials True action potentials Occur automatically when the resting membrane more positive −40 millivolts Frequency: 1-10/sec The higher the slow wave potential, the greater the frequency of spikes Duration: 10 to 20 milliseconds Generated by calcium-sodium channels large numbers of calcium ions entering along with smaller numbers of sodium ions much slower to open and close long duration of the action potentials  Electrical Activity of GI Smooth Muscle Changes in Voltage of the Resting Membrane Potential (−56 millivolts) If less negative (depolarization) -> muscle fibers become more excitable stretching of the muscle, stimulation by Ach and specific GI hormones If more negative (hyperpolarization) -> muscle fibers become less excitable effect of norepinephrine or epinephrine and stimulation of the sympathetic nerves Calcium Ions and Muscle Contraction Significant quantities of Ca ions enter the fibers and cause most of the contraction during the spike potentials Slow waves do not cause Ca ions to enter the smooth muscle fiber -> no contractions Tonic Contraction of Some Gastrointestinal Smooth Muscle Continuous, not associated with basic electrical rhythm ⇅ in intensity, lasts several minutes or hours Caused by continuous repetitive spike, hormones, continuous Ca entry  The Enteric Nervous System The entire nervous system of the GI Sympathetic and parasympathetic fibers tract connect to both plexuses 100 millions neurons Enhance or inhibit ENS functions Lies in the wall of the gut Sensory nerve endings from the GI Controls GI movements and epithelium secretion afferent fibers 2 plexuses both plexuses Myenteric / Auerbach’s SNS prevertebral ganglia Outer, between longitudinal spinal cord and circular muscle layers vagus nerves Submucosal / Meissner’s elicit local reflexes within the gut wall or Inner, in the submucosa reflexes relayed to the gut from the prevertebral ganglia or the basal regions of the brain  Differences between Plexuses Myenteric Plexus Submucosal Plexus Linear chain of neurons extending to Controls function of inner wall of the entire GI tract each segment of the intestine Controls muscle activity Integrates sensory signals from the increased gut wall tonic contraction epithelium to control more intense rhythmical contractions local intestinal secretion increased rate of rhythm of contraction local absorption increased velocity of excitatory waves local contraction of the submucosal more rapid movement of the gut peristaltic waves muscle infolding of the GI mucosa Partial inhibitory effect (VIP) Pyloric sphincter Ileocecal valve sphincter  Autonomic Control of the GI Tract Esophagus, stomach, pancreas, Cranial fibers Vagus nerves small intestine, first half of colon Postganglionic Parasympathetic nerves in the 2 Nervous System plexuses Sacral fibers Pelvic nerves Distal half of colon ✓Parasympathetic stimulation -> increases ENS activity (+ defecation)  Autonomic Control of the GI Tract Celiac ganglion Sympathetic Post-ganglionic Pre-ganglionic Nervous System Sympathetic chains fibers innervate fibers (T5-L2) all gut Mesenteric ganglia ✓Sympathetic stimulation -> ENS inhibition ✓Norepinephrine ✓direct effect on smooth muscle ✓indirect effect by inhibiting ENS neurons  Gastrointestinal Reflexes 1. Reflexes entirely integrated entirely in ENS secretion, peristalsis, mixing contractions 2. Reflexes transmitting signals long distances to other areas of the GI tract (gut -> prevertebral ganglia -> GI tract) Gastro-colic reflex Colon evacuation induced be the presence of food in the stomach Entero-gastric reflex Inhibition of gastric motility when food passes into the small intestine Colono-ileal reflex Inhibition of emptying ileal contents into the colon 3. Reflexes from the gut to the spinal cord or brain stem and back to the GI tract stomach / duodenum -> brain stem -> stomach via vagus nerve control gastric motility and secretion pain reflexes -> inhibition of the GI tract colon / rectum -> spinal cord -> colon / rectum defecation reflex powerful colonic, rectal, and abdominal contractions Hormonal Control of GI Motility Hormonal Control of GI Motility ✓Gastrin ✓Cholecystokinin ✓Secretin  Functional Types of Movements in the GI Tract 1. Propulsive movements = Peristalsis Stimulated by gut distension Contractile rings appears 2-3cm behind and moves forward (5-10cm) Mediated by the myenteric plexus “Law of the Gut” = receptive relaxation Gut relaxes several cm downstream to allow food be propelled more easily  Functional Types of Movements in the GI Tract 2. Mixing movements Peristaltic contractions -> mixing forward progression blocked by a sphincter Peristaltic wave churns the intestinal contents Local intermittent constrictive contractions occur every few cm last 5 - 30 sec new constrictions at other points “chopping” and “shearing” Propulsion and Mixing of Food  Ingestion of Food Mastication (chewing) Swallowing Breaks the indigestible cellulose Voluntary stage membranes initiates swallowing Increases the surface area where Pharyngeal stage digestive enzymes act involuntary Prevents excoriation food passes into the esophagus Facilitates food transition through Esophageal stage GI tract involuntary food passes into the stomach Pharyngeal Stage of Swallowing  Pharyngeal Stage of Swallowing Palatopharyngeal folds Soft palate moves approximate each other upward to close forming a sagittal slit posterior nares well masticated food can pass muscular wall of pharynx vocal cords contracts approximated food propelled by peristalsis into Upper larynx is pulled upward the esophagus esophageal and anteriorly sphincter relaxes epiglottis swings backward trachea  Nervous Initiation of the Pharyngeal Stage of Swallowing Stimulation of epithelial swallowing areas (mainly on the tonsillar pillars) -> trigeminal and glossopharyngeal nerves -> medulla oblongata -> tractus solitarius -> reticular substance of the medulla and lower portion of the pons (deglutition or swallowing center) -> 5th, 9th, 10th, 12th cranial nerves -> pharynx and upper esophagus Pharyngeal stage of swallowing ✓ a reflex act ✓ initiated by voluntary movement of food into the back of the mouth ✓ excites involuntary pharyngeal sensory ✓ elicit the swallowing reflex  Esophageal Stage of Swallowing Conducts food rapidly from pharynx to the stomach Primary Continuation of the peristaltic wave that begins in the pharynx peristalsis Passes from the pharynx to the stomach in 8 - 10 sec -> esophagus distention by retained food Secondary peristalsis Continues until all food into the stomach Initiated by myenteric NS and reflexes that begin in the pharynx and are transmitted upward through vagal afferent fibers to the medulla and back again to the esophagus through glossopharyngeal and vagal efferent nerve fibers  Lower Esophageal Sphincter The esophageal circular muscle located at the lower end of the esophagus extending upward about 3 centimeters above its juncture with the stomach functions as a broad lower esophageal sphincter Remains tonically constricted prevent significant reflux of stomach contents into the esophagus Receptive relaxation of LES when a peristaltic wave arrives at its level allows easy propulsion of the swallowed food into the stomach Achalasia the sphincter does not relax satisfactorily  Motor Functions of Stomach Slow emptying of the Storage of large chyme from the quantities of food until Mixing of this food stomach into the small the food can be with gastric secretions intestine processed in the until it forms a suitable rate for proper stomach, duodenum, semifluid mixture digestion and and lower intestinal chyme absorption by the tract small intestine Basic Gastric Anatomy  Storage Function of the Stomach Food forms concentric circles in the stomach the newest food lies closest to the esophagus When food stretches the stomach -> “vagovagal reflex” reduced tone in the muscular wall that bulges progressively accommodating greater and greater quantities of food 0.8 - 1.5 liters  Mixing and Propulsion of the Food in the Stomach Basic electrical rhythm digestive juices -> immediately into contact with stored food mixing waves -> move from mid to upper stomach to the antrum every 15 to 20 sec more intense in the antrum forcing the antral contents under high pressure toward the pylorus Retropulsion = antral contents squeezed upstream toward the body of the stomach Not small enough to pass through the pylorus Pylorus contracts as wave arrives Chyme the resulting mixture of food and secretions that passes in the small intestine murky semifluid or paste Hunger Contractions intense contractions when the stomach is empty for several hours rhythmical peristaltic contractions in the body of the stomach. may cause mild pain in the pit of the stomach begin 12 -24 hours after the last ingestion of food greatest intensity 3 - 4 days in starvation  Stomach Emptying Intense antral peristaltic contractions Pyloric pump Decreased constriction of the pyloric sphincter Allows passage of food particles mixed in the chyme to fluid consistency  Regulation of Stomach Emptying Gastric factors Duodenal factors Gastric wall distension Entero-gastric Nervous Reflexes Increases pyloric pump action Inhibit pyloric pump Increase pyloric sphincter tone Gastrin secreted by antral mucosa Influenced by Increases pyloric pump action Duodenal distention Irritated duodenal mucosa Acidity and osmolality of the chyme Breakdown products in the chyme Cholecystokinin  Movements of the Small Intestine Mixing contractions Induced by wall distension Segmentation of the small intestine Chain of sausages Chopping the chyme 2-3 times per minute Maximum frequency 12/min  Movements of the Small Intestine Propulsive movements Peristaltic waves stimulated by Gastro-enteric reflex Gastrin, CCK, insulin, motilin, serotonin Gastro-ileal reflex Secretin - glucagon inhibit intestinal motility Peristaltic rush powerful and rapid peristalsis after intense irritation of mucosa e.g. severe infection  Function of the Ileocecal Valve Prevent backflow of fecal contents from the colon Protrudes into the lumen Forcefully closed when excess pressure in the cecum Ileocecal sphincter thickened circular muscle several cm upstream from the ileocecal valve Mildly constricted Gastro-ileal reflex intensifies peristalsis in the ileum emptying of ileal contents into the cecum Facilitates absorption 1,5 – 2 liters/d empty into the cecum  Principals Functions of the Colon Absorption of Storage of water and fecal matter electrolytes from until it can the chyme to be expelled form solid feces  Movements of the Colon Mixing Movements – Haustrations Baglike sacs due to combined contractions of the circular and longitudinal strips (teniae coli) of muscle peak intensity in 30 sec, disappears during the next 60 sec minor amount of forward propulsion material is exposed to the mucosal surface of the large intestine, and fluid and dissolved substances are progressively absorbed 80 - 200 ml of feces expelled / day  Movements of the Colon Propulsive Movements – Mass Movements 8 - 15 hours: ileocecal valve -> rectum Mass movements responsible for further propulsion 1-3 times/days 15 minutes during the first hour after breakfast constrictive ring occurs in response to a distended or irritated point in the colon 20 or more centimeters of colon distal to the constrictive ring contract as a unit “en masse” propulsion in this segment persist for 10 - 30 min Gastrocolic Reflex Duodenocolic Reflexes Irritation  Defecation Reflexes Intrinsic Reflex Initiated when feces enter the rectum Spread through the myenteric plexus Initiate peristaltic waves in the descending colon, sigmoid, and rectum -> anus -> internal anal sphincter relaxation Voluntarily relaxation of external anal -> defecation  Defecation Reflexes Parasympathetic Defecation Reflex assists internal reflex sacral segments of the spinal cord intensify the peristaltic waves relax the internal anal sphincter effective in emptying the colon from splenic flexure to anus