Gastrointestinal physiology 1 M.Bayat Ph.D GI tract motility Gastrointestinal Smooth Muscle Functions as a Syncytium. Cajal cell Slow Waves. Most gastrointestinal contractions occur rhythmically, and this rhythm is determined mainly by the frequency of so-called slow waves. These waves, are not action potentials. Instead, they are slow, undulating changes in the resting membrane potential. The precise cause of the slow waves is not completely understood, although they appear to be caused called the interstitial cells of Cajal, that are believed to act as electrical pacemakers for smooth muscle cells. Slow wave frequency Their intensity usually varies between 5 and 15 millivolts, and their frequent gastrointestinal tract from 3 to 12 per minute: ranges in different parts of the human 3-5 in the body of the stomach, 12 in the duodenum, 8 or 9 in the terminal ileum. 6-8 in the colon Therefore, the rhythm of contraction of the body of the stomach usually is about 3 per minute, of the duodenum about 12 per minute, and of the ileum 8 to 9 per minute. 1-10 /sec AP in Nerve fiber=0.3 ms Skeletal muscle=10-15 ms Smooth muscle spike=10-20 ms Cardiac muscle= ms Myosin light chain kinas phosphatase Enteric nervous system Enteric Nervous System Auerbachs plexus Meissners plexus M3 Contraction of GI wall 22 relaxation of GI wall T5-L2 To GIT M3 receptor Indirectly by ENS 11 22 Directly innervates vessels, mucosal muscle, sphincter Directly has contraction effect indirectly has relaxation effect by ENS Gastrointestinal Reflexes 1. Reflexes that are integrated entirely within the gut wall enteric nervous system. These include reflexes that control much gastrointestinal secretion, peristalsis, mixing contractions, local inhibitory effects, 2. Reflexes from the gut to the paravertebral sympathetic ganglia and then back to the gastrointestinal tract. The gastrocolic reflex The enterogastric reflexes The colonoileal reflex 3. Reflexes from the gut to the spinal cord or brain stem and then back to the gastrointestinal tract. Reflexes from the stomach and duodenum to the brain stem and back to the stomach Pain reflexes that cause general inhibition Defecation reflexes 3. MMC, Migrating motor complex 20 Peristaltic Reflex and the Law of the Gut. contractile ring causing the peristalsis normally begins on the orad side of the distended segment and moves toward the distended segment, pushing the intestinal contents in the anal direction for 5 to 10 centimeters before dying out. At the same time, the gut sometimes relaxes several centimeters downstream toward the anus, which is called receptive relaxation, thus allowing the food to be propelled more easily anally than orad. This complex pattern does not occur in the absence of the myenteric plexus. Therefore, the complex is called the myenteric reflex or the peristaltic reflex. 21 Peristaltic movement 22 23 In large intestine hustration and mass movement 24 25 Effectual peristalsis requires an active myenteric plexus Mixing Movements In some areas, the peristaltic contractions themselves cause most of the mixing (blocked by a sphincter) local intermittent constrictive contractions These constrictions usually last only 5 to 30 seconds; then new constrictions occur at other points in the gut 26 Retropulsion 27 28 29 The maximum frequency of the segmentation contractions in the small intestine is determined by the frequency of electrical slow waves maximum frequency of the segmentation contractions in duodenum is also about 12 per minute, these contractions are not effective without background excitation by parasympathetic 30 Movements of the Colon mixing movements Haustrations propulsive movements mass 31 32 the ileocecal sphincter thickened circular muscle called the ileocecal sphincter. slows emptying of ileal contents into the cecum. Normally, only 1500 to 2000 milliliters of chyme empty into the cecum each day. Gastroileal reflex? 33 Initiation of Mass Movements by Gastrocolic and Duodenocolic Reflexes. Appearance of mass movements after meals is facilitated by gastrocolic and duodenocolic reflexes. distention of the stomach and duodenum Need extrinsic autonomic nerves Irritation in the colon can also initiate intense mass movements(ulcerative colitis) 34 Mixing MovementsHaustrations. same manner that segmentation movements the circular muscle contracts, At the same time, the longitudinal muscle of the colon, which is aggregated into three longitudinal strips called the teniae coli, contracts. move slowly toward the anus The combination of constricted and relaxed portions of the colon create haustrations (bag-like pouches). Haustrations mix the chyme or fecal material and provide slow persistent forward movement. 35 A mass movement is a modified type of peristalsis First, a constrictive ring occurs in response to a distended or irritated point in the colon Then, rapidly, the 20 or more centimeters of colon distal to the constrictive ring lose their haustrations and instead contract as a unit, propelling the fecal material in this segment en masse further down the colon. 36 Propulsive MovementsMass Movements. Much of the propulsion in the cecum and ascending colon results from the slow but persistent haustral contractions From the transvers colon to the sigmoid, mass movements one to three times each day, in many people especially for about 15 minutes during the first hour after eating breakfast. Occasionally (1-3 times/day), a modified contraction will propel the contents forward en masse. This occurs by a peristaltic contraction pushing the contents through 20 cm or more of colon in which the haustrations are relaxed. Irritation can cause mass movements 37 Defecation Most of the time, the rectum is empty of feces. When a mass movement forces feces into the rectum, the desire for defecation occurs immediately, 38 Smooth muscle Striated muscle Migrating motor complexes (MMCs). Migrating motor complexes (MMCs). Note that the complexes move down the gastrointestinal tract at a regular rate during fasting, that they are completely inhibited by a meal, and that they resume 90120 minutes after the meal min 25 min 10 min 39 1.MMC are waves of activity in a regular cycle during fasting state. 2.MMC facilitate transportation of indigestible substances such as bone, fiber, and foreign bodies from the stomach, through the small intestine, past the ileocecal sphincter, and into the colon.stomachsmall intestineileocecal sphinctercolon 3.MMC originates in the stomach during the interdigestive phase (between meals) and is responsible for the rumbling experienced when hungry.interdigestive phaserumbling 4.MMC is thought to be partially regulated by motilin, which is initiated in the stomach as a response to vagal stimulation, and does not directly depend on extrinsic nervesmotilinvagal 5.MMC could relax and open pyloric sphincter to 3-4 cm 6.Morphine, has been found at relatively low doses to stimulate phase III of the migrating motor complex.Morphine 7.serves to transport bacteria from the small intestine to the large intestine, and to inhibit the migration of colonic bacteria into the terminal ileum.bacteriasmall intestinelarge intestine Migrating motor complexes (or migrating myoelectric complex or migratory motor complex or migratory myoelectric complex or "magical muscle contraction" or MMC) 40 41 Trimebutine is one drug that as part of its therapeutic action causes a premature activation of phase III of the migrating motor complex in the digestive tract. It also serves to transport bacteria from the small intestine to the large intestine, and to inhibit the migration of colonic bacteria into the terminal ileum.bacteriasmall intestinelarge intestine swallowing Therefore, even after paralysis of the brain stem swallowing reflex, food fed by tube or in some other way into the esophagus still passes readily into the stomach. 42 sensory 5,9 motor nerve 5,9,10,12 striated muscle. smooth muscle lower two thirds Skeletal motor Nerve in 9,10 cranial nerve Parasympatic Vagus 43 44 swallowing 45 Voluntary oral stage 46 47 48 The actions of Swallowing center in pharyngeal stage 1.Soft palate 2.Palatopharyngeal fold 3.Epiglotis 4.Vocal cord 5.Larynx 6.UES 7.Pharyngeal muscle 8.Respiration 49 Swallowing center sensory 5,9 motor nerve 5,9,10,12 50 51 Distention of the esophagus by the moving bolus initiates another wave called secondary peristalsis. Frequently, repetitive secondary peristalsis is required to clear the esophagus of the bolus. 52