GI Tract Motility - PDF Lecture Notes

GI TRACT MOTILITY OBJECTIVES 1. Compare and contrast the properties of skeletal and smooth muscle. 2. Describe the coordinated series of events that move material from the mouth to the stomach. 3. Identify the factors that initiate and control stomach motilit...

GI TRACT MOTILITY OBJECTIVES 1. Compare and contrast the properties of skeletal and smooth muscle. 2. Describe the coordinated series of events that move material from the mouth to the stomach. 3. Identify the factors that initiate and control stomach motility and gastric emptying. 4. Explain how ingested material is mechanically broken down and moved along the small intestine. 5. Describe the sequence of events in the large intestine that lead to defecation. 1. SKELETAL AND SMOOTH MUSCLE Muscle classified as skeletal, cardiac and smooth … 1. Skeletal / cardiac muscle termed striated … due to arrangement of thick (myosin) and thin (actin) filaments i. One unit of thick/thin filaments … sarcomere 1. Intestinal lining very susceptible to damage by treatments that 1. SKELETAL AND SMOOTH MUSCLE Muscle classified as skeletal, cardiac and smooth … 1. Smooth muscle also possesses thick and thin filaments but does not have the striations … i. Thin filaments anchored either to the plasma membrane or to cytoplasmic dense bodies ii. Thick/thin filaments oriented diagonally to the long axis of the cell iii. Filament contraction (sliding filament mechanism as in striated muscle) leads to shortening-ballooning of the cell 1. Intestinal lining very susceptible to damage 1. SKELETAL AND SMOOTH MUSCLE Muscle classified as skeletal, cardiac and smooth … 2. Skeletal muscle cross-bridge formation Ca2+-dependent … i. Low cytosolic Ca2+.. Tropomyosin covers the myosin binding site on thin filament actin ii. High cytosolic Ca2+.. Ca2+ binds to troponin.. tropomyosin moves and uncovers myosin binding site.. iii. Cross-bridge forms.. muscle contracts … (a) Low cytosolic Ca2+, relaxed muscle (b) High cytosolic Ca2+, activated muscle Cross-bridge binding Tropomyosin Troponin Ca2+ sites are exposed Actin Actin binding site Cross-bridge binds Energized cross-bridge to actin and cannot bind to actin generates force 1. SKELETAL AND SMOOTH MUSCLE Muscle classified as skeletal, cardiac and smooth … 2. Smooth muscle cross-bridge formation also Ca2+-dependent but … i. ↑ cytosolic Ca2+ → ↑ Ca2+-calmodulin complex activity which …. ii. ↑ myosin light-chain kinase activity which …. iii. ↑ phosphorylation of myosin leading to cross-bridge formation → contraction iv. Relaxation dependent on myosin dephosphorylation by myosin light-chain phosphatase 2. Inhibits the release of gastrin and histamine 1. SKELETAL AND SMOOTH MUSCLE Muscle classified as skeletal, cardiac and smooth … 3. Skeletal muscle Ca2+ source primarily the sarcoplasmic reticulum i. Release initiated by motor neurons.. every muscle fiber innervated ii. ↑ activity → ↑ acetylcholine release → ↑ muscle cell depolarization … → t-tubule dihydropyridine (DHP) receptor activation → → ryanodine receptor activation → ↑ cytosolic Ca2+ 2. Inhibits the release of gastrin and histamine 1. SKELETAL AND SMOOTH MUSCLE Muscle classified as skeletal, cardiac and smooth … 3. Smooth muscle Ca2+ source … the sarcoplasmic reticulum and extracellular Ca2+ i. No t-tubules.. SR Ca2+ released by plasma membrane depolarization and 2nd messengers ii. Extracellular Ca2+ enters via voltage-gated and ligand-gated Ca2+ channels iii. Contractility affected by multiple effectors 1. SKELETAL AND SMOOTH MUSCLE Muscle classified as skeletal, cardiac and smooth … 3. Smooth muscle Ca2+ source … the sarcoplasmic reticulum and extracellular Ca2+ i. No t-tubules.. SR Ca2+ released by plasma membrane depolarization and 2nd messengers ii. Extracellular Ca2+ enters via voltage-gated and ligand-gated Ca2+ channels iii. Contractility affected by multiple effectors iv. Single-unit smooth muscle.. all muscle cells undergo synchronous activity … Action potentials triggered in some cells transmitted to all others via gap junctions Autonomic nerve fiber Varicosities Gap junctions 2. GI TRACT MOTILITY … MOUTH, PHARYNX AND ESOPHAGUS 1. Chewing … i. Controlled by somatic nerves to the skeletal muscles of the mouth and jaw (voluntary) and ii. Mechanoreceptors sense pressure of food against gums/hard palate → inhibit muscles holding the jaw closed → ↓ pressure on mechanoreceptors → new cycle of contraction ….. iii. Saliva … i. Secreted by three pairs of salivary glands ii. Secretion ↑ due to food 1) sight/smell 2) taste (chemoreceptors) 3) volume (mechanoreceptors) iii. ↑ secretion due to ↑ blood flow to glands caused by ↑ parasympathetic nerve activity 2. Swallowing (deglutition)… i. Food / liquid pushed to the rear of the mouth by the tongue activates pressure receptors in the walls of the pharynx … sends afferent impulses to the swallowing center in the medulla oblongata and … i. As food passes to pharynx, the soft palate elevates.. Prevents food entering nasal cavity 2. Swallowing (deglutition)… i. Food / liquid pushed to the rear of the mouth by the tongue activates pressure receptors in the walls of the pharynx … sends afferent impulses to the swallowing center in the medulla oblongata and … i. As food passes to pharynx, the soft palate elevates.. Prevents food entering nasal cavity ii. Swallowing center impulses.. 1) inhibit respiration.. 2) close the glottis iii. As food passes further into the pharynx, the epiglottis tilts back to cover the glottis.. Prevents aspiration of food into the trachea iv. The upper esophageal sphincter relaxes … food descends into the esophagus 3. Swallowing … the esophageal phase … i. Both ends of the esophagus normally closed by sphincters … i. Upper esophageal sphincter... skeletal muscle ii. Lower esophageal sphincter... smooth muscle ii. With upper esophageal sphincter relaxation … food descends into upper esophagus and … 1) the sphincter closes.. 2) the glottis opens.. 3) breathing resumes Upper esophageal sphincter Lower esophageal sphincter Trachea Esophagus Diaphragm Stomach 3. Swallowing … the esophageal phase … iii. Food moved toward stomach by progressive wave of muscle contraction.. peristaltic waves (one wave takes 9-10 sec to move from upper esophageal sphincter → stomach) iv. If a large food bolus does not reach the stomach … Esophageal mechanoreceptors → afferent signals to swallowing center → → secondary peristalsis v. Lower esophageal sphincter remains relaxed during swallowing Upper esophageal sphincter Lower esophageal sphincter Trachea Esophagus Diaphragm Stomach 3. Swallowing … the esophageal phase … vi. Between meals … closure of the lower esophageal sphincter prevents acidic stomach contents from entering the esophagus.. gastroesophageal reflux.. heartburn Reflux often occurs in pregnancy due to … i. Increased abdominal pressure caused by the fetus which also … ii. Pushes terminal portion of the esophagus through the diaphragm into the thoracic cavity Large meals can also cause reflux Upper esophageal sphincter Lower esophageal sphincter Trachea Esophagus Diaphragm Stomach 3. GI TRACT MOTILITY … THE STOMACH i. Two primary regions of the stomach.. the body and the antrum (thicker smooth muscle.. mixes contents) ii. Volume of the empty stomach = 50 mL; can increase to 1.5 L with ingestion of a meal …. iii. Little increase in internal pressure due to relaxation of the smooth muscle in the body and antrum before arrival of food.. receptive relaxation iv. Mediated by nitic oxide and serotonin released from enteric neurons. 3. GI TRACT MOTILITY … THE STOMACH With arrival of food … i. Stomach produces peristaltic waves of muscle contraction … body → antrum ii. Antral contractions much stronger resulting in … i. Closure of the pyloric sphincter ii. Stomach contents mixed and forced back toward the body of the stomach … retropulsion 2. Inhibits the release of gastrin and histamine 3. GI TRACT MOTILITY … THE STOMACH The peristaltic waves … i. Rhythm (3 /minute) generated by pacemaker cells in longitudinal muscle layer Spontaneous depolarization/repolarization cycles (slow waves) … This basic electrical rhythm conducted through the muscle layer via gap junctions ii. These basal depolarizations are too small to cause significant contractions but … i. Excitatory neurotransmitters and/or hormones cause greater depolarization resulting in.. ii. Generation of action potentials … the more APs the greater the contraction Membrane potential (mV) Action potentials Threshold potential Slow Slowwaves waves –60 Membrane depolarization Time Smooth muscle tension 0 Time 3. GI TRACT MOTILITY … THE STOMACH iii. Action potentials generated by … i. Gastrin ii. Short and long neural reflexes triggered by stomach distension (the vagovagal reflex) 2. Inhibits the release of gastrin and histamine Membrane potential (mV) Action potentials Threshold potential Slow Slowwaves waves –60 Membrane depolarization Time Smooth muscle tension 0 Time 4. REGULATION OF GASTRIC EMPTYING i. In the duodenum.. distension, ↑ acidity, fat*, amino acids, hypertonic solutions decrease gastric emptying via … i. Short and long neural reflexes ii. Increased secretion and therefore plasma levels of enterogastrones (secretin, CCK) ii. The CNS can regulate motility independent of reflexes … ↑ parasympathetic activity → ↑ motility ↑ sympathetic activity → ↓ motility 5. GI TRACT MOTILITY … THE SMALL INTESTINE i. During the digestive phase no peristaltic waves in the small intestine rather.. i. Periods of contraction-relaxation of short segments … segmentation Ensures mixing of the chyme ii. Segmenting movement initiated by pacemaker cells.. Interstitial cells of Cajal (located in the circular smooth muscle) iii. Contraction frequency decreases along small intestine (duodenum 12 /min; ileum 9/min) Promotes slow movement of the lumen contents toward the large intestine 5. GI TRACT MOTILITY … THE SMALL INTESTINE ii. Site-selective reflexes regulate contractile activity … i. The gastroileal reflex.. ↑ gastric emptying → ↑ segmentation intensity in the ileum ii. The intestino-intestinal reflex.. ↑↑ intestine distension/wall injury/bacterial infections → stop motility the release of gastrin and histamine 5. GI TRACT MOTILITY … THE SMALL INTESTINE iii. After absorption of the meal … i. Segmentation stops and replaced by peristaltic activity ….. migrating myoelectrical complex (MMC) ii. Begins in the distal stomach … repeated waves travel ~ 2 feet … die out iii. Next wave begins a little further down … takes ~ 2 hours to reach the large intestine iv. The MMC … i. Moves any undigested material in the small intestine into the large intestine ii. Prevents colonization of bacteria in the small intestine iii. Initiated by the intestinal hormone motilin Wave 1 Wave 2 Wave 3 Wave 3 6. GI TRACT MOTILITY …THE LARGE INTESTINE i. The large intestine … i. Approximately 5 feet long with three distinct segments … 1) cecum.. 2) colon (ascending, transverse and descending).. 3) rectum. ii. Separated from the small intestine by the iliocecal valve (sphincter) iii. Absorbs most of the 1500 mL of (primarily liquid) chyme delivered each day iv. Concentrates fecal material prior to defecation v. Contains bacteria which digest undigested polysaccharides to short-chain fatty acids 6. GI TRACT MOTILITY …THE LARGE INTESTINE i. In the large intestine… i. Segmentation much slower than in the small intestine (one every 30 min) ii. 3-4 times / day.. intense wave of contraction from transverse segment → rectum.. mass movement iii. Terminus of rectum.. the anus.. normally closed by.. i. Internal anal sphincter (smooth muscle) ii. External anal sphincter (skeletal muscle.. therefore, under voluntary control) 6. GI TRACT MOTILITY …THE LARGE INTESTINE i. In the large intestine… iv. Distension of the rectum walls by mass movement of fecal material … initiates the defecation reflex with.. i. Contraction of the rectum ii. Relaxation of the internal AS and initial contraction of the external AS iii. Increased motility of the sigmoid colon v. Eventually increased internal rectal pressure triggers reflex relaxation of external AS.. Leading to defecation 6. GI TRACT MOTILITY …THE LARGE INTESTINE i. In the large intestine… vi. Reflex relaxation of the external AS can be overridden.. voluntary control vii.Sustained distension of the rectum initiates a reverse movement pushing feces back into the colon and the urge to defecate subsides until the next mass movement viii. Defecation assisted by … i. Deep breath ii. Contraction of abdominal and thoracic muscles … Valsalva maneuver 5. GI TRACT MOTILITY END