GI Motility 2023-1 Student Version PDF

Carla Romo Jaramillo MD Anesthesiology / Critical Care MASTER LECTURE 2023-1 GASTROINTESTINAL MOTILITY OBJECTIVES • Definition of motility. • Describe the mechanism of contraction of the GI tract. • Understand the function and components of chewing. • Differentiate the phases of swallowing. • Des...

Carla Romo Jaramillo MD Anesthesiology / Critical Care MASTER LECTURE 2023-1 GASTROINTESTINAL MOTILITY OBJECTIVES • Definition of motility. • Describe the mechanism of contraction of the GI tract. • Understand the function and components of chewing. • Differentiate the phases of swallowing. • Describe and compare the characteristics of the motility in the esophagus, stomach, small and large intestine. • Define mass movements. • Describe the process of defecation. MOTILITY - DEFINITION • Refers to contraction and relaxation of the walls and sphincters of the GI tract. • Grinds, mixes and fragments ingested food to prepare it for digestion and absorption. • Propels the food along the GI tract. COMPOSITION • Striated muscle – pharynx, upper 1/3 of the esophagus and external anal sphincter. • Smooth muscle – almost all the contractile tissue of the GI tract. Unitary smooth muscle –gap junctions. • Circular – contraction – shortening of the ring – decreases the diameter. • Longitudial – contraction – shortening in the longitudinal direction – decreases the lenght. TYPES OF CONTRACTIONS • Phasic contractions: Periodic contractions followed by relaxation. Esophagus, gastric antrum, and small intestine (mixing and propulsion). • Tonic contractions: Constant level of contraction without relaxation. Orad region of the stomach and in the lower esophageal, ileocecal, and internal anal sphincters. SPHINCTERS Specialized regions of circular muscle that separate two adjacent regions. At rest → POSITIVE PRESSURE (prevent anterograde and retrograde flow) For contents to move it must relax and transiently lower its pressure. Changes in sphincter pressure are coordinated with contractions of the smooth muscle of the adjacent organs via reflexes (ex. Swallowing réflex). The GI Blues: Heartburn, Acid Reflux and Indigestion Byron Cryer, M.D. Dallas VA Medical Center REMEMBER: Slow waves are not action potentials but rather oscillating depolarization and repolarization of the membrane potential of the smooth muscle. CHARACTERISTICS OF SLOW WAVES • Frequency: Intrinsic rate varies along the GI tract, 3 (stomach) -12 (duodenum) per minute. • Origin: Interstitial cells of Cajal – low resistence Gap junctions → determinates the rate. Slow wave + Contraction mechanism: • Depolarizing: Opening of Ca2+ channels – inward (during the plateau of the slow wave) • Plateau: Still open Ca2+ channels. • Repolarizing: Opening of K+ channels – outward. No neural or hormonal influence *But neural and hormonal activity do modulate production of action potentials and the strength of contractions CHEWING • Functions: • Mixes food with saliva. • Reduce the size of food particles. • Amylase- CH digestion. • Components: • Voluntary. • Involuntary: Reflexes initiated by food in the mouth. • Sensory information – mechanoreceptors -> brain stem -> stimuli of muscles for chewing. SWALLOWING • Voluntarily + reflex control. Food in the mouth Somatosensory receptors near the pharynx Vagus and glossopharyngeal nerves Medullary swallowing center Motor output of the striated muscle of the pharynx and upper esophagus PHASES OF SWALLOWING • Oral: the tongue forces (voluntary) a bolus of food toward the pharynx. • Pharyngeal: pharynx – esophagus. 1.Soft palate is pulled upward. 2.Epiglottis cover the opening of the larynx. 3.UES relaxes -> food to the esophagus. 4.Peristaltic wave of contraction of the pharynx. • Esophageal motility. Swallowing initiates pharyngeal and esophageal peristalsis and sphincter relaxation • UES is controlled by swallowing center in the medulla (through V, IX, X, XII) • Stimuli: pharyngeal stimulation and distension of esophageal wall. • LES specialized smooth muscle cells. Relaxes after UES return to its resting pressure ESOPHAGEAL MOTILITY 1.UES opens (SR) -> once the bolus enters the esophagus -> UES closes. 2.Primary peristaltic contraction (SR) -> push the bolus along. 3.Opening of the LES (vagus nerve – VIP – relaxation) • Receptive relaxation. • As soon as the bolus enters the orad stomach, the LES contracts. 4.If 1ary contraction doesn’t clear the esophagus = 2dary contraction (ENS). • Starts at the point of distention. GASTRIC MOTILITY - COMPONENTS 1.Relaxation of the orad (receive the food bolus from the esophagus). 2.Contractions (reduce the size of the bolus and mix it with gastric secretions – initiate digestion). 3.Gastric emptying (propels chyme into the small intestine). FUNCTIONAL REGIONS OF THE STOMACH 19 CONTROL OF THE STOMACH • Parasympathetic stimulation and gastrin/motilin: Increase the frequency of action potentials and the force of gastric contractions. • Sympathetic stimulation and secretin/GIP: decrease the frequency of action potentials and the force of contractions. • During fasting – motilin – migrating myoelectric complexes (periodic gastric contractions every 90 minutes) -> clear the stomach. Jet-like retropulsion through the orifice of the antral contraction triturates solid particles PYLORIC PUMP GASTRIC EMPTYING • After a meal -> 1.5 L in the stomach of solids, liquids, and gastric secretions. • Emptying the stomach to the duodenum takes 3 hours. • The emptying must provide adequate time for neutralization of gastric H+ in the duodenum and adequate time for digestion and absorption. • To enter the duodenum, solids must be reduced to particles of 1 mm or less. • Slow or inhibit gastric emptying: fat (CCK) and H+ ions (ENS). SMALL INTESTINE MOTILITY • Serves to: • Mix the chyme with digestive enzymes and pancreatic secretions. • Expose the nutrients to the intestinal mucosa for absorption. • Propel the unabsorbed chyme along the small intestine into the large intestine. • Slow waves: • Duodenum: 12 waves per minute. • Ileum: 9 waves per minute. • Also have migrating myoelectric complexes. MYGRATING MYOELECTRIC COMPLEX (MMC ) 24 FACTORS THAT REGULATE MMC • ENTERIC NERVOUS SYSTEM • MOTILIN • Synthetized in the duodenal mucosa • Released just before phase III of the MMC 25 ANS – SMALL INTESTINE • Parasympathetic: Vagus nerve – increase contractions. • Sympathetic: Celiac and superior mesenteric ganglia – decreases contraction. PERISTALSIS • Food sensed by enterochromaffin-like cells of the intestinal mucosa. • Release serotonin -> binds to receptors on intrinsic primary afferent neurons -> initiate the peristaltic reflex in that segment of small intestine. • Behind the bolus cause excitation of circular muscle and inhibition of longitudinal muscle= the intestine narrows and lengthens. • In front of the bolus cause inhibition to circular muscle and excitation of the longitudinal and the intestine widens and shortens. EXAM LIKE QUESTION! All the following events are likely to occur during emesis EXCEPT one. Which one is the EXCEPTION? A.Antiperistalsis B.Opening of the lower esophageal sphincter C.A deep breath D.Contraction of the diaphragm E.Opening of the glottis VOMITING • Afferent information comes to the vomiting center from: the vestibular system, the back of the throat, the gastrointestinal tract and the fourth ventricle. • Vomiting reflex: • • • • • • • Abolition of the gastric and small intestinal slow wave activity. Reverse peristaltic contractions (intestine -> stomach). Forced inspiration to increase abdominal pressure. Larynx upward and forward. Relaxation of the LES. Closure of the glottis. Forceful expulsion of gastric/duodenal content. LARGE INTESTINAL MOTILITY • The contents of the large intestine (feces), are destined for excretion. • Small intestine -> cecum and proximal colon -> ileocecal sphincter contracts preventing reflux. • Fecal material then moves from the cecum through the colon, to the rectum, and on to the anal canal. Segmentation contractions occur in the cecum and proximal colon. In the large intestine, the contractions are associated with characteristic saclike segments called haustra. MASS MOVEMENTS • Move the contents of the large intestine over long distances. • 1 to 3 times per day. • Water absorption occurs in the distal colon -> semisolid and increasingly difficult to move. • A final mass movement propels the fecal contents into the rectum, where they are stored until defecation occurs. EXAM LIKE QUESTION! A 24 year old student consumes a meal consisting of 50% carbohydrates, 30% proteins, and 20% fats. The student feels the urge to defecate 20 minutes after consuming the meal. Which of the following best describes a direct action that promotes the urge to defecate in this student? A.Relaxation of the pylorus B.Relaxation of the duodenum C.Distention of the jejunum D.Distention of the rectal wall E.Contraction of the external sphincter DEFECATION • Rectum fills with feces -> the wall of the rectum contracts and the internal anal sphincter relaxes in the rectosphincteric reflex -> EAS contracted. • Once the rectum fills to 25% of its capacity, there is an urge to defecate. • The EAS is relaxed voluntarily and the smooth muscle of the rectum contracts to create pressure. SUMMARY • Motility is the contraction and relaxation of the walls and sphincters of the GI tract. • There are two types of contractions: tonic and phasic. • Chewing is the firts step in and is divided in voluntary and involuntary phases. • Swallowing has a voluntary part followed by the reflex control. • Esophagel motility starts with the opening of the UES, continues with a primary peristaltic contraction and the opening of the LES. SUMMARY • Gastric motility is composed by relaxation of the orad, contractions of the stomach and gastric emptying. • The small intestine motility mix the chyme with digestive enzymes and pancreatic secretions using segmentation and peristaltic contraction. • The enterochromaffin cells detect food and release serotonin. • Mass movements move the content of the large intestine over long distances. RECOMMENDED READINGS • Linda S. Costanzo (2018), 6th edition. Physiology, chapter 8, pages 348-356.