General Principles and Functions of the Alimentary Tract PDF

General Principles and Functions of the Alimentary Tract YBOD200 Dr Simo Zulu Email: [email protected]  Learning Objectives 1.Discuss the general principles of gastrointestinal motility. 2.Define and explain the enteric nervous system. 3.Discuss the...

General Principles and Functions of the Alimentary Tract YBOD200 Dr Simo Zulu Email: [email protected]  Learning Objectives 1.Discuss the general principles of gastrointestinal motility. 2.Define and explain the enteric nervous system. 3.Discuss the hormonal control of gastrointestinal motility. 4.Compare and contrast neural and hormonal control over the movements and secretions of the various digestive organs. 5.Name and discuss the functional types of movements in the gastrointestinal tract. 6.Define and discuss splanchnic circulation. Gastrointestinal System Gastrointestinal system involves multiple coordinated Secretory, Enzymatic and Motor processes.  Gastrointestinal (GIT) Wall Serosa (adventitia) - Squamous mesothelial cells that secrete a viscous fluid. Smooth muscle - Longitudinal and Circular layer. Submucosa - Submucosal plexus. Mucosa - Epithelium, lamina propria, and muscularis mucosae.  Gastrointestinal Smooth Muscle Functions 1. Longitudinal muscle layer - Extend longitudinally down the intestinal tract. 2. Circular muscle layer - Extend around the gut. Smooth Muscle fibers are connected through gap junctions.(review Smooth Muscle Physiology) Muscle layers functions as a Syncytium. Longitudinal and Circular smooth muscle control propulsion and mixing of the chyme.  Electrical Activity of Gastrointestinal Smooth Muscle GIT smooth muscles are continuously excited by Slow Intrinsic Electrical Stimuli. Basic types of electrical waves: Slow waves Spikes Potential Slow waves are not action potentials but “rise and fall” in smooth muscle RMP.  Interstitial Cells of Cajal Interstitial cells of Cajal - Act as electrical pacemakers for smooth muscle cells and are thought to generate slow waves. Interstitial cells of Cajal (ICC) ion channels periodically open and generate inward currents that produces a slow wave.  Spike Potentials GIT Smooth muscle RMP (−50 and −60 mV). Spike potentials are triggered if membrane potential rises above −40 mV. Higher slow wave potential = high frequency of the spike potentials. Picture sourced: Netter's Essential Physiology; Susan Mulroney; Adam Myers  Gastrointestinal Smooth Muscle Rhythmic Contraction GIT contractions occur Rhythmically. Contraction rhythm is determined mainly by the frequency of “slow waves” of smooth muscle membrane potential.  GIT Smooth Muscle Contraction Smooth muscle contraction occurs in response to Ca2+ influx. Slow waves do not trigger Ca2+ influx but only cause Na+ influx. ▪ Therefore, the slow waves by themselves do not cause muscle contraction. Spike Potential Stimuli  QUIZ In the presence of drug X, application of ACh to a bundle of gastric muscle cells causes membrane action potentials, an increase followed by a decrease in intracellular free calcium, but no contractile response. Drug X most likely is inhibiting which step in the contraction-relaxation process? a. Activation of myosin light chain kinase b. Binding of ACh with membrane receptors c. Opening of membrane calcium channels d. Opening of sarcoplasmic reticulum calcium channels Smooth Muscle Contraction Enteric Nervous System  Gastrointestinal Tract Intrinsic Nervous System GIT intrinsic nervous system called the Enteric Nervous System. ▪ Beginning in the oesophagus and ending at the anus. ▪ Controls GIT movements (motility) and Secretions.  Enteric Nervous System 1. Myenteric plexus (Auerbach’s plexus) ▪ Outer plexus lying between the longitudinal and circular muscle layers. ▪ Controls GIT movements. 2. Submucosal plexus (Meissner’s plexus) ▪ Inner plexus and lies in the submucosa. ▪ Controls gastrointestinal secretion and local blood flow.  Enteric Nervous System Connected to the autonomic systems but function independently.  Parasympathetic Control of the Gastrointestinal Tract Parasympathetic system (PNS) to GIT has 2 divisions. Cranial division Sacral division Sacral parasympathetics function - execute the defecation reflexes. PNS secrete Acetylcholine → Excites enteric nervous system → Increase GIT activity.  Sympathetic Control of the Gastrointestinal Tract SNS secrete mainly Norepinephrine. Stimulation inhibits GIT activity and enteric nervous system. Strong stimulation of the SNS inhibit GIT movements → Impeding movement of food through the GIT.  GIT Sensory Stimulation Stimulation: ▪ Irritation of the gut mucosa ▪ Excessive gut distention ▪ Chemical substances in the gut. Sensory signals - excitatory or inhibitory on intestinal movements or intestinal secretion.  QUIZ Stimulation of an intrinsic nerve in the intestine causes contraction of an intestinal muscle cell through the release of which neurotransmitter? a. Acetylcholine (ACh) b. Nitric oxide (NO) c. Norepinephrined d. Somatostatine Gastrointestinal Reflexes  Gastrointestinal Reflexes Reflexes integrated with the enteric nervous system and gut wall. Control GIT: ▪ Secretion ▪ Peristalsis ▪ Mixing contractions ▪ Local inhibitory effects  GIT and Sympathetic Ganglia Reflexes Gastrocolic Reflex ▪ Signals from the stomach stimulating evacuation of the colon. ▪ Distention of the stomach by food increases → Increase motility of the colon. ▪ Responsible for the urge to defecate following a meal.  GIT and Sympathetic Ganglia Reflexes Enterogastric Reflexes ▪ Signals from the colon and small intestine to inhibit stomach motility and stomach secretion. Colonoileal Reflex ▪ Inhibit emptying of ileal contents into the colon.  GIT and Parasympathetic Reflexes Vagovagal Reflex ▪ Vagus nerve signals from the stomach and duodenum to the brain stem and back to the stomach. ▪ Control gastric motor and secretory activity.  GIT and Parasympathetic Reflexes Defecation reflexes ▪ Signals from the colon and rectum to the spinal cord and back again to produce the powerful colonic, rectal, and abdominal contractions required for defecation.  Vomiting Reflex Sensory signals that initiate vomiting originate from upper GIT. (Pharynx, Oesophagus, Stomach, and Upper portions of the small intestines) Motor signals from the vomiting center trigger the actual vomiting. Drugs e.g., Morphine can directly stimulate Chemoreceptor trigger zone of the vomiting center. Vomiting Reflex  Hormonal Control of Gastrointestinal Motility  Hormonal Control of Gastrointestinal Motility Hormones control GIT secretion and motility. Hormones act on target cells with specific receptors for the hormone.  Gastrin Gastrin is secreted by the “G” cells. Site of secretion ▪ Antrum of the stomach, duodenum, and jejunum Action: ▪ Stimulate gastric acid secretion. ▪ Growth of the gastric mucosa.  Gastrin Gastrin is secreted in response to: ▪ Ingestion of a meal ▪ Stomach distention ▪ Products of proteins  Cholecystokinin (CCK) Cholecystokinin (CCK) is secreted by “I” cells. Site of secretion ▪ Mucosa of the duodenum, jejunum and ileum. Stimuli ▪ Fat, acids, and proteins in the intestinal contents.  Cholecystokinin (CCK) CCK Action ▪ Pancreatic enzyme and bicarbonate secretion ▪ CCK stimulate gallbladder to contract and release bile into the small intestine. ▪ Bile emulsify fatty substances – fat digestion and absorption.  Cholecystokinin (CCK) CCK slows gastric emptying to allow adequate time for fat digestion in the upper intestinal tract. CCK inhibits appetite. Stimulate sensory nerve in the duodenum to send signal (vagus nerve) to inhibit feeding centers in the brain.  Secretin Secretin is the 1st gastrointestinal hormone to be discovered. Secretin is secreted by the “S” cells. Site of secretion: Mucosa of the small intestine. Stimuli ▪ Fats and acid - Gastric juice emptying into the duodenum from the stomach.  Secretin Action Stimulate pepsin secretion. Pancreatic and Biliary secretion of bicarbonate to neutralize acid in the small intestine. Inhibitory gastrin release and gastric acid secretion.  Gastric Inhibitory Peptide (GIP) GIP is called Glucose-dependent insulinotropic peptide secreted by K cells. Site of secretion - Duodenum and Jejunum Stimuli ▪ Fatty acids and amino acids (response less to carbohydrate).  Gastric Inhibitory Peptide (GIP) Slows emptying of gastric contents into the duodenum when the upper small intestine is already overloaded with food. Stimulates insulin release Inhibit gastric acid secretion  Motilin Motilin is secreted by the stomach and upper duodenum (Mo cells) during FASTING. Site of secretion: duodenum and jejunum. The only known function motilin is to increase gastrointestinal motility.  Motilin Released periodically and stimulates waves of gastrointestinal motility called Interdigestive Myoelectric Complexes. Interdigestive myoelectric complexes occur in the stomach and small intestine every 90 minutes during fasting.  QUIZ Intravenous injection of a hormone initiates a phase of intense sequential contractions of the proximal duodenum that appears to migrate slowly toward the cecum. Which hormone was most likely injected? a. CCK b. Gastrin c. Motilin d. Secretin Summary of GIT Hormonal Control Picture sourced: Netter's Essential Physiology; Susan Mulroney; Adam Myers Functional Movements in the Gastrointestinal Tract  Functional Movements in the Gastrointestinal Tract Two types of GIT digestive movements: Propulsive movements ▪ Move food forward along the tract and rate of propulsion allow digestion and absorption. Mixing movements ▪ Keep the intestinal contents until thoroughly mixed.  Propulsive Movements Propulsive movements — Peristalsis. Peristalsis mechanism: A contractile ring is formed by the circular muscle around the gut and then moves forward.  Peristalsis Mechanism Law of the Gut” ▪ Peristaltic waves move toward the anus - Receptive Relaxation. Peristalsis can “theoretically” occur in either direction from a stimulated point. Peristalsis toward the mouth is inhibited.  Mixing Movements Mixing Movements - Segmentation Contraction Intermittent segmentation contractions lasting between 5 to 30 seconds occur in the gut wall. Splanchnic Circulation  Splanchnic Circulation Splanchnic circulation includes blood flow through the gut, spleen, pancreas, and liver. Blood passes through liver sinusoids (lined by Reticuloendothelial cells.  Reticuloendothelial cells Remove bacteria and preventing direct transport of potentially harmful agents into the body. Fats are absorbed into the intestinal lymphatics not carried in the portal blood.  Blood flow during GIT Activity Vasodilator substances are released from the mucosa of the intestinal tract. ▪ Vasodilator substances - cholecystokinin, vasoactive intestinal peptide, gastrin, and secretin. Gastrointestinal glands also release powerful vasodilating kinins - kallidin and bradykinin.  Blood flow during GIT Activity Decreased O2 concentration in the gut wall also increase intestinal blood flow. Increased mucosal and gut wall metabolic rate lowers the O2 concentration → increase intestinal blood flow.  Gastrointestinal Disorders Diarrhea - Chyme and fecal matter move too quickly through the colon. Irritation of the lower GI tract e.g., bacteria or virus infection of inflammatory diseases. Cholera is disease caused by the bacterium Vibrio cholerae. Chronic diarrhea results in dehydration and loss of HCO3− → METABOLIC ACIDOSIS.  Gastrointestinal Disorders Congenital absence of the myenteric plexus ▪ Weakness or absence of peristalsis in affect portion of the GIT. Atropine (Muscarinic antagonist) ▪ Paralyze the myenteric plexus cholinergic nerve endings → Weak or blocked peristalsis in the entire gut.  Gastrointestinal Disorders Vomiting (emesis) - Reverse peristalsis Cause loss of H+ and K+ (from stomach acid). Chronic vomiting → Dehydration + Metabolic alkalosis + Hypokalemia.