Digestive System Stomach Part 3 Gastric Motility PDF

Digestive System Part 3: Gastric Motility Dr Seley Gharanei, Assistant Professor Cell and Tissue Biomedicine [email protected] MB ChB Phase I Block 1 Health, Metabolism and Learning outcomes  Describe the movements of the stomach and how they are...

Digestive System Part 3: Gastric Motility Dr Seley Gharanei, Assistant Professor Cell and Tissue Biomedicine [email protected] MB ChB Phase I Block 1 Health, Metabolism and Learning outcomes  Describe the movements of the stomach and how they are controlled  Define gastric slow waves  Define action potentials  Describe the regulation of the pyloric sphincter in the passage of the contents of the stomach to the duodenum  Gastric Motility Allows the stomach to: 1. Serve as a reservoir for a large volume of food Receptive Relaxation 2. Fragment the food into smaller particles and mix it with gastric secretions Mixing 3. Empty gastric contents into the duodenum at a controlled rate Emptying Q1: What region of the stomach is called the Orad region and how is it different from  Gastric Motility 1. Receptive Relaxation The empty stomach has a volume of approx. 50 ml. In the empty state the stomach is contracted, and its mucosa and submucosa are highly folded Berne and Levy, Principles of Physiology (Mosby) into rugae. Q2: Which layer of the stomach wall facilitates the peristaltic Gastric Motility 1. Receptive Relaxation  When food is swallowed, the smooth muscle cells of the muscularis mucosae of the wall of the orad region of the stomach relax.  This enables the stomach to increase in volume to as much as 1.5 litres with little increase in pressure. Q3: What is the difference between the extrinsic and intrinsic innervation of the  Gastric Motility 1. Receptive Relaxation  Is mediated by the vagus nerve.  The vagus nerve co-ordinates with the stomach’s enteric nerve plexuses.  The enteric neurons release nitric oxide (NO) and serotonin, which mediate the relaxation of the smooth muscle cells of the muscularis mucosae. Q4: Where are the enteric Berne and Levy, Principles of Physiology (Mosby)  Gastric motility Allows the stomach to serve as a reservoir for a large volume of food  The predominant motor activity of the orad region is the accommodation of ingested Orad region food.  Because the muscle layers are thin, Caudad contractions of the orad region are normally region weak.  Gastric contents often remain in relatively undisturbed layers for an hour or more after eating. Q5: What is Bolus? Gastric Motility 2. Mixing  Gastric contractions (peristaltic waves) begin in the body of the stomach and moves towards the antrum.  As the wave approaches the antrum it causes a powerful contraction. The pyloric sphincter closes.  Most of the contents of the antrum are forced backwards into the body of the stomach. This is termed retropulsion. Peristaltic waves The muscularis, in most regions, consists of 2 layers of smooth muscle:  inner layer of circular muscle  outer layer of longitudinal muscle  Peristaltic waves are achieved by coordinated contractions and relaxations of the circular and longitudinal layers of smooth muscles in the Muscularis externa  Peristalsis  Waves of alternating contractions and relaxations of smooth muscle layers that mix and squeeze the contents through hollow tubes.  Move a bolus along the length of the GI tract. Human Anatomy 3rd Edition, Prentice Hall 2001 The muscularis of the stomach has three layers of muscle  In addition to the circular and longitudinal smooth muscle layers, the muscularis of the stomach includes an inner oblique smooth muscle layer:  This enables the stomach to contract in 3 directions, allowing it to grip food as it churns and grind it with digestive juices.  Electrophysiology of Gastrointestinal Smooth Muscle How are GI peristaltic waves generated?  Normal gastrointestinal motility results from coordinated contractions of smooth muscle.  The contractions derive from 2 basic patterns of electrical activity across the membranes of smooth muscle cells: slow waves and action potentials. For more P1 B0 CTB Plasma Membrane and Membrane information Transport on Greater detail in Block 2 membrane “Catch Up Biology” Chapter 16  Slow Waves  The smooth muscle cells undergo spontaneous cycles of depolarization and repolarization known as basic electrical rhythm of the GI tract or slow waves.  Slow wave frequency varies by region of the GI tract.  Gastric slow waves occur at a frequency of approx. 3 per minute.  The rhythm of gastric slow waves is generated in the pacemaker zone. Berne and Levy, Principles of Physiology (Mosby)  Slow waves do not elicit contractions. They coordinate contractions by controlling the appearance of action potentials. Q6: What are the pacemaker cells in the Action Potentials  Excitatory neurotransmitters and hormones further depolarize the membrane, thereby increasing the amplitude of the slow wave.  If the peak of a slow wave rises above a certain threshold, the cell fires one or more action potentials.  The intensity of the stimulus (e.g. the concentration of the hormone or neurotransmitter) determines number of action potentials fired at the peak of the slow wave. Action Potentials  Action potentials elicit muscle contraction.  The number of action potentials occurring at the peak of a slow wave determines the strength of the muscle contraction. Gastric Motility 3. Emptying  Gastric emptying is accomplished by the coordinated contractile activity of the stomach, pylorus and proximal small intestine.  Regulation of gastric emptying allows for optimal digestion and absorption of ingested material.  The Pyloric Sphincter  Is a ring of smooth muscle and connective tissue between the gastric antrum and the duodenum.  Allows the regulated emptying of gastric contents at a rate consistent with the ability of the duodenum to process chyme. Berne and Levy, Principles of Physiology (Mosby)  Prevents regurgitation of duodenal contents back into the stomach. Control of the Pyloric Sphincter By the autonomic nervous system:  Sympathetic nerve fibres increase the constriction of the sphincter.  Parasympathetic: Inhibitory vagal fibres relax the sphincter. Excitatory vagal fibres constrict the Bysphincter. hormones: Gastrin, cholecystokinin (CCK), gastric inhibitory peptide (GIP) and secretin all elicit constriction of the sphincter.  Gastric emptying is regulated in response to the nature of the  Materialsduodenal that are high incontents fat digestion products, highly acidic or very hypertonic all decrease the rate of gastric emptying.  Receptors on the mucosae of the duodenum and jejunum sense acidity, osmotic pressure, certain fats, amino acids and peptides and cause the release of intestinal hormones.  Intestinal hormones inhibit antral contractions and/or elicit constriction of the pyloric sphincter, thereby decreasing the rate of gastric emptying. Q7: What is a hypertonic  Summary  Gastric motility  slow waves  Action potentials  Control of the pyloric sphincter  Recommended Reading Clinical Key Student (https://www.clinicalkey.com/student): Physiology. Seventh Edition. Costanzo, L. S. 2022. Elsevier. Costanzo, Linda S., PhD, Physiology, Chapter 8, 339-393. Medical Sciences, Naish and Syndercombe Court, Chapter 15: The Alimentary System https://0-www-clinicalkey-com.pugwash.lib.warwick.ac.uk/student/ content/book/3-s2.0-B9780702073373000158 Sections Stomach and duodenum Gastric and duodenal musculature Gastric motility Gastric emptying  Answers Q1: What region of the stomach is called the Orad region and how is it different from the Caudat region? The regions covering cardia, fundus and the body of the stomach are called the orad regions. The smooth muscles in this are thin, therefore the contractions are weak, and food can stay in this area undisturbed for up to an hour. Whereas the smooth muscle in the caudat region are thick, allowing powerful contractions. Q2: Which layer of the stomach wall facilitates with the peristaltic movements? The muscularis layer of the wall is responsible for the peristaltic movements. Q3: What is the difference between the extrinsic and intrinsic innervation of the GI tract? The extrinsic innervations consists of the sympathetic and the parasympathetic divisions and the intrinsic innervations refer to the enteric nervous system of the GI tract that consists of the two nerve networks: the myenteric plexus and the submucosal plexus. Q4: Where are the enteric neurons/plexus located? The enteric neurons are located within the walls of the GI tract organs. The submucosal plexes are located in the submucosal layer and the myenteric plexus are located between the circular and the longitudinal smooth muscles of the muscularis layer. Q5: What is Bolus? Bolus is the masticated food passing through the oesophagus to the stomach. Q6: What are the pacemaker cells in the gut? The pacemaker cells in the gut are called Interstitial cells of Cajal (ICC). They have special properties that make them unique in their ability to generate and propagate slow waves in gastrointestinal muscles. The electrical slow wave activity determines the characteristic frequency of phasic contractions of the stomach, intestine and colon. Q7: What is a hypertonic material/solution?