Gastric Motility PDF

Section 5: Gastric Motility I. Gastric Motility (Simple Stomach) – Also applies to ruminant abomasum Motor functions of the stomach: 1. Reception of ingested food 2. Storage of contents (accomodation) 3. Mixing-grinding of contents 4. Controlled propulsion into duo...

Section 5: Gastric Motility I. Gastric Motility (Simple Stomach) – Also applies to ruminant abomasum Motor functions of the stomach: 1. Reception of ingested food 2. Storage of contents (accomodation) 3. Mixing-grinding of contents 4. Controlled propulsion into duodenum Gastric digestion: Formation of acid chyme (liquid of dissolved food +HCl+pepsin) Fig. 20. Ingesta formed into concentric spheres A. Gastric motor regions 1. The proximal stomach: Basic motility pattern is tonic not phasic -No peristalsis -Tonic contractions (domination by circular smooth muscle layer) 31  a. Motility and functions of the proximal stomach 1) Receive and store ingested food. Occurs by two processes: a) Receptive relaxation: Continuation of 1° or 2° esophageal peristalsis Swallowing Center (CNS) Vagus Activated inhibitory effector neurons b) Accommodation: Tension in the stomach wall can increase significantly without an increase in intragastric pressure. Gastric tension receptor → CNS → Vagus → Activated inhibitory effector neurons Fasting condition: Pressure = Wall tension/Radius of stomach = 8/4= 2 Filled condition: Pressure = Wall tension/Radius of stomach = 16/8= 2 2) Propulsion of food - Steady pressure toward antrum after filled. 2. Distal stomach: Basic motility pattern is peristalsis driven by a pacemaker in mid-region of stomach. a. Motility and functions of the distal stomach 1) Propulsion of content through antrum and emptying via pylorus 2) Mixing and grinding of gastric contents 3) Retropulsion of solid matter A. Propulsion: Peristaltic ring A B begins at mid-stomach Wave towards peristalsis B. Emptying: Peristaltic ring at Fig. 21 mid-antrum (pylorus open) C D C. Retropulsion: Peristaltic ring at dist. antrum (pylorus narrow) 32 C. Control of gastric motility: Fed Pattern (distal stomach) 1. Stimulatory Control: Stimulus = Gastric volume/presence of food *Stimulatory control of gastric secretion very similar Fig. 22. 3000 Gastric Volume (cm ) 3 2500 2000 1500 1000 500 0 0 2 4 6 8 10 12 Time (h) Rate of gastric emptying in 160-kg pony fed at 12-hour intervals. Stimulatory control of gastric motility/emptying mediated by: a. Neural activity Afferent pathway: Mechanoreceptors monitor gastric volume Chemoreceptors detect amino acids, pH, osmolarity Activation of: Enteric nervous afferents Interneurons ENS effectors Visceral afferents CNS Vagus parasympathetic ENS effectors Response: Increased gastric motility and emptying b. Hormonal: Gastrin Stimuli: Cholinergic neurons Gastrin releasing peptide (GRP) neurons- Responds to filling Amino acids (dietary, acid hydrolysis, pepsin) Increased antral pH Response: Increased gastric motility and emptying (action at smooth mm.) 33 2. Feedback (Inhibitory ) Control Feedback control of gastric motility/emptying (distal stomach) mediated by: a. Neural activity: Enterogastric long reflex Several receptors stimulate afferent pathway (via general visceral afferents): 1) Duodenal osmoreceptors - Mucosal receptors monitor osmolality of duodenal contents: Duodenal hypertonicity - Less gastric emptying Duodenal hypotonicity - More gastric emptying 2) Duodenal pH receptors - Mucosal receptors monitor luminal pH Duodenal pH < 3.5 - 4.0 - Less gastric emptying Duodenal pH > 3.5 - 4.0 - More gastric emptying 3) Stretch receptors: Monitor filling of duodenum Duodenum full - Less gastric emptying Duodenal empty - More gastric emptying Reduced gastric motility/emptying is mediated by: 1) **Activation of sympathetic efferents 2) Deactivation of parasympathetic efferents b. Hormonal: Cholecystokinin and Incretins CCK: Released by these stimuli in intestinal lumen Lipids of 12 -18 C atoms Amino acids Action: Partial agonist at gastrin receptor GLP-1,2: Release stimulated by CHO in intestinal lumen Action: Reduce gastric motility Main effect by GLP-1 34  3. Rate of gastric emptying associated with type of diet Supplemental Fig. 12 Liquid empties faster than solid Low fat diets (CHO, Protein) High fat diets D. Control of Gastric Motility: Fasting pattern (interdigestive period = between meals) Interdigestive motility complex: CNS timing mechanism MMCs Stomach and small bowel cleared of indigestible content which is emptied into the LI Migrating motor complex (three phases) Begin at antrum Entrained peristalsis Propagated to ileum Phase I - Slow waves with no contraction Phase II - Intermittant spiking at the crests of slow waves Phase III - Spiking at each crest of slow waves: Train of peristal. contractions -Motilin provides neurocrine cholinergic action in Phase III -Pylorus remains open to clear stomach Phases repeat at 2-hr intervals Immediately interrupted by eating 35 D. Hunger contractions Occur 12 - 24 hr after stomach empties After 24 - 72 hrs, intense rhythm of peristaltic contractions fuse into tetanic contractions 36 Objectives: 1. Describe the motor functions of the proximal and distal stomach. 2. Contrast the motility patterns of the proximal and distal stomach. 3. Know how gastric accommodation prevents increased gastric pressure. 4. Know the afferent and efferent paths to stimulatory control of gastric motility. 5. Know the site of production, site of action, stimuli and effect of gastrin and CCK. 6. Know the afferent and efferent paths to feedback control of gastric motility. 7. Describe the sequence and function of the migrating motor complex. 8. Describe the development of hunger contractions. Related Question: 1. How does gastric bloat in large breeds of dogs cause death? 37