Electrical Activity of Gastrointestinal Smooth Muscle

1. Electrical Activity of Gastrointestinal Smooth Muscle

3. The GI smooth muscle acts as a functional syncytium.

4. Characteristic features of the basic electrical activity of GIT & its relation to smooth muscle contractile activity under physiologic conditions

5. Characteristic features of the basic electrical activity of GIT • Slow waves spike potential • Spike potentials • Voltage of the resting membrane potential of the gastrointestinal smooth muscle can be made to change to different levels

6. Slow Waves • Rhythmical changes in membrane potential caused by variations in sodium conductance • Slow waves are unique to GI muscle • Intensity usually varies between 5 and 15 mv • Frequency ranges in different parts of the human GIT from 3 to 12 /min • Cause • Complex interactions among the smooth muscle cells and specialized cells Interstitial cells of Cajal - pacemaker cells

8. Spike Potentials • True action potentials • Occur when slow waves reach threshold • Cause SM contraction • Frequency • Affected by nervous / hormonal stimuli •  frequency  stronger contraction

9. Each time the peaks of the slow waves temporarily become more positive than -40 millivolts, spike potentials appear on these peaks The higher the slow wave potential rises, the greater the frequency of the spike potentials, usually ranging between 1 and 10 spikes per second. Figure 62-3; Guyton & Hall

10. AP of the gastrointestinal smooth muscle • Channels responsible for the AP allow large numbers of calcium ions to enter along with smaller numbers of sodium ions and therefore are called calcium-sodium channel • These are much slower to open and close than are the rapid Na+ channels of large nerve fibers • Accounts for the long duration of the action potentials

11. Changes in Voltage of the R M P • Resting MP averages about -56 millivolts • Multiple factors can change this level • Factors that depolarize the –excitable— • Stretching of the muscle • Stimulation by acetylcholine • Stimulation by parasympathetic nerves that secrete acetylcholine • Stimulation by several specific gastrointestinal hormones.

12. Important factors that make the membrane potential more negative—hyperpolarize the membrane and make the muscle fibers less excitable— • Effect of norepinephrine or epinephrine on the fiber membrane • Stimulation of the sympathetic nerves that secrete mainly norepinephrine at their endings

13. Calcium Ions and Muscle Contraction • Occurs in response to entry of calcium ions • Calcium ions, acting through a calmodulin control mechanisms

14. Neural Control of GI Tract • Intrinsic Control - Enteric nervous system • Myenteric (Auerbach’s) plexus • Submucosal (Meissner’s) plexus • Extrinsic Control - Autonomic nervous system • Parasympathetic - mainly stimulates (Ach) • Sympathetic - mainly inhibits (NE)

15. Physiological anatomy of enteric nervous system

17. Enteric Nervous System (ENS) • Location - gut wall from esophagus to anus

18. ENS - Myenteric Plexus • Location - • Esophagus to anus • Between longitudinal and circular SM layers • Function - controls GI motility • Stimulatory influences - •  tonic contraction (tone) •  contraction frequency / intensity (propulsion) • Inhibitory influences • Decreased Sphincter tone (relax) - pyloric sphincter, ileocecal sphincter, LES

19. Figure 62-4; Guyton & Hall

20. ENS - Submucosal Plexus • Location - Mucosal layer from esophagus to anus • Function - Local control • Secretion • Absorption • Contraction of muscularis mucosa

21. Parasympathetic Innervation • Cranial Division -(Vagus N.) - first half of gut • Sacral Division -(Pelvic N.) - second half of gut • Neurons- preganglionic - long - postganglionic - short, entirely in ENS Synapse with ENS neurons (mainly) • Stimulation -Excites ENS (in general)

22. Sympathetic Innervation • Preganglionic Neurons-Originate at T5-L2 (cell bodies) • Postganglionic Neurons (long) • Originate in ganglia • Innervate entire gut • stimulation of the sympathetic nervous system inhibits activity of the gastrointestinal tract causing many effects opposite to those of the parasympathetic system • Direct effect of secreted norepinephrine to inhibit intestinal tract smooth • Muscle • Inhibitory effect of norepinephrine on the neurons of the entire enteric • nervous system

24. Neurotransmitters • Preganglionic efferent neurons - acetylcholine • Postganglionic efferent neurons • PNS - acetylcholine • SNS - norepinephrine • Enteric nervous system (many others) • Excitatory - acetylcholine, substance P • Inhibitory - VIP, NO

25. Sensory Afferent Neurons • Stimulation of afferent neurons • Distention of gut wall • Non-specific irritation of gut mucosa • chemical stimuli • Stimulation - can excite or inhibit • Intestinal movements • Intestinal secretions

26. Figure 62-4; Guyton & Hall

27. Gastrointestinal Reflexes • Reflexes that are integrated entirely within the gut wall enteric nervous system • Control GI secretion, peristalsis, mixing contractions • Reflexes from the gut to the prevertebral sympathetic ganglia and then back to the gastrointestinal tract • Gastrocolic reflex • Enterogastric reflexes • Colonoileal reflex

28. Reflexes from the gut to the spinal cord or brain stem and then back to the gastrointestinal tract • Reflexes from the stomach and duodenum to the brain stem and back to the stomach • Pain reflexes that cause general inhibition of the entire gastrointestinal tract • Defecation reflexes

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