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 Slow waves are unique to GI muscle
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 Frequency
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. Function - controls GI motility
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

23. Organ
Effect of Sympathetic Stimulation
Effect of Parasympathetic Stimulation
Gut
Lumen
Sphincter
Decreased peristalsis and tone
Increased tone (most times)
Increased peristalsis and tone
Relaxed (most times)

24. Neurotransmitters Postganglionic efferent neurons
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

29. Thank you