The GI smooth muscle acts as a functional syncytium.
Slide 4
Characteristic features of the basic electrical activity of GIT
& its relation to smooth muscle contractile activity under
physiologic conditions
Slide 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
Slide 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
Slide 7
Slide 8
Spike Potentials True action potentials - Occur when slow waves
reach threshold - Cause SM contraction Frequency - Affected by
nervous / hormonal stimuli - frequency stronger contraction
Slide 9
Figure 62-3; Guyton & Hall 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.
Slide 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
Slide 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.
Slide 12
Important factors that make the membrane potential more
negativehyperpolarize 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
Slide 13
Calcium Ions and Muscle Contraction Occurs in response to entry
of calcium ions Calcium ions, acting through a calmodulin control
mechanisms
Slide 14
Neural Control of GI Tract Intrinsic Control - Enteric nervous
system - Myenteric (Auerbachs) plexus - Submucosal (Meissners)
plexus Extrinsic Control - Autonomic nervous system -
Parasympathetic - mainly stimulates (Ach) - Sympathetic - mainly
inhibits (NE)
Slide 15
Physiological anatomy of enteric nervous system
Slide 16
Slide 17
Enteric Nervous System (ENS) Location - gut wall from esophagus
to anus
Slide 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
Slide 19
Figure 62-4; Guyton & Hall
Slide 20
ENS - Submucosal Plexus Location - Mucosal layer from esophagus
to anus Function - Local control - Secretion - Absorption -
Contraction of muscularis mucosa
Slide 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)
Slide 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
Slide 23
OrganEffect 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)
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
Slide 26
Figure 62-4; Guyton & Hall
Slide 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
Slide 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