• The GI smooth muscle acts as a functional
syncytium.
Characteristic features of the basic electrical
activity of GIT & its relation to smooth muscle
contractile activity under physiologic
conditions
Characteristic features of the basic
electrical activity of GIT
• Slow waves
• Spike potentials
spike potential
• Voltage of the resting membrane potential of the
gastrointestinal smooth muscle can be made to
change to different levels
•
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
Spike Potentials
• True action potentials
- Occur when slow waves reach threshold
- Cause SM contraction
• Frequency
- Affected by nervous / hormonal stimuli
-  frequency  stronger contraction
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
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
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.
• 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
Calcium Ions and Muscle Contraction
• Occurs in response to entry of calcium ions
• Calcium ions, acting through a calmodulin control
mechanisms
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)
Physiological anatomy of enteric
nervous system
Enteric Nervous System (ENS)
• Location
- gut wall from esophagus to anus
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
Figure 62-4; Guyton & Hall
ENS - Submucosal Plexus
• Location - Mucosal layer from esophagus to
anus
• Function - Local control
-
Secretion
- Absorption
- Contraction of muscularis mucosa
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)
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
Organ
Effect of Sympathetic
Stimulation
Effect of Parasympathetic
Stimulation
Decreased peristalsis and tone
Increased tone (most times)
Increased peristalsis and tone
Relaxed (most times)
Gut
Lumen
Sphincter
Neurotransmitters
• Preganglionic efferent neurons - acetylcholine
• Postganglionic efferent neurons
- PNS - acetylcholine
- SNS - norepinephrine
• Enteric nervous system (many others)
- Excitatory - acetylcholine, substance P
- Inhibitory - VIP, NO
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
Figure 62-4; Guyton & Hall
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
• 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