ELECTRICAL ACTIVITY OF GASTROINTESTINAL SMOOTH MUSCLE

• 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 potential• Spike potentials

• 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

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.

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

GutLumenSphincter

Decreased peristalsis and toneIncreased tone (most times)

Increased peristalsis and toneRelaxed (most times)

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

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