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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.

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slide4

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
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
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
Spike Potentials
  • True action potentials
    • Occur when slow waves reach threshold
    • Cause SM contraction
  • Frequency
    • Affected by nervous / hormonal stimuli
    •  frequency  stronger contraction
slide9

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
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
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.
slide12

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
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
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)
enteric nervous system ens
Enteric Nervous System (ENS)
  • Location - gut wall from esophagus to anus
ens myenteric plexus
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
ens submucosal plexus
ENS - Submucosal Plexus
  • Location - Mucosal layer from esophagus to anus
  • Function - Local control
      • Secretion
      • Absorption
      • Contraction of muscularis mucosa
parasympathetic innervation
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
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
neurotransmitters
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
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
gastrointestinal reflexes
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
slide28

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