1.2 Electrical Activity of GI Smooth Muscle

• What type of muscle is found in the gastrointestinal tract?

  Smooth muscle
• What is the nature of the electrical activity in gastrointestinal smooth muscle?

  It is almost continual and intrinsic
• What are the two basic types of electrical waves in gastrointestinal smooth muscle?

  Slow waves and spikes
• How does the resting membrane potential of gastrointestinal smooth muscle affect motor activity?

  It can change to different levels
• What is the typical intensity range of slow waves in gastrointestinal smooth muscle?

  5 to 15 millivolts
• What determines the rhythm of gastrointestinal contractions?

  The frequency of slow waves
• What is the frequency range of slow waves in the gastrointestinal tract?

  3 to 12 per minute
• How does the frequency of slow waves differ in various parts of the gastrointestinal tract?

  3 in stomach, 12 in duodenum, 8-9 in ileum
• What is the frequency of slow waves in the body of the stomach?

  About 3 per minute
• What is the frequency of slow waves in the duodenum?

  About 12 per minute
• What is the frequency of slow waves in the terminal ileum?

  About 8 to 9 per minute
• What are slow waves in smooth muscle cells believed to be caused by?

  Complex interactions among smooth muscle cells
• What role do interstitial cells of Cajal play in smooth muscle function?

  They act as electrical pacemakers for smooth muscle cells
• Where are interstitial cells of Cajal located?

  Between the smooth muscle layers
• How do interstitial cells of Cajal communicate with smooth muscle cells?

  Through synaptic-like contacts
• What causes the cyclic changes in membrane potential in interstitial cells of Cajal?

  Unique ion channels that periodically open
• What do the inward (pacemaker) currents in interstitial cells of Cajal generate?

  Slow wave activity
• Do slow waves cause muscle contraction in the gastrointestinal tract?

  No, they mainly excite spike potentials
• What do spike potentials do in relation to muscle contraction?

  They excite the muscle contraction
• In which part of the gastrointestinal tract do slow waves potentially cause muscle contraction?

  In the stomach
• What is the role of interstitial cells of Cajal in the gastrointestinal tract?

  They act as electrical pacemakers
• What are spike potentials in gastrointestinal smooth muscle?

  True action potentials
• When do spike potentials occur in gastrointestinal smooth muscle?

  When resting membrane potential exceeds −40 mV
• What is the normal resting membrane potential in gastrointestinal smooth muscle fibers?

  Between −50 and −60 millivolts
• What happens when the peaks of slow waves become more positive than −40 millivolts?

  Spike potentials appear on these peaks
• How does the frequency of spike potentials relate to the slow wave potential?

  The higher the slow wave, the greater the frequency
• What is the typical frequency range of spike potentials?

  1 to 10 spikes per second
• How long do spike potentials last in gastrointestinal muscle compared to large nerve fibers?

  10 to 40 times longer
• What is the duration of each gastrointestinal spike potential?

  10 to 20 milliseconds
• What primarily causes action potentials in nerve fibers?

  Rapid entry of sodium ions
• What type of channels are responsible for action potentials in gastrointestinal smooth muscle fibers?

  Calcium-sodium channels
• How do calcium-sodium channels differ from sodium channels in nerve fibers?

  They open and close more slowly
• What accounts for the long duration of action potentials in gastrointestinal smooth muscle?

  The slowness of calcium-sodium channel opening
• What role do calcium ions play during the action potential in gastrointestinal muscle fibers?

  They cause the intestinal muscle fibers to contract