Cardiac Muscle Excitability and Action Potential
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Questions and Answers

During which phase of the cardiac action potential does the resting membrane potential across the myocardial cell membrane reach about -90 mV?

  • Phase 0 (Depolarization) (correct)
  • Phase 2 (Plateau)
  • Phase 3 (Complete repolarization)
  • Phase 1 (Early repolarization)
  • What causes the rapid upstroke of the action potential during Phase 0 of the cardiac action potential?

  • Release of Ca2+ from the sarcoplasmic reticulum
  • Inward Ca2+ current through L-type Ca channels
  • Rapid Na+ influx by voltage gated Na+ channels (correct)
  • Outward K+ current resulting in repolarization
  • Which phase of the cardiac action potential is characterized by inward Ca2+ current balanced by outward K+ current, resulting in little change in membrane potential?

  • Phase 2 (Plateau) (correct)
  • Phase 0 (Depolarization)
  • Phase 3 (Complete repolarization)
  • Phase 1 (Early repolarization)
  • What triggers muscle contraction in cardiac muscle during the plateau phase of the action potential?

    <p>Release of Ca2+ from the sarcoplasmic reticulum</p> Signup and view all the answers

    What occurs during Phase 3 (Complete repolarization) of the cardiac action potential?

    <p>L-type Ca channels close, but K+ currents still exist</p> Signup and view all the answers

    What prevents tetany in cardiac muscle during the action potential?

    <p>Long duration of the action potential</p> Signup and view all the answers

    During which phase does the cardiac muscle not respond to any stimulus?

    <p>Phase 2</p> Signup and view all the answers

    What is the nature of the heart's ability to beat at regular intervals?

    <p>Myogenic</p> Signup and view all the answers

    Which node has the highest rhythmicity of discharge of cardiac impulses?

    <p>Sino-atrial node (SAN)</p> Signup and view all the answers

    What happens to heart rate as a result of sympathetic supply?

    <p>Increases</p> Signup and view all the answers

    What is the function of the enzymes Na+/K+-ATPase & Na+/Ca2+ exchange during Phase 4?

    <p>Removal of excess Na+ &amp; Ca2+ from the cell</p> Signup and view all the answers

    Which factor decreases heart rate as a negative chronotropic factor?

    <p>Alkalosis</p> Signup and view all the answers

    What is essential for the heart's rhythmicity due to its mainly aerobic metabolism?

    <p>$O_2$ supply</p> Signup and view all the answers

    Which phase corresponds to late phase 3 and allows the heart cell to begin responding to excitation again?

    <p>Phase 3</p> Signup and view all the answers

    Innervation of the heart by sympathetic supply increases rhythmicity of the whole heart including which part?

    <p>Ventricles</p> Signup and view all the answers

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