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Cardiac Action Potentials quiz #1

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  • What are the key differences between the action potentials of cardiac pacemaker cells and cardiac contractile cells, and how do these differences contribute to the heart's function?
    Cardiac pacemaker cells have a slow depolarization phase (pacemaker potential) due to simultaneous sodium inflow and potassium outflow through the same channel, which sets the heart's rhythm (autorhythmicity). They lack a true resting potential and continuously cycle through depolarization and repolarization. In contrast, cardiac contractile cells have a rapid depolarization (from sodium inflow), followed by a plateau phase where calcium inflow and slow potassium outflow prolong depolarization, extending the absolute refractory period. This ensures the heart muscle fully contracts and relaxes between beats, preventing tetany and allowing effective blood pumping.
  • Describe the sequence of ion movements during an action potential in a cardiac contractile cell and explain the significance of the plateau phase.
    In a cardiac contractile cell, the action potential begins with rapid depolarization as sodium channels open and sodium enters the cell. This is followed by the plateau phase, where calcium channels open (calcium enters) and some potassium channels open (potassium exits), causing positive ions to move in opposite directions and maintaining depolarization. Finally, repolarization occurs when calcium channels close and more potassium channels open, allowing potassium to exit rapidly. The plateau phase prolongs the absolute refractory period, ensuring the heart muscle has time to contract and relax before the next beat, which is essential for proper cardiac function.
  • What is the main function of cardiac pacemaker cells in the heart?
    Cardiac pacemaker cells set the rhythm of the heart by generating action potentials on their own, a property called autorhythmicity.
  • How does the action potential of a cardiac pacemaker cell differ in shape from that of a cardiac contractile cell?
    Pacemaker cells have a slow depolarization (pacemaker potential) followed by a rapid depolarization and repolarization, while contractile cells have a rapid depolarization, a plateau phase, and then repolarization.
  • Which ions are primarily responsible for the slow depolarization (pacemaker potential) in pacemaker cells?
    The slow depolarization is due to sodium ions entering and potassium ions exiting the cell simultaneously through the same channel, with more sodium entering than potassium leaving.
  • What ion movement causes the rapid depolarization phase in cardiac contractile cells?
    Rapid depolarization in contractile cells is caused by sodium ions quickly entering the cell through voltage-gated sodium channels.
  • What is the significance of the plateau phase in the action potential of cardiac contractile cells?
    The plateau phase, caused by calcium entering and potassium leaving the cell, prolongs the absolute refractory period, ensuring the heart muscle fully contracts and relaxes between beats.
  • Why do cardiac pacemaker cells lack a true resting potential?
    Pacemaker cells continuously cycle through depolarization and repolarization, so as soon as they repolarize, the slow depolarization (pacemaker potential) begins again.
  • How does the absolute refractory period in cardiac contractile cells prevent tetany?
    The prolonged absolute refractory period prevents the heart muscle from being restimulated before it has relaxed, thus avoiding sustained contraction (tetany).
  • Which three ions are involved in the action potentials of both cardiac pacemaker and contractile cells?
    Sodium (Na+), calcium (Ca2+), and potassium (K+) ions are all involved in the action potentials of both cell types.
  • Which electrolyte plays a crucial role in cardiac muscle contraction, particularly during the plateau phase of the cardiac action potential?
    Calcium ions (Ca2+) are essential for cardiac muscle contraction, as their inflow during the plateau phase of the cardiac action potential prolongs depolarization and enables effective contraction.