Cardiac Action Potentials and SA Node Overview
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Questions and Answers

What occurs during Phase 0 of the action potential?

  • Closure of Ca2+ channels
  • Efflux of K+ ions
  • Influx of Na+ ions
  • Opening of L-type Ca2+ channels (correct)

    • During which phase does repolarization occur?

  • Phase 2
  • Phase 4
  • Phase 1
  • Phase 3 (correct)

    • What is the primary ionic movement during Phase 4?

  • Inward flux of K+
  • Slow depolarization towards threshold (correct)
  • Closure of L-type Ca2+ channels
  • Efflux of Na+

    • What effect does the closure of delayed K+ channels have during the pacemaker potential?

    <p>Facilitates slow influx of Na+ ions</p> Signup and view all the answers

    What initiates the upstroke of the next action potential?

    <p>Reaching the threshold at -40 mV</p> Signup and view all the answers

    Which ionic channel does not contribute to the stable resting membrane potential in the AV node?

    <p>Inward rectifier potassium channels</p> Signup and view all the answers

    What is the main purpose of Funny (If) channels during the pacemaker potential?

    <p>To allow a slow inward Na+ current</p> Signup and view all the answers

    How does calcium influx influence the action potential in the SA node?

    <p>It generates the upstroke due to depolarization</p> Signup and view all the answers

    What triggers the rapid opening of the activation gate (m gate) in Na+ channels?

    <p>A depolarization that exceeds threshold potential</p> Signup and view all the answers

    During which phase of the action potential are both the activation gate (m gate) and inactivation gate (h gate) open?

    <p>Phase 0</p> Signup and view all the answers

    What occurs during repolarization of the action potential?

    <p>The inactivation gate (h gate) closes</p> Signup and view all the answers

    What is the state of the Na+ channels immediately after completing repolarization?

    <p>Activation gate is closed, inactivation gate is open</p> Signup and view all the answers

    What is the immediate effect of Na+ ions entering the cell?

    <p>The interior of the cell becomes less negative</p> Signup and view all the answers

    What happens when the membrane potential is below threshold potential?

    <p>The activation gate remains closed and inactivation gate is open</p> Signup and view all the answers

    What primarily determines the rapid inward flow of Na+ ions during depolarization?

    <p>Electrochemical gradient favoring Na+ entry</p> Signup and view all the answers

    Why can Na+ channels not be reactivated immediately after repolarization?

    <p>The resting membrane potential has not been reestablished</p> Signup and view all the answers

    What is the order of excitation in the heart?

    <p>Endocardium to epicardium</p> Signup and view all the answers

    Why does the epicardium have a shorter duration of action potential compared to the endocardium?

    <p>It has more potassium channels expressed</p> Signup and view all the answers

    What prevents cardiac muscle from tetanizing?

    <p>The long refractory period</p> Signup and view all the answers

    What characterizes the Relative Refractory Period (RRP) in cardiac muscle?

    <p>A stronger than normal stimulus is required to generate another action potential</p> Signup and view all the answers

    What is the duration of the action potentials in cardiac muscle?

    <p>200-350 msec</p> Signup and view all the answers

    What is primarily responsible for the membrane excitation in cardiac muscle?

    <p>Na+, K+, and Ca2+ channels</p> Signup and view all the answers

    What is the effect of a channel having equal permeability to both Na+ and K+?

    <p>The reversal potential lies between the Nernst potentials of both ions.</p> Signup and view all the answers

    Which layer of the heart repolarizes first?

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

    What triggers the opening of the inactivation gate in cardiac muscle?

    <p>Membrane potential falling to -75 mV</p> Signup and view all the answers

    What is the primary characteristic of the fast voltage gated Na+ channels in cardiac tissue?

    <p>They open and activate at negative voltages.</p> Signup and view all the answers

    Why can't cardiac muscle contractions summate like skeletal muscle contractions?

    <p>Because cardiac muscle action potentials have longer durations</p> Signup and view all the answers

    What occurs when the fast voltage gated Na+ channels open?

    <p>Na+ enters the cell, generating an inward current.</p> Signup and view all the answers

    What is indicated by the term 'mixed conductance channels'?

    <p>Channels that allow more than one type of ion to pass.</p> Signup and view all the answers

    What is the general impact of ion channels on the electrical properties of tissues?

    <p>They facilitate the movement of charged ions across membranes.</p> Signup and view all the answers

    What is the reversal potential for a channel that has equal permeability to Na+ and K+?

    <p>-15 mV</p> Signup and view all the answers

    Why are ion channels necessary for action potentials in cardiac cells?

    <p>They enable rapid depolarization of the membrane.</p> Signup and view all the answers

    What is the primary function of the SA node in the heart?

    <p>To dominate the frequency of firing throughout the heart</p> Signup and view all the answers

    What happens to the AV node when the SA node generates an action potential?

    <p>The AV node’s pacemaking activity is reset by the SA node's action potential</p> Signup and view all the answers

    Which of the following best describes overdrive suppression?

    <p>The inhibition of pacemaker activity of secondary and tertiary pacemakers by the SA node</p> Signup and view all the answers

    What characteristic of the atrium contributes to a shorter duration of action potentials?

    <p>Fewer L-type calcium channels present compared to other cardiac tissues</p> Signup and view all the answers

    Which of the following cells can act as a pacemaker in the heart?

    <p>Any cell possessing ion channels that allow for phase 4 depolarization</p> Signup and view all the answers

    Study Notes

    Cardiac Action Potentials

    • The speed of the action potential's rising phase is affected by sodium channels, which open rapidly at negative voltages.
    • Activation and inactivation gates control the sodium channel.
    • The activation gate (m gate) opens rapidly, allowing sodium to flow into the cell, while the inactivation gate (h gate) closes more slowly, resulting in a brief period (1-2ms) where both gates are open during the upstroke (phase 0).
    • During repolarization (phase 3), the inactivation gate (h gate) closes, preventing sodium from flowing even though the activation gate is still open.
    • Sodium channels are in a refractory state and cannot be reactivated until the resting membrane potential is reached.

    SA Nodal Action Potential

    • The SA Node is the primary pacemaker of the heart with a firing frequency of 60-100 bpm.
    • The SA node has a slow upstroke (phase 0) due to opening of L-type calcium channels and influx of calcium.
    • Repolarization (phase 3) occurs due to the opening of potassium channels, allowing potassium to flow out of the cell, and the closure of calcium channels.
    • During phase 4, the resting membrane potential is unstable, with a slow influx of sodium due to the opening of Funny (If) channels.
    • Once threshold potential (-40mV) is reached, L-type calcium channels open, generating the upstroke (phase 0) of the next action potential.

    Cardiac Ion Channels

    • Ion channels facilitate the movement of ions across the cell membrane and play crucial roles in the heart’s action potential.
    • Sodium, potassium and calcium channels are essential for membrane excitation in cardiac muscle.
    • Mixed conductance channels allow multiple ions to pass through.
    • Sodium channels open rapidly at negative voltages, allowing a large influx of sodium into the cell, leading to depolarization.

    Cardiac Muscle Contractions

    • Cardiac muscle contractions cannot be tetanized due to the long refractory period, which is longer than the contractile phase.
    • This prevents summation of successive action potentials, ensuring a period of diastole for the heart to fill with blood.

    Intrinsic Pacemaker Activity of the Heart

    • The SA Node is the primary pacemaker of the heart.
    • AV Node and Purkinje fibers are secondary and tertiary pacemakers, respectively.
    • Overdrive suppression occurs when the SA Node resets the firing rates of other pacemaker cells, resulting in a uniform heart rhythm.
    • The SA Node has the fastest intrinsic firing frequency, dominating the other pacemaker cells.

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    Description

    Explore the intricacies of cardiac action potentials and the role of the SA node as the heart's pacemaker. This quiz covers sodium channels' functions, activation and inactivation gates, and the phases of action potentials, essential for understanding heart rhythms. Test your knowledge on these vital physiological processes.

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