Cardiac Action Potential Overview
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Questions and Answers

What is the firing frequency of idioventricular pacemakers in the Purkinje fibers when the AV node fails?

  • 30–40 impulses per minute (correct)
  • 60–80 impulses per minute
  • 40–60 impulses per minute
  • 20–30 impulses per minute

    • What characterizes phase 1 of the cardiac action potential?

  • Rapid influx of calcium ions
  • Closure of sodium channels and potassium efflux (correct)
  • Opening of calcium channels
  • Stable resting membrane potential

    • What is the primary function of the plateau phase (phase 2) of the cardiac action potential?

  • Increased potassium ion efflux
  • Rapid depolarization of the cell
  • Completion of the resting membrane potential
  • Prolonged action potential duration (correct)

    • Which statement accurately describes phase 3 of the cardiac action potential?

    <p>Closure of calcium channels and increased potassium permeability occur</p> Signup and view all the answers

    What is the average resting membrane potential of myocardial fibers?

    <p>-90 mV</p> Signup and view all the answers

    Which ion's influx is primarily responsible for maintaining the plateau phase of the action potential?

    <p>Calcium ions</p> Signup and view all the answers

    What is one benefit of the plateau phase for cardiac cells?

    <p>It allows for synchronous contraction of heart chambers.</p> Signup and view all the answers

    During what phase does the cell's membrane return to its resting potential?

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

    What is the role of the sinoatrial (SA) node in the heart's conduction system?

    <p>To serve as the primary pacemaker of the heart</p> Signup and view all the answers

    Which structure in the heart has the highest frequency of action potentials?

    <p>SA node</p> Signup and view all the answers

    What happens when the SA node generates an action potential?

    <p>The AV node enters a refractory period</p> Signup and view all the answers

    What distinguishes Purkinje fibers from myocytes?

    <p>Purkinje fibers are larger and have fewer striations</p> Signup and view all the answers

    Which of the following parts of the conduction system acts only if the SA node is damaged?

    <p>AV node</p> Signup and view all the answers

    Why does the AV node rarely initiate contractions?

    <p>It enters a refractory period when stimulated by the SA node</p> Signup and view all the answers

    What characterizes the resting membrane potential (RMP) of living cardiac muscle cells?

    <p>It is negative inside relative to the outside</p> Signup and view all the answers

    What is the primary function of the conduction system of the heart?

    <p>To efficiently transmit action potentials through the heart</p> Signup and view all the answers

    What characterizes the absolute refractory period (ARP) in cardiac muscle cells?

    <p>It corresponds to the contraction phase of the cardiac muscle.</p> Signup and view all the answers

    What is the resting membrane potential of the sinoatrial (SA) node?

    <p>-60 mV</p> Signup and view all the answers

    During which phases does the absolute refractory period occur?

    <p>Phase 1, 2, and part of phase 3.</p> Signup and view all the answers

    Which ion influx primarily contributes to the gradual depolarization of the SA node?

    <p>Sodium ions (Na+)</p> Signup and view all the answers

    What is a key feature of the relative refractory period (RRP)?

    <p>A larger than normal stimulus is required to initiate an action potential.</p> Signup and view all the answers

    What happens to the membrane potential of SA node cells during action potential termination?

    <p>It drops to about -55 to -60 mV.</p> Signup and view all the answers

    Which structures can act as latent pacemakers if the SA node is suppressed?

    <p>AV node and bundle of His.</p> Signup and view all the answers

    What physiological event is triggered by the rapid entry of Ca+2 and Na+ ions during action potential?

    <p>Rapid depolarization of the membrane.</p> Signup and view all the answers

    Study Notes

    Cardiac Action Potential

    • All living cells have a resting membrane potential (RMP), expressed in millivolts (mV). Inside the cell is negative relative to the outside.
    • Membrane potential changes due to ion flow (ions moving into or out of the cell).
    • Depolarization occurs when positive charge flows into the cell, making the membrane potential less negative.
    • The sinoatrial (SA) node initiates the heart's excitation signal (action potential).
    • The conduction system in the heart consists of the SA node, AV node, bundles of atrial fibers, the bundle of His, and Purkinje fibers.
    • The SA node is located at the junction of the superior vena cava and the right atrium.
    • The AV node is in the right posterior portion of the interatrial septum.
    • Three bundles of atrial fibers connect the SA node to the AV node.
    • The bundle of His and its branches are components of the conduction system.
    • Purkinje fibers are specialized conducting cells.
    • Modified cardiac muscle cells form the conduction system, with fewer striations and indistinct boundaries compared to typical cardiac muscle.
    • Purkinje fibers are large, with fewer mitochondria, and specialized for conduction.

    Phases of Cardiac Action Potential

    • Phase 0 (Depolarization): Voltage-gated sodium channels open, allowing rapid sodium influx, depolarizing the cell. Membrane potential reaches approximately +20 mV.
    • Phase 1 (Initial Repolarization): Sodium channels close; the cell begins repolarizing. Potassium ions leave the cell through open potassium channels.
    • Phase 2 (Plateau): Calcium ion permeability increases, while potassium permeability decreases. A combination of decreased potassium efflux and increased calcium influx maintain the action potential plateau.
    • Phase 3 (Rapid Repolarization): Calcium channels close; potassium permeability increases significantly allowing potassium ions to rapidly leave the cell, returning the membrane potential to its resting level.
    • Phase 4 (Resting Membrane Potential): The cell returns to resting membrane potential, which averages approximately -90 mV.

    Action Potential of Cardiac Muscle Cells

    • Myocardial fibers have a resting membrane potential of about -90 mV.
    • Stimulation from the SA node initiates a propagated action potential, responsible for muscle contraction.
    • The AV node can serve as a secondary (latent) pacemaker if the SA node is damaged.
    • Ventricular contraction occurs only after the completion of systole (contraction) of the cardiac muscle.

    Pacemaker Cells

    • The SA node is the primary pacemaker of the heart, initiating normal sinus rhythm (NSR).
    • The normal resting membrane potential of the SA node is -55 to -60 mV (compared to -85 mV in ventricular fibers).
    • Sodium channels in SA cells are slightly leaky ("less negative"). Sodium influx contributes to gradual depolarization.
    • Calcium-sodium channels open when the membrane potential reaches a threshold (~40 mV), causing a rapid rise in the membrane potential to ~0mV.
    • Potassium efflux during repolarization returns the membrane potential to -55 to -60 mV.

    Refractory Period

    • Absolute Refractory Period (ARP): A period during which the membrane cannot be re-excited, regardless of the stimulus strength.
    • Relative Refractory Period (RRP): A period during which the membrane can be re-excited, but only by a larger-than-normal stimulus.

    Benefits of the Plateau Phase

    • Prolongation of action potential duration
    • Exposure to high cytoplasmic Ca²+ concentration, increasing the force of contraction.
    • Increased time for all cardiac cells to contract simultaneously.

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    Cardiovascular Physiology (Lec 2) PDF

    Description

    Explore the fundamental concepts of cardiac action potentials and the heart's conduction system. Understand how the resting membrane potential influences depolarization and the role of the SA and AV nodes. This quiz covers essential pathways like the bundle of His and Purkinje fibers that contribute to cardiac function.

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