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Cardiac Action Potentials Overview
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Cardiac Action Potentials Overview

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@SoulfulMarigold

Questions and Answers

What primarily drives changes in transmembrane potential in myocardial cells?

  • Oxygen levels in the blood
  • Changes in temperature
  • Variation in heart rate
  • Movement of ions across the membrane (correct)

    • If a cardiac muscle cell membrane were only permeable to potassium ions (K+), what would the membrane potential be approximately?

  • -90 mV (correct)
  • +130 mV
  • +60 mV
  • 0 mV

    • What happens to the transmembrane voltage when there is a balance in current flow across the membrane?

  • It reaches zero
  • It becomes unstable
  • It increases indefinitely
  • It remains constant (correct)

    • During an action potential, what primarily changes the membrane's relative permeability?

    <p>Ion channel opening and closing</p> Signup and view all the answers

    Which of the following ions contributes positively to the resting membrane potential of a cardiac muscle cell?

    <p>Ca2+</p> Signup and view all the answers

    What is the Nernst potential for sodium ions (Na+) in a myocardial cell?

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

    Which scenario indicates that an action potential is likely to occur in myocardial cells?

    <p>Increased permeability to Na+ ions</p> Signup and view all the answers

    What is the significance of ion permeability changes during the cardiac action potential?

    <p>It allows for the generation of an action potential</p> Signup and view all the answers

    What characterizes the supranormal period (SNP) in ventricular tissue?

    <p>More depolarized state than normal</p> Signup and view all the answers

    What triggers the upstroke in the action potential of the SA and AV nodal cells?

    <p>Opening of voltage-dependent L-type Ca2+ channels</p> Signup and view all the answers

    What is the maximum diastolic potential approximately in SA nodal cells?

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

    How does the action potential amplitude in SA and AV nodal cells compare to fast fibers?

    <p>It is decreased due to slower depolarization</p> Signup and view all the answers

    What is one consequence of the slower rate of depolarization in SA and AV nodal fibers?

    <p>Decreased conduction velocity</p> Signup and view all the answers

    What can result from the generation of abnormally timed action potentials during the SNP?

    <p>Ventricular dysrhythmia</p> Signup and view all the answers

    Which phase is absent in the SA and AV nodal action potential?

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

    At what threshold level do L-type calcium channels open during the action potential phase?

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

    What role do inward rectifying potassium channels (Kir 2.1) play in cardiac muscle cells?

    <p>They maintain high K+ conductance during resting conditions.</p> Signup and view all the answers

    During which phase of the cardiac action potential do the inward rectifying potassium channels (Kir 2.1) close?

    <p>During depolarization</p> Signup and view all the answers

    What is the primary effect of K+ currents flowing out of cardiac cells during repolarization?

    <p>They lead to hyperpolarization of the cell.</p> Signup and view all the answers

    What is the function of delayed rectifying K+ channels (lK) during the cardiac action potential?

    <p>They slowly open in response to depolarization to facilitate repolarization.</p> Signup and view all the answers

    How does the closure of inward rectifying potassium channels (Kir 2.1) affect the plateau phase of the cardiac action potential?

    <p>It leads to sustained depolarization and prolongs the plateau phase.</p> Signup and view all the answers

    What is the approximate membrane potential (Em) at rest due to the high K+ conductance from Kir 2.1 channels?

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

    What triggers the opening of delayed rectifying K+ channels (lK)?

    <p>Membrane depolarization.</p> Signup and view all the answers

    How many distinct types of K+ channels are present in cardiac muscle that facilitate outward K+ current?

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

    What phenomenon occurs when myocardial cells outside specialized fibers develop automaticity?

    <p>Ectopic beats</p> Signup and view all the answers

    Which mechanism does acetylcholine utilize to slow the intrinsic pacemaker activity in the heart?

    <p>Inhibits adenyl cyclase activity</p> Signup and view all the answers

    What is the effect of the vagus nerve on heart rate?

    <p>Decreases heart rate</p> Signup and view all the answers

    How does decreased cAMP affect the phase 4 depolarization in the SA node?

    <p>Decreases the steepness of depolarization</p> Signup and view all the answers

    What type of receptors does acetylcholine act on in the heart to exert parasympathetic effects?

    <p>M2 cholinergic receptors</p> Signup and view all the answers

    What change in action potentials occurs as a result of increased parasympathetic stimulation?

    <p>Decreased frequency of action potentials</p> Signup and view all the answers

    What current is affected by acetylcholine in the SA node, leading to slower depolarization?

    <p>If current</p> Signup and view all the answers

    Which of the following is NOT a consequence of acetylcholine's actions in the heart?

    <p>Increased heart rate</p> Signup and view all the answers

    What property allows the SA node to be the primary pacemaker of the heart?

    <p>It generates action potentials at approximately 60-100 times per minute.</p> Signup and view all the answers

    What happens to secondary and tertiary pacemakers when the SA node generates action potentials?

    <p>They become suppressed due to overdrive suppression.</p> Signup and view all the answers

    How does the pacemaker potential affect the heart rate?

    <p>It leads to progressive slow depolarization.</p> Signup and view all the answers

    What occurs if the SA node fails?

    <p>A backup pacemaker with the next highest intrinsic rate will take over.</p> Signup and view all the answers

    What is the intrinsic firing rate of AV nodal cells?

    <p>40-60 beats per minute</p> Signup and view all the answers

    Which of the following correctly describes the Purkinje fibers?

    <p>They have the lowest intrinsic firing rate among pacemaker cells.</p> Signup and view all the answers

    What is 'overdrive suppression' in the context of heart pacemakers?

    <p>The effect when the SA node pacemaker activity suppresses slower pacemakers.</p> Signup and view all the answers

    What distinguishes the electrical activity of the SA node from other cardiac cells?

    <p>It has a faster rate of phase 4 depolarization.</p> Signup and view all the answers

    Study Notes

    Cardiac Action Potentials Overview

    • Myocardial cells have an electric potential due to charge distribution across their membranes, enabling current flow.
    • Changes in transmembrane potential relate directly to current movement; a balanced current keeps voltage constant.
    • Cardiac contractions originate from action potentials, involving ion flow through specific channels: Na+, K+, and Ca2+.

    Membrane Potential Dynamics

    • Membrane potential varies based on permeability to ions:
      • If only Na+ permeable, potential approaches +60 mV (Nernst for Na+).
      • If only K+ permeable, potential approaches -90 mV (Nernst for K+).
      • Ca2+ permeability results in a potential of +130 mV (Nernst for Ca2+).
    • During action potentials, membrane permeability shifts, causing depolarization and subsequent repolarization.

    Potassium Current in Cardiac Action Potentials

    • Outward K+ currents contribute to repolarization, making the interior of the cell more negative.
    • At rest, ungated K+ channels maintain high permeability, holding the membrane potential near EK+ (-90 mV).
    • Four K+ channels contribute to varying outward K+ currents:
      • Inward rectifying potassium channels (Kir 2.1)
      • Transient outward potassium channels (Ito)
      • Delayed rectifying K+ currents (IK) classified into:
        • Rapid component (IKr)
        • Slow component (IKs)

    Inward Rectifying Potassium Channels (Kir 2.1)

    • Voltage-gated channels that maintain K+ conductance under resting conditions.
    • Close near depolarization end and reopen during repolarization.
    • Closure during plateau phase is critical for preventing premature repolarization.

    Delayed Rectifying K+ Channels (IK)

    • Closed at resting membrane potential; open gradually during depolarization.
    • More rapid opening near end of plateau phase to initiate repolarization.

    Supranormal Period (SNP)

    • Occurs when ventricular tissue is depolarized, leading to a hyper-excitable state.
    • Results in a decreased action potential amplitude but may still elicit propagated responses.
    • Can cause rhythm abnormalities due to mis-timed action potentials.

    Ionic Basis of SA and AV Node Action Potentials

    • Pacemaker cells in the SA and AV nodes exhibit automatic action potentials and lack a true resting membrane potential.
    • Maximum diastolic potential reaches around -60 mV.
    • The SA node generates action potentials at 60-100 beats per minute, making it the primary pacemaker.

    Setting Heart Rate

    • SA node cells enable the heart to beat independently due to intrinsic electrical activity.
    • Phase 4 depolarization progressively approaches the threshold for action potential generation.
    • AV node and Purkinje fibers exhibit slower intrinsic rates (40-60 and 20-40 beats per minute, respectively).

    Overdrive Suppression

    • Faster pacing from the SA node suppresses other pacemaker cells, ensuring a unified heart rate.
    • Should the SA node fail, backup pacemakers take over based on their intrinsic rates.

    Automaticity and Ectopic Beats

    • Cardiac tissue injury can lead to ectopic beats due to the development of automaticity in non-specialized myocardial cells.

    Effects of Parasympathetic Nervous System on Heart Rate

    • The vagus nerve stimulates the heart, releasing acetylcholine, which lowers pacemaker activity through M2 cholinergic receptors.
    • Activation of M2 receptors decreases the If current, causing a slower depolarization rate.
    • Reduced cAMP levels lead to a less steep phase 4 depolarization, decreasing action potentials per unit time, thus lowering heart rate.

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    Description

    Explore the dynamics of cardiac action potentials in myocardial cells. This quiz covers the role of membrane potential, ion flow, and the significance of Na+, K+, and Ca2+ currents in cardiac function. Test your knowledge on how these elements contribute to cardiac contractions and repolarization.

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