Cardiac Action Potential Phases Quiz

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Excitability is the ability of cardiac muscle to respond to an adequate stimulus by generating an action potential followed by a relaxation.


Resting membrane potential (RPM) refers to the electrical potential difference across the membrane under resting condition.


Action potential in excitable cells involves a rapid change in the membrane potential that spreads slowly along the membrane.


Phase 0 of ventricular action potential is characterized by initial rapid repolarization.


The ionic basis of ventricular action potential includes the movement of sodium ions into the cell during Phase 0.


Phase 1 of ventricular action potential is a phase of brief initial depolarization.


During Phase 0 of the ventricular action potential, Na+ ions exit the cell due to the opening of fast Na+ channels.


The resting membrane potential during Phase 4 is +100 mV.


The plateau phase in the cardiac muscle action potential is due to the influx of K+ and Cl- ions.


The rapid repolarization in Phase 3 is caused by the immediate closing of Na+ channels.


The cardiac muscle remains excitable throughout the entire contraction phase.


Isometric contraction results in a change in muscle length during contraction.


The vagal tone increases the heart rate from 70-80/min to 90-100/min.


The conductivity velocity of the AV Node is higher than that of the SA Node.


AV Nodal delay is important as it allows for proper atrial filling.


Purkinje fibers have a slower conductivity velocity than the Bundle of His.


A block in transmission of impulse is referred to as 'ventricular block'.


The right pump of the heart is formed by the left ventricle and left atrium.


Study Notes

Ionic Basis of Ventricular Action Potential

  • Phase 0: Initial rapid depolarization due to Na+ inflow from opening of fast Na+ channels.
  • Phase 1: Brief initial repolarization due to opening of transient K+ channels.
  • Phase 2: Prolonged plateau due to balance between influx of Na+ and Ca+ and outflux of K+ from opening of slow Ca++-Na+ channels.
  • Phase 3: Rapid repolarization due to delayed opening of K+ channels.
  • Phase 4: Resting membrane potential of -100 mv.

Long Refractory Period

  • Cardiac muscle action potential has a long refractory period due to the plateau phase.
  • This ensures the heart cannot be re-stimulated during contraction phase.
  • Non-tentanizing, meaning the heart remains non-excitable for the entire contraction phase.
  • Prevents sustained state of contraction (tetanus) which could lead to stopping of circulation.
  • Absolute refractive period (ARP) and relative refractive period (RRP) occur during the plateau phase.

Properties of Cardiac Muscle

  • Contractility: ability to contract when stimulated by electrical impulse.
  • Two types of muscle contraction:
    • Isometric contraction: increases muscle tension without changing muscle length.
    • Heart has two major syncytium: atrial syncytium and ventricular syncytium.


  • Ability to respond to an electrical stimulus by generating an action potential followed by a contraction.
  • Resting membrane potential (RPM): electrical potential difference across the membrane under resting condition.
  • Action potential: rapid change in membrane potential that spreads rapidly along the membrane.


  • Ability to transmit an electrical stimulus from cell to cell throughout the myocardium.
  • Velocity of conduction:
    • SA Node: 1 m/sec.
    • AV Node: 0.05 m/sec.
    • Bundle of His: 1 m/sec.
    • Purkinje fibers: 4 m/sec.
  • SA Node initiates cardiac impulse, which spreads through atria to reach AV Node.
  • Conduction velocity slows down in AV Node, causing AV Nodal delay (120-200ms in human).
  • This delay allows for proper ventricular filling.
  • Block in transmission of impulse is known as "heart block".

Heart Pumps

  • The atrium and ventricles of the heart join together to form two major heart pumps:
    • Right pump: formed by right ventricle and right atrium.
    • Left pump: formed by left ventricle and left atrium.

Test your knowledge on the ionic basis of ventricular action potential phases, including rapid depolarization, plateau, and late rapid repolarization. Understand the role of Na+, K+, and Ca++ channels in each phase.

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