Cardiac Impulse Propagation Quiz
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Questions and Answers

What is the primary role of the sino-atrial (SA) node in the cardiac conduction system?

  • To transmit impulses from the atria to the ventricles
  • To slow down conduction velocity in the ventricles
  • To act as the primary pacemaker of the heart (correct)
  • To inhibit conduction through the atrio-ventricular (AV) node
  • Which structure in the cardiac conduction system normally provides the only electrical connection between the atria and ventricles?

  • Atrioventricular (AV) node (correct)
  • Bundle of His
  • Atrial muscle tissue
  • Sino-atrial (SA) node
  • What contributes to the Purkinje fibers having the fastest conduction velocity among cardiac tissues?

  • Connection to the sino-atrial (SA) node
  • Presence of gap junctions and numerous Na+ channels (correct)
  • Lower concentration of Na+ ions
  • Small diameter of the cells
  • How does the sodium-potassium pump affect the resting membrane potential of cardiac cells?

    <p>Establishes a slightly negative resting membrane potential</p> Signup and view all the answers

    What mechanism is primarily responsible for maintaining high intracellular K+ concentration?

    <p>Sodium-potassium pump activity</p> Signup and view all the answers

    Which of the following best describes the conduction velocity of ventricular myocardium?

    <p>Fast, at about 1 meter/sec</p> Signup and view all the answers

    What is a notable effect of digitalis (digoxin) on cardiac cells?

    <p>Inhibits the sodium-potassium pump activity</p> Signup and view all the answers

    What contributes to the preferential conduction of impulses from the sino-atrial node to the atrioventricular node?

    <p>Specialized internodal pathways</p> Signup and view all the answers

    What primarily determines the final resting membrane potential (Vm)?

    <p>The flux of ions through ion channels</p> Signup and view all the answers

    Which statement best describes the equilibrium potential (EX) for an ion?

    <p>It is the voltage where chemical and electrostatic forces balance.</p> Signup and view all the answers

    In relation to ion conductance in myocytes, which ion channel is most conductive at rest?

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

    What happens to Na+ ion flow when the membrane potential (Vm) exceeds +72 mV?

    <p>Na+ flows out of the cell</p> Signup and view all the answers

    What factor primarily influences the total membrane conductance (Gm)?

    <p>The number of open ion channels</p> Signup and view all the answers

    What is the primary effect of the outward potassium 'leak' current in cardiac myocytes?

    <p>It stabilizes the resting membrane potential close to EK</p> Signup and view all the answers

    Which of the following best describes the relationship between conductance and the resting membrane potential?

    <p>Vm is the sum of equilibrium potentials weighted by conductance.</p> Signup and view all the answers

    Which factor does NOT influence the direction of ion flow through an open ion channel?

    <p>Size of the cell membrane surface</p> Signup and view all the answers

    What is the primary factor that influences the resting membrane potential (Vm) across the cell?

    <p>K+ gradient across the cell</p> Signup and view all the answers

    How much leakage of intracellular K+ ions is required to cause Vm = -70 mV at an intracellular [K+] of 120 mM?

    <p>0.004%</p> Signup and view all the answers

    Which state does a fast Na+ voltage-gated channel NOT enter during its functioning?

    <p>Completely depolarized</p> Signup and view all the answers

    Which gate is responsible for inactivation in the fast Na+ voltage-gated channel?

    <p>h gate</p> Signup and view all the answers

    What has to happen for the myocyte to recover from the inactivation of Na+ channels?

    <p>Repolarization to a more negative potential</p> Signup and view all the answers

    What drives the flow of Na+ ions into the cell during depolarization?

    <p>Electro-chemical gradient</p> Signup and view all the answers

    What occurs if Vm becomes more positive than the equilibrium potential for Na+ (ENa)?

    <p>Na+ will flow out of the cell</p> Signup and view all the answers

    Which aspect describes the opening of the 'm' gate in the fast Na+ voltage-gated channels?

    <p>Voltage-dependent and very fast</p> Signup and view all the answers

    What occurs during the relative refractory period?

    <p>An action potential can occur with a slower upstroke.</p> Signup and view all the answers

    Which cardiac tissue type has the largest diameter?

    <p>Purkinje cells</p> Signup and view all the answers

    What is the conduction velocity of medium diameter cardiac tissues?

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

    What is a characteristic feature of large diameter cardiac tissues?

    <p>Very rapid conduction</p> Signup and view all the answers

    Which statement best describes the structure-function relationship of small diameter cardiac tissues?

    <p>They act as pacemaker tissues.</p> Signup and view all the answers

    Which tissue type has the least contractile activity?

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

    What type of connection do small diameter cardiac tissues have?

    <p>Few connections</p> Signup and view all the answers

    Which feature is least attributed to medium diameter cardiac tissues?

    <p>Very rapid conduction</p> Signup and view all the answers

    What is the primary role of L-type Ca2+ channels during the cardiac action potential?

    <p>They maintain the plateau phase and increase intracellular Ca2+.</p> Signup and view all the answers

    During which phase of the action potential does the increase in IK potassium current occur?

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

    What contributes to the diastolic depolarization in pacemaker cells?

    <p>Inward cation currents that cause less negative Vm.</p> Signup and view all the answers

    Which statement about T-type Ca2+ channels is correct?

    <p>They inactivate rapidly and affect the early phases of action potentials.</p> Signup and view all the answers

    What is the primary characteristic of phase 4 in cardiac action potentials?

    <p>Resting membrane potential without conductance changes.</p> Signup and view all the answers

    What role does the 'funny' current play in cardiac electrophysiology?

    <p>It leads to a slow inward cation current aiding pacemaker activity.</p> Signup and view all the answers

    Which phase lacks fast Na+ channels in cardiac action potentials?

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

    How does the IK current behave during phase 4?

    <p>It slowly decreases its conductance.</p> Signup and view all the answers

    Study Notes

    Cardiac Impulse Propagation

    • The sino-atrial (SA) node acts as the heart's pacemaker, initiating electrical impulses.
    • Internodal pathways in the atria facilitate efficient conduction of the impulse from the SA node to the atrioventricular (AV) node.
    • The AV node serves as the only electrical connection between the atria and ventricles, preventing impulse spread around the AV valves.

    Bundle of His and Purkinje Fibers

    • The Bundle of His, located below the AV node, conducts the impulse to the ventricles.
    • Purkinje fibers, boasting the fastest conduction velocity due to their large diameter and abundance of sodium channels and gap junctions, contribute to the efficient spread of the impulse within the ventricles.
    • Together, the Bundle of His and Purkinje fibers form the His-Purkinje system.

    Resting Membrane Potential

    • The sodium-potassium pump, an active transporter using ATP, pumps out 3 sodium ions (Na+) and brings in 2 potassium ions (K+) to maintain ionic gradients and establish a negative resting membrane potential.
    • The resting membrane potential is primarily influenced by the potassium equilibrium potential (EK) due to the high permeability of the membrane to potassium.
    • Minor changes in extracellular potassium concentration have significant effects on cardiac electrical conduction.

    Electrical Excitation of a Myocyte (Action Potential)

    • The action potential in cardiac myocytes is characterized by distinct phases driven by voltage-gated ion channels.
    • Fast sodium channels contribute to the rapid depolarization phase (phase 0) of the action potential.
    • The "m" activation gate opens rapidly in response to depolarization, while the "h" inactivation gate closes more slowly, contributing to the repolarization phase.
    • Long-lasting calcium channels (L-type) maintain the plateau phase of the action potential, increasing intracellular calcium concentration essential for excitation-contraction coupling.

    Pacemaker Activity (Slow Action Potentials)

    • Pacemaker cells, lacking fast sodium channels, rely solely on calcium currents for depolarization (phase 0) of their action potentials.
    • Phase 4 in pacemaker cells is characterized by a consistent slow depolarization until threshold, driven by the "funny" (If) current, calcium current (ICa), and potassium current (IK).

    Refractory Periods

    • The effective refractory period (ERP) represents the time during which a new action potential cannot be generated, ensuring proper synchronization of heartbeats.
    • The relative refractory period follows the ERP, during which a new action potential is possible but requires a stronger stimulus to trigger.

    Structure-Function Relationships in Cardiac Tissues

    • Cardiac tissues differ in size, shape, and number of gap junctions, resulting in varying conduction velocities and functional roles.
    • SA node and AV node tissues are characterized by small size, fewer gap junctions, and slow conduction velocities, reflecting their pacemaker function.
    • Atrial and ventricular muscle cells exhibit medium size, abundant gap junctions, and rapid conduction velocities, enabling efficient contraction and force generation.
    • His bundle and Purkinje fibers possess large size, a high degree of connectivity via many gap junctions, and very rapid conduction velocities, facilitating rapid impulse transmission throughout the ventricular network.

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    Description

    Test your knowledge on how electrical impulses are propagated in the heart. This quiz covers the roles of the SA node, AV node, Bundle of His, and Purkinje fibers, as well as the resting membrane potential and ionic transport mechanisms. Perfect for students of cardiac physiology.

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