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Questions and Answers
What is the primary role of the sino-atrial (SA) node in the cardiac conduction system?
Which structure in the cardiac conduction system normally provides the only electrical connection between the atria and ventricles?
What contributes to the Purkinje fibers having the fastest conduction velocity among cardiac tissues?
How does the sodium-potassium pump affect the resting membrane potential of cardiac cells?
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What mechanism is primarily responsible for maintaining high intracellular K+ concentration?
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Which of the following best describes the conduction velocity of ventricular myocardium?
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What is a notable effect of digitalis (digoxin) on cardiac cells?
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What contributes to the preferential conduction of impulses from the sino-atrial node to the atrioventricular node?
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What primarily determines the final resting membrane potential (Vm)?
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Which statement best describes the equilibrium potential (EX) for an ion?
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In relation to ion conductance in myocytes, which ion channel is most conductive at rest?
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What happens to Na+ ion flow when the membrane potential (Vm) exceeds +72 mV?
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What factor primarily influences the total membrane conductance (Gm)?
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What is the primary effect of the outward potassium 'leak' current in cardiac myocytes?
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Which of the following best describes the relationship between conductance and the resting membrane potential?
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Which factor does NOT influence the direction of ion flow through an open ion channel?
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What is the primary factor that influences the resting membrane potential (Vm) across the cell?
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How much leakage of intracellular K+ ions is required to cause Vm = -70 mV at an intracellular [K+] of 120 mM?
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Which state does a fast Na+ voltage-gated channel NOT enter during its functioning?
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Which gate is responsible for inactivation in the fast Na+ voltage-gated channel?
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What has to happen for the myocyte to recover from the inactivation of Na+ channels?
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What drives the flow of Na+ ions into the cell during depolarization?
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What occurs if Vm becomes more positive than the equilibrium potential for Na+ (ENa)?
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Which aspect describes the opening of the 'm' gate in the fast Na+ voltage-gated channels?
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What occurs during the relative refractory period?
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Which cardiac tissue type has the largest diameter?
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What is the conduction velocity of medium diameter cardiac tissues?
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What is a characteristic feature of large diameter cardiac tissues?
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Which statement best describes the structure-function relationship of small diameter cardiac tissues?
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Which tissue type has the least contractile activity?
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What type of connection do small diameter cardiac tissues have?
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Which feature is least attributed to medium diameter cardiac tissues?
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What is the primary role of L-type Ca2+ channels during the cardiac action potential?
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During which phase of the action potential does the increase in IK potassium current occur?
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What contributes to the diastolic depolarization in pacemaker cells?
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Which statement about T-type Ca2+ channels is correct?
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What is the primary characteristic of phase 4 in cardiac action potentials?
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What role does the 'funny' current play in cardiac electrophysiology?
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Which phase lacks fast Na+ channels in cardiac action potentials?
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How does the IK current behave during phase 4?
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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.
