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A comprehensive exam on cardiac electrophysiology, focusing on the ventricular action potential. it delves into the ionic mechanisms underlying each phase of the action potential, including depolarization, plateau, and repolarization. the questions cover key concepts such as ion channel function, action potential duration, and the role of various ion currents in maintaining cardiac rhythm. the exam is valuable for students studying cardiac electrophysiology, providing a thorough assessment of their understanding of fundamental principles.
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1.Which phase of the ventricular myocyte action potential is characterized by rapid depolarization due to the opening of fast sodium channels? A. Phase 0 B. Phase 1 C. Phase 2 D. Phase 3 Correct Answer: A Explanation: Phase 0 represents the rapid upstroke of the action potential, mainly due to the influx of Na⁺ through fast sodium channels. 2.During which phase of the ventricular action potential is the plateau maintained primarily by the balance of inward Ca²⁺ current and outward K⁺ current? A. Phase 0 B. Phase 1 C. Phase 2 D. Phase 3 Correct Answer: C Explanation: Phase 2 is the plateau phase where L-type calcium channels allow Ca²⁺ influx, balanced by K⁺ efflux. 3.Which ion channel is primarily responsible for the repolarization phase (Phase
Explanation: In contrast to ventricular myocytes, SA node cells rely on Ca²⁺ influx via L-type calcium channels for the upstroke. 6.Which of the following best describes the role of the T-type calcium channel in cardiac pacemaker cells? A. Initiates rapid depolarization B. Contributes to early repolarization C. Participates in the late diastolic depolarization D. Is responsible for the plateau phase Correct Answer: C Explanation: T-type calcium channels contribute to the pacemaker potential by aiding the late diastolic depolarization before the threshold is reached. 7.What is the primary ionic mechanism underlying the effective refractory period in cardiac cells? A. Inactivation of sodium channels B. Activation of potassium channels C. Deactivation of calcium channels D. Persistent sodium influx Correct Answer: A Explanation: The effective refractory period is largely due to the inactivation of fast sodium channels, preventing premature re-excitation. 8.Which of the following best explains the term “action potential duration (APD)” in cardiac cells? A. The time from resting potential to peak depolarization B. The total time from the start of depolarization to complete repolarization C. The duration of the refractory period only D. The interval between successive action potentials Correct Answer: B Explanation: APD is defined as the time from the beginning of the depolarization phase until the cell repolarizes completely. 9.In a ventricular action potential, the “notch” observed at the beginning of repolarization (Phase 1) is due to the transient outward potassium current. What is this current often designated as? A. I_Na B. I_CaT C. I_to D. I_K Correct Answer: C Explanation: I_to, the transient outward potassium current, is responsible for the early repolarization “notch” in the action potential. 10.The term “afterdepolarizations” refers to abnormal depolarizations that occur during or after repolarization. Which type is associated with early afterdepolarizations (EADs)? A. Occur during Phase 0 B. Occur during Phase 2 or 3
C. Ion channels are only activated chemically D. Channel function is independent of the membrane potential Correct Answer: B Explanation: Voltage-dependent channels change their probability of being open or closed based on the membrane potential. 16.Which of the following best explains the term “ion channel gating”? A. The rate at which ions diffuse through the channel B. The process by which channels open or close in response to stimuli C. The electrical gradient across the cell membrane D. The binding of ions to the channel pore Correct Answer: B Explanation: Gating is the process of opening and closing of ion channels, controlled by voltage, ligands, or mechanical forces. 17.Which phase of the cardiac action potential is most sensitive to the effects of Class I antiarrhythmic drugs? A. Phase 0 B. Phase 1 C. Phase 2 D. Phase 4 Correct Answer: A Explanation: Class I antiarrhythmic drugs block sodium channels and primarily affect Phase 0, the rapid depolarization phase. 18.The concept of “charge density” in electrophysiology refers to: A. The amount of electrical charge per unit area on a cell membrane B. The total number of ions in the cell C. The speed of the action potential propagation D. The threshold potential for depolarization Correct Answer: A Explanation: Charge density describes how much electrical charge exists per unit area of the cell membrane, influencing excitability. 19.Which of the following changes would most likely decrease the conduction velocity in myocardial tissue? A. Increased gap junction density B. Enhanced sodium channel availability C. Fibrosis leading to cell uncoupling D. Increased resting membrane potential Correct Answer: C Explanation: Fibrosis disrupts cell-to-cell coupling, reducing conduction velocity and promoting arrhythmogenic substrates. 20.In terms of electrophysiological properties, what is meant by “source–sink mismatch”? A. An imbalance between depolarizing current (source) and the load of adjacent tissue (sink) B. A mismatch in calcium and sodium concentrations
C. A discrepancy between action potential amplitude and duration D. A difference between intracellular and extracellular ion levels Correct Answer: A Explanation: Source–sink mismatch refers to the balance between the current generated by an active cell (source) and the electrical load of the neighboring cells (sink), critical for successful impulse propagation. 21.Which current is predominantly responsible for the “funny” pacemaker activity in SA nodal cells? A. I_CaL B. I_Na C. I_f D. I_Kr Correct Answer: C Explanation: The I_f current, carried by hyperpolarization-activated cyclic nucleotide–gated channels, is responsible for pacemaker activity in the SA node. 22.What is the effect of increasing extracellular potassium concentration ([K⁺]o) on the resting membrane potential of cardiac myocytes? A. Hyperpolarization B. Depolarization C. No change D. Initial depolarization followed by hyperpolarization Correct Answer: B Explanation: Elevated [K⁺]o reduces the gradient for K⁺ efflux, causing depolarization of the resting membrane potential. 23.Which factor primarily determines the threshold potential in cardiac cells? A. The density of potassium channels B. The availability of sodium channels C. The resting membrane potential D. The magnitude of the inward calcium current Correct Answer: C Explanation: The threshold potential is the membrane voltage at which a sufficient number of sodium channels activate; it is largely determined by the resting membrane potential relative to channel activation properties. 24.In ventricular myocytes, which of the following most significantly contributes to the “plateau” phase? A. A balance between inward Ca²⁺ and outward K⁺ currents B. Rapid activation of sodium channels C. Sustained activation of potassium channels only D. The activity of chloride channels Correct Answer: A Explanation: The plateau is maintained by a balance between the inward L-type Ca²⁺ current and outward K⁺ currents. 25.Which property of cardiac tissue best describes “anisotropy”? A. Uniform conduction in all directions
A. Phase 0 B. Phase 3 C. Phase 4 D. Phase 2 Correct Answer: C Explanation: Phase 4, the diastolic depolarization, is modulated by autonomic input (sympathetic and parasympathetic) altering the slope of the pacemaker potential. 31.Which of the following best explains “electromechanical coupling” in cardiac muscle? A. The conversion of electrical signals into mechanical contraction B. The process of electrical signal propagation C. The generation of the action potential D. The synchronization of pacemaker cells Correct Answer: A Explanation: Electromechanical coupling refers to the mechanism whereby electrical excitation (action potentials) triggers mechanical contraction of the heart muscle. 32.Which of these statements about calcium-induced calcium release (CICR) in cardiac myocytes is correct? A. It refers to the release of Ca²⁺ from the nucleus B. It is the mechanism where Ca²⁺ influx triggers further Ca²⁺ release from the sarcoplasmic reticulum C. It only occurs in pacemaker cells D. It is independent of L-type Ca²⁺ channels Correct Answer: B Explanation: CICR is a process in which the entry of Ca²⁺ through L-type Ca²⁺ channels triggers a larger release of Ca²⁺ from the sarcoplasmic reticulum, essential for contraction. 33.Which channel is responsible for the initial rapid depolarization in Purkinje fibers? A. L-type calcium channels B. Fast sodium channels C. T-type calcium channels D. Inward rectifier potassium channels Correct Answer: B Explanation: Purkinje fibers rely on fast sodium channels for the rapid depolarization phase of the action potential. 34.During which phase of the action potential is the cell most refractory to a new stimulus? A. Phase 0 B. Early Phase 3 C. Late Phase 4 D. Plateau phase Correct Answer: B Explanation: Early Phase 3 is within the effective refractory period when most sodium channels are inactivated, preventing a new action potential.
35.In a normal ventricular action potential, which phase corresponds with rapid repolarization? A. Phase 0 B. Phase 1 C. Phase 2 D. Phase 4 Correct Answer: B Explanation: Phase 1 is characterized by a brief rapid repolarization due to the transient outward potassium current. 36.What effect does a mutation that slows the inactivation of the sodium channel have on the action potential? A. Shortened action potential duration B. Prolonged depolarization and increased risk of arrhythmia C. No effect on conduction D. Enhanced resting potential stability Correct Answer: B Explanation: Slowed inactivation of sodium channels can prolong the depolarization phase, potentially leading to afterdepolarizations and arrhythmogenesis. 37.Which of the following best describes “phase 4” in ventricular myocytes? A. Rapid depolarization B. Repolarization C. Resting membrane potential D. Plateau phase Correct Answer: C Explanation: Phase 4 represents the resting membrane potential in ventricular myocytes, where the cell is at rest until the next action potential. 38.The term “excitation–contraction coupling” in cardiac cells primarily refers to the relationship between: A. Ion channel gating and gap junctions B. Electrical activation and the initiation of contraction C. Calcium influx and sodium efflux D. Action potential generation and repolarization Correct Answer: B Explanation: Excitation–contraction coupling is the process by which an action potential triggers Ca²⁺ release, leading to myocardial contraction. 39.Which of the following factors does NOT directly influence the duration of the cardiac action potential? A. Ion channel kinetics B. Membrane capacitance C. Intercellular coupling D. Ion gradients across the membrane Correct Answer: C Explanation: While intercellular coupling affects conduction, the intrinsic duration of
B. Enhanced automaticity C. Early afterdepolarizations D. Delayed afterdepolarizations Correct Answer: D Explanation: Digitalis toxicity often leads to calcium overload in cardiac cells, resulting in DADs that can trigger arrhythmias. 45.In reentrant arrhythmias, the “wavelength” of the reentrant circuit is determined by: A. The size of the heart B. The product of conduction velocity and refractory period C. The density of pacemaker cells D. The strength of the depolarizing current Correct Answer: B Explanation: The reentrant circuit wavelength is the product of conduction velocity and refractory period; a short wavelength facilitates reentry. 46.Which of the following is a typical substrate for reentrant arrhythmias? A. Uniform tissue with no scar B. Areas of fibrosis and heterogeneous conduction C. Increased sodium channel density D. Enhanced gap junction connectivity Correct Answer: B Explanation: Fibrosis and heterogeneous conduction create areas of slow conduction and unidirectional block, which are substrates for reentry. 47.Triggered activity due to early afterdepolarizations is most often precipitated by: A. Hyperkalemia B. Hypokalemia C. Hypercalcemia D. Increased vagal tone Correct Answer: B Explanation: Hypokalemia can prolong repolarization and predispose to early afterdepolarizations, thereby triggering arrhythmias. 48.Which condition is most commonly associated with increased automaticity in cardiac cells? A. Ischemia B. Hypoxia C. Acidosis D. All of the above Correct Answer: D Explanation: Ischemia, hypoxia, and acidosis can all enhance automaticity by destabilizing membrane potentials and promoting spontaneous depolarizations. 49.“Triggered activity” in cardiac tissue refers to arrhythmias that are: A. Caused by reentry circuits only B. Initiated by abnormal afterdepolarizations C. Due solely to enhanced automaticity
D. A direct result of increased conduction velocity Correct Answer: B Explanation: Triggered activity is the result of afterdepolarizations (either early or delayed) that reach threshold and trigger premature action potentials. 50.Which of the following interventions is most likely to suppress arrhythmias due to DADs? A. Beta-adrenergic stimulation B. Calcium channel blockade C. Sodium channel blockade D. Potassium channel activation Correct Answer: B Explanation: Calcium channel blockers can reduce intracellular Ca²⁺ overload and thereby decrease the likelihood of DADs. 51.Which of the following describes “unidirectional block” in the context of reentrant arrhythmias? A. A block that prevents conduction in both directions B. A block that prevents conduction in one direction while allowing it in the opposite direction C. A block that shortens the refractory period D. A block that increases conduction velocity Correct Answer: B Explanation: Unidirectional block occurs when an impulse can travel in one direction but is blocked in the reverse direction, which is essential for reentry to occur. 52.The term “functional block” in arrhythmogenesis refers to: A. A fixed anatomical barrier B. A temporary conduction block due to refractoriness C. An irreversible conduction defect D. An area of enhanced conduction Correct Answer: B Explanation: A functional block is a transient conduction block created by regions of refractory tissue, not by permanent anatomical obstacles. 53.Which of the following is the primary mechanism by which ischemia predisposes to arrhythmias? A. Increased repolarization reserve B. Enhanced conduction velocity C. Heterogeneous electrical conduction and altered resting potentials D. Uniform prolongation of the action potential Correct Answer: C Explanation: Ischemia leads to metabolic disturbances that cause heterogeneous conduction, altered resting potentials, and regional differences in refractoriness, all predisposing to arrhythmia. 54.Which electrolyte abnormality is most likely to precipitate torsades de pointes via early afterdepolarizations?
the action potential duration and refractory period, thus suppressing reentrant arrhythmias. 59.Which mechanism is most commonly implicated in atrial fibrillation initiation? A. Early afterdepolarizations B. Delayed afterdepolarizations C. Focal ectopic activity and reentry D. Enhanced gap junction coupling Correct Answer: C Explanation: Atrial fibrillation often results from focal ectopic discharges (triggered activity) that initiate reentrant circuits in a vulnerable atrial substrate. 60.The term “electrical remodeling” in arrhythmogenesis refers to: A. Structural changes in the heart B. Changes in ion channel expression and function in response to sustained arrhythmia C. Enhanced conduction via gap junctions D. Permanent loss of myocardial tissue Correct Answer: B Explanation: Electrical remodeling involves alterations in ion channel density, kinetics, and function, which can be induced by persistent arrhythmias and alter the substrate for future arrhythmias. 61.Which structure in the heart is known as the “pacemaker” due to its highest intrinsic firing rate? A. Atrioventricular (AV) node B. Bundle of His C. Sinoatrial (SA) node D. Purkinje fibers Correct Answer: C Explanation: The SA node has the highest intrinsic firing rate and sets the pace for the heart’s rhythm. 62.Anisotropic conduction in cardiac tissue is primarily due to which of the following? A. Homogeneous ion channel distribution B. Uniform cell size C. The orientation of myocardial fibers and gap junction distribution D. Consistent refractory periods Correct Answer: C Explanation: Anisotropy is due to the directional alignment of myocardial fibers and the variable distribution of gap junctions, leading to different conduction velocities in different directions. 63.Which of the following statements best describes “electrotonic interaction” in cardiac tissue? A. The active propagation of an action potential
B. Passive spread of voltage changes between cells through gap junctions C. The process of repolarization via potassium channels D. The formation of reentrant circuits Correct Answer: B Explanation: Electrotonic interactions involve the passive spread of electrical current between adjacent cells via gap junctions, influencing excitability and conduction. 64.Which of the following factors can influence conduction velocity in cardiac tissue? A. Gap junction density B. Cell size C. Action potential upstroke velocity D. All of the above Correct Answer: D Explanation: Conduction velocity is affected by multiple factors including gap junction density, cell size, and the rapidity of phase 0 depolarization. 65.The term “safety factor” in conduction refers to: A. The minimal number of ion channels required for depolarization B. The ratio of available depolarizing current to the threshold current needed to excite adjacent tissue C. The duration of the refractory period D. The amplitude of the action potential Correct Answer: B Explanation: The safety factor is the ratio of the available depolarizing current (source) to the current required to bring neighboring cells to threshold (sink), ensuring reliable conduction. 66.In cardiac tissue, a decrease in gap junctional conductance would most likely result in: A. Increased conduction velocity B. Decreased conduction velocity C. Unchanged conduction velocity D. Enhanced automaticity Correct Answer: B Explanation: Reduced gap junction conductance impairs electrical coupling between cells, slowing conduction. 67.Which region of the heart is most susceptible to conduction block due to its complex fiber orientation? A. The left ventricle B. The right atrium C. The atrioventricular (AV) node region D. The ventricular apex Correct Answer: C Explanation: The AV node region has complex fiber orientation and slower conduction properties, making it more susceptible to conduction block.
73.Which property of Purkinje fibers contributes to their role in rapid conduction? A. Low gap junction density B. High sodium channel density and fast conduction C. Slow depolarization D. Enhanced automaticity Correct Answer: B Explanation: Purkinje fibers have a high density of fast sodium channels, facilitating rapid conduction throughout the ventricular myocardium. 74.“Heterogeneity of refractoriness” in the heart can predispose to: A. Uniform conduction B. Reentrant arrhythmias C. Decreased automaticity D. Shortened conduction intervals Correct Answer: B Explanation: Variability in refractory periods among different regions of the heart creates conditions favorable for reentry. 75.Which of the following best describes “conduction velocity restitution”? A. The relationship between conduction velocity and tissue refractoriness B. The recovery of conduction velocity after an arrhythmia C. The effect of increased gap junction density on conduction D. The influence of heart rate on the duration of the action potential Correct Answer: A Explanation: Conduction velocity restitution refers to how conduction velocity changes as a function of the diastolic interval (or tissue recovery), which can influence arrhythmia dynamics. 76.In healthy cardiac tissue, what is the approximate conduction velocity in the Purkinje system? A. 0.1 m/s B. 0.5 m/s C. 2-4 m/s D. 5-10 m/s Correct Answer: C Explanation: Conduction velocity in the Purkinje system is generally in the range of 2–4 m/s, significantly faster than in the working myocardium. 77.Which ion channel contributes significantly to the steep upstroke of the action potential in working myocardium? A. L-type calcium channels B. T-type calcium channels C. Fast sodium channels D. Delayed rectifier potassium channels Correct Answer: C Explanation: Fast sodium channels are responsible for the rapid depolarization (Phase 0) in working myocardial cells.
78.In the context of conduction, the “sink” refers to: A. The region generating the action potential B. The region that receives the depolarizing current C. The refractory tissue D. The area of fastest conduction Correct Answer: B Explanation: The “sink” is the tissue that is being depolarized by the current flowing from the “source” region. 79.A reduction in sodium channel density in cardiac tissue would most likely result in: A. Increased conduction velocity B. Slowed conduction velocity C. Enhanced repolarization D. Decreased refractory period Correct Answer: B Explanation: Reduced sodium channel density decreases the upstroke velocity, slowing conduction across the myocardium. 80.Which of the following best characterizes “electrical heterogeneity” in the heart? A. Uniform action potential durations across all regions B. Variation in action potential durations and conduction properties across different regions C. Consistent refractory periods D. Homogeneous distribution of ion channels Correct Answer: B Explanation: Electrical heterogeneity refers to differences in electrophysiological properties (such as APD and conduction velocity) between different regions, which can contribute to arrhythmogenesis. 81.In atrial tissue, compared to ventricular tissue, the action potential duration is generally: A. Longer B. Shorter C. The same D. More variable Correct Answer: B Explanation: Atrial myocytes typically have a shorter action potential duration compared to ventricular myocytes. 82.Which of the following is most likely to enhance conduction in the heart? A. Reduced gap junction expression B. Increased cell-to-cell coupling C. Increased fibrosis D. Decreased sodium channel function Correct Answer: B Explanation: Enhanced cell-to-cell coupling (e.g., through increased gap junction expression) improves conduction velocity.
88.“Decremental conduction” is a phenomenon most commonly associated with which tissue? A. Atrial myocardium B. Ventricular myocardium C. Atrioventricular (AV) node D. Purkinje fibers Correct Answer: C Explanation: The AV node exhibits decremental conduction, meaning that increased stimulation rates result in progressively slower conduction. 89.Which of the following best explains the importance of the “refractory period” in preventing arrhythmias? A. It enhances the action potential amplitude B. It prevents premature re-excitation of cardiac tissue C. It increases conduction velocity D. It shortens the action potential duration Correct Answer: B Explanation: The refractory period prevents premature stimulation of cardiac cells, thereby reducing the risk of arrhythmia development. 90.In the context of conduction, what is meant by “source–sink mismatch”? A. A balance between depolarizing and repolarizing currents B. An imbalance where the source current is insufficient to excite the sink C. A condition of excessive conduction velocity D. The equal distribution of ion channels Correct Answer: B Explanation: Source–sink mismatch occurs when the depolarizing current (source) cannot adequately overcome the electrical load (sink) of the adjacent tissue, leading to conduction block. 91.Which factor most significantly contributes to the initiation of reentrant arrhythmias in structurally abnormal hearts? A. Uniform conduction B. Increased conduction velocity C. Scar formation and fibrosis D. Enhanced sodium channel density Correct Answer: C Explanation: Scar tissue and fibrosis create regions of slow or blocked conduction that are critical for reentrant circuits to develop. 92.Which property is most critical in determining the success of impulse propagation in cardiac tissue? A. The amplitude of the action potential B. The duration of the refractory period C. The balance between the depolarizing source and the excitability of the sink D. The density of L-type calcium channels Correct Answer: C Explanation: Successful impulse propagation depends on the depolarizing current (source) being sufficient to bring the adjacent tissue (sink) to threshold.
93.In terms of tissue excitability, which statement is true? A. A lower threshold for activation leads to decreased excitability B. A higher threshold for activation leads to increased excitability C. A lower threshold for activation leads to increased excitability D. The threshold does not affect excitability Correct Answer: C Explanation: Increased excitability means that a lower current is needed to reach the threshold for action potential generation. 94.Which of the following best describes “conduction heterogeneity” in the context of arrhythmogenesis? A. Uniform conduction across the myocardium B. Variability in conduction properties that can lead to localized conduction block C. Increased gap junction density throughout the heart D. Homogeneous ion channel expression Correct Answer: B Explanation: Conduction heterogeneity refers to regional differences in conduction velocity and refractory periods, which can predispose to arrhythmias by creating conduction blocks. 95.Which electrophysiological parameter is most directly affected by the density of gap junctions? A. Action potential amplitude B. Conduction velocity C. Resting membrane potential D. Calcium transient amplitude Correct Answer: B Explanation: The density and function of gap junctions directly affect the conduction velocity between cardiac cells. 96.In a reentrant circuit, reducing the refractory period would likely lead to: A. A shorter circuit wavelength and a higher propensity for reentry B. A longer circuit wavelength and reduced reentry C. No change in the reentrant circuit D. Complete termination of the reentry Correct Answer: A Explanation: A shorter refractory period reduces the wavelength (conduction velocity × refractory period), making it easier for a reentrant circuit to sustain itself. 97.Which of the following best describes the “pace‐mapping” technique used in electrophysiology studies? A. Recording spontaneous electrical activity from the SA node B. Delivering pacing stimuli at various sites to match the morphology of clinical arrhythmia C. Measuring the conduction velocity along the Purkinje fibers D. Assessing the refractory period of myocardial tissue Correct Answer: B Explanation: Pace‐mapping involves pacing from different endocardial sites and