What principle of cardiac conduction is demonstrated when embryonic heart cells synchronize in a petri dish?
All cells contract independently unless stimulated by an external force.
The fastest cell controls the rhythm by spreading its impulse to slower cells.
Slower cells override faster cells to establish a uniform contraction rate.
The presence of electrical impulses forces all cells to contract at different rates.
The Correct Answer is B
A. All cells contract independently unless stimulated by an external force: Cardiac myocytes possess intrinsic automaticity, allowing them to generate action potentials spontaneously. However, when coupled electrically, they do not remain completely independent; they influence each other’s activity through gap junctions.
B. The fastest cell controls the rhythm by spreading its impulse to slower cells: In a network of electrically coupled cardiac cells, the cell with the highest intrinsic firing rate (typically pacemaker-like cells) sets the rhythm. Its action potential propagates through gap junctions to slower cells, synchronizing contractions across the tissue. This phenomenon demonstrates the principle of overdrive suppression and explains how the sinoatrial node establishes the dominant heart rate in vivo.
C. Slower cells override faster cells to establish a uniform contraction rate: Slower cells cannot override faster pacemaking cells because electrical impulses flow from faster to slower cells, not the reverse. Slower cells follow the rhythm set by the fastest pacemaker.
D. The presence of electrical impulses forces all cells to contract at different rates: Coupled cardiac cells do not maintain different contraction rates when electrically connected; instead, the fastest cell entrains the others, producing a synchronized contraction, which is essential for coordinated cardiac pumping.
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Related Questions
Correct Answer is {"dropdown-group-1":"B"}
Explanation
A. Aortic: The aortic valve is a semilunar valve located between the left ventricle and the aorta. It opens during ventricular systole to allow blood ejection into the systemic circulation and closes during diastole to prevent backflow. Its closure produces the S2 heart sound, not S1.
B. Atrioventricular: The atrioventricular (AV) valves, comprising the tricuspid and mitral (bicuspid) valves, are located between the atria and ventricles. During ventricular contraction (systole), these valves close to prevent backflow into the atria. The closure of the AV valves generates the first heart sound (S1), commonly described as "lub," marking the onset of systole.
C. Semilunar: The semilunar valves, including the aortic and pulmonary valves, prevent backflow from the arteries into the ventricles. Their closure occurs at the end of ventricular systole and produces the second heart sound (S2), which is distinct from the "lub" of S1.
D. Pulmonary: The pulmonary valve is another semilunar valve located between the right ventricle and the pulmonary artery. It opens during right ventricular systole and closes during diastole. Its closure contributes to the S2 heart sound rather than S1.
Correct Answer is D
Explanation
A. The sodium ion channels open, allowing Na+ to enter the cell: Opening of sodium channels occurs during the depolarization phase of cardiac action potentials, not repolarization. The influx of Na+ rapidly raises the membrane potential, initiating the action potential and triggering subsequent calcium influx for contraction.
B. The calcium ion channels open, allowing Ca2+ to enter the cell: Calcium channels open primarily during the plateau phase (phase 2) of the cardiac action potential, allowing Ca2+ entry to sustain contraction. This occurs before repolarization and contributes to excitation-contraction coupling rather than the return to resting membrane potential.
C. Sodium ion channels close, preventing Na+ from entering: Sodium channel closure occurs immediately after depolarization and contributes to the refractory period, but it does not itself drive repolarization. The cell requires potassium efflux to restore the resting membrane potential.
D. Potassium ion channels open, allowing K+ to leave the cell: During repolarization (phase 3) of cardiac conducting cells, voltage-gated potassium channels open, permitting K+ to exit the cytoplasm. The outward movement of positively charged K+ restores the negative resting membrane potential, terminating the action potential and preparing the cell for the next depolarization.
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