Which statement accurately describes the resting membrane potential of contractile cells in the heart?
Atrial cells have a resting potential of -80 mV, while ventricular cells have a resting potential of -90 mV
Both atrial and ventricular cells have a resting potential of -70 mV
Atrial cells have a resting potential of-90 mV, while ventricular cells have a resting potential of -80 mV
Both atrial and ventricular cells have a resting potential of-60 mV
The Correct Answer is A
A. Atrial cells have a resting potential of -80 mV, while ventricular cells have a resting potential of -90 mV: Cardiac contractile cells maintain a negative resting membrane potential due to selective permeability to potassium ions and the activity of the Na+/K+ ATPase pump. Atrial myocytes have a resting potential around -80 mV, while ventricular myocytes are slightly more negative at approximately -90 mV.
B. Both atrial and ventricular cells have a resting potential of -70 mV: A resting potential of -70 mV is characteristic of neurons, not cardiac contractile cells. Cardiac myocytes require a more negative resting potential to maintain proper excitability and ensure effective action potential generation for coordinated contraction.
C. Atrial cells have a resting potential of -90 mV, while ventricular cells have a resting potential of -80 mV: This reverses the actual values of atrial and ventricular cells. Ventricular cells are more polarized than atrial cells at rest due to higher potassium conductance and greater expression of inward-rectifier potassium channels.
D. Both atrial and ventricular cells have a resting potential of -60 mV: A resting potential of -60 mV is typical of pacemaker (autorhythmic) cells, such as those in the sinoatrial node, rather than contractile myocytes. Contractile cells require a more negative resting potential to maintain a stable resting state before depolarization.
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Naxlex Comprehensive Predictor Exams
Related Questions
Correct Answer is A
Explanation
A. Both ventricles: Stroke volume (SV) is the volume of blood ejected by a ventricle during a single contraction, measured from end-diastolic volume minus end-systolic volume. The left ventricle pumps blood into systemic circulation, while the right ventricle pumps blood into the pulmonary circulation, making SV a measure relevant to both ventricles.
B. Left ventricle: The left ventricle pumps oxygenated blood into the aorta for systemic circulation. While left ventricular SV is often emphasized clinically due to systemic perfusion, stroke volume is conceptually defined for both ventricles, not exclusively the left.
C. Right ventricle: The right ventricle ejects deoxygenated blood into the pulmonary trunk for gas exchange in the lungs. Like the left ventricle, it contributes to the stroke volume, so SV is not limited solely to the right ventricle.
D. Both atria: The atria function as filling chambers that contribute to ventricular preload by delivering blood through the atrioventricular valves. While atrial contraction assists ventricular filling, stroke volume specifically refers to blood ejected from the ventricles, not the atria.
Correct Answer is A
Explanation
A. It marks the boundary of the sarcomere and anchors thin filaments: The Z-line, also called the Z-disc, is a dense protein structure that defines the lateral boundary of each sarcomere in cardiac muscle. It serves as the anchoring site for actin (thin) filaments and plays a critical role in transmitting the force generated during sarcomere shortening across the myocardial tissue, contributing to coordinated contraction.
B. It shortens during contraction to generate force: The Z-line itself does not shorten; rather, the sarcomere shortens as thin and thick filaments slide past each other. The Z-lines move closer together during contraction, but they act as structural anchors, not active generators of force.
C. It houses mitochondria for ATP production: Mitochondria are located in the cytoplasm between myofibrils, not within the Z-line. They provide ATP necessary for cross-bridge cycling, but their presence is independent of the Z-line structure.
D. It serves as a storage site for calcium ions: Calcium ions are primarily stored in the sarcoplasmic reticulum and released during excitation-contraction coupling. The Z-line does not function as a calcium reservoir; its role is structural and mechanical within the sarcomere.
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