What defines the boundaries of a sarcomere in a cardiomyocyte?
Z-line to Z-line
A-line to A-line
I-band to I-band
M-line to M-line
The Correct Answer is A
A. Z-line to Z-line: A sarcomere, the functional contractile unit of a cardiomyocyte, is defined as the region between two adjacent Z-lines (or Z-discs). The Z-lines anchor the thin filaments (actin) and mark the lateral boundaries of the sarcomere. During contraction, the sarcomere shortens as actin and myosin filaments slide past each other, generating tension and ultimately producing myocardial contraction.
B. A-line to A-line: The A-band corresponds to the length of the thick filaments (myosin) within a sarcomere and does not represent the full functional unit. It includes both overlapping regions with thin filaments and the central H-zone, but A-band boundaries do not define sarcomere length.
C. I-band to I-band: The I-band contains only thin filaments and is bisected by the Z-line. It shortens during contraction, but its boundaries alone do not encompass the entire sarcomere, making it an incomplete reference for the sarcomere’s limits.
D. M-line to M-line: The M-line lies at the center of the sarcomere, anchoring thick filaments. While it is important for structural integrity, using M-line to M-line does not define the full sarcomere; it represents only the midpoint rather than the full functional contractile unit.
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Naxlex Comprehensive Predictor Exams
Related Questions
Correct Answer is B
Explanation
Correct answer: False
Afterload refers to the resistance the ventricles must overcome to eject blood into the aorta and pulmonary artery during systole. Vasoconstriction of blood vessels increases systemic vascular resistance, which raises afterload and makes it more difficult for the heart to pump blood effectively. Conversely, vasodilation decreases vascular resistance, lowering afterload and reducing cardiac workload. Anatomically, the aorta and systemic arteries are the primary sites where afterload is influenced, and physiologically, increased afterload can lead to higher ventricular pressure, reduced stroke volume, and increased myocardial oxygen demand.
Correct Answer is A
Explanation
A. It binds and stores oxygen for aerobic metabolism: Myoglobin is an oxygen-binding protein located within the cytoplasm of cardiac and skeletal muscle cells. It serves as an intracellular oxygen reservoir, facilitating rapid oxygen delivery to mitochondria during periods of high metabolic demand. This supports sustained aerobic metabolism and continuous ATP production necessary for cardiac contraction.
B. It transports glucose into heart cells: Glucose transport into cardiomyocytes is mediated by glucose transporters (GLUT1 and GLUT4), not by myoglobin. Myoglobin’s role is specifically related to oxygen handling, not nutrient transport.
C. It generates electrical impulses for contraction: Electrical impulses in the heart are generated by pacemaker cells in the sinoatrial node and conducted through the cardiac conduction system. Myoglobin has no role in depolarization or action potential propagation.
D. It breaks down fatty acids into ATP: Fatty acid oxidation occurs in mitochondria through beta-oxidation, producing ATP. Myoglobin does not catalyze this process; its primary function is oxygen storage and delivery to support mitochondrial metabolism.
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