What is the significance of the Z-line in cardiac muscle contraction?
It marks the boundary of the sarcomere and anchors thin filaments.
It shortens during contraction to generate force.
It houses mitochondria for ATP production.
It serves as a storage site for calcium ions.
The Correct Answer is A
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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Related Questions
Correct Answer is D
Explanation
A. It is caused only by infections: Heart failure has multiple causes, including chronic hypertension, coronary artery disease, myocardial infarction, valvular disorders, and cardiomyopathies. While infections such as myocarditis can contribute, they are not the sole cause.
B. It always results in a heart attack: Heart failure is a condition of impaired cardiac function, whereas a heart attack (myocardial infarction) is an acute event caused by coronary artery blockage. Heart failure may develop after a heart attack, but it does not always result from one.
C. It does not affect blood circulation: Heart failure directly impairs the heart’s ability to pump blood effectively, reducing cardiac output and often causing fluid buildup in the lungs and systemic circulation. Circulatory compromise is a hallmark of the condition.
D. It is a condition where the heart can't pump blood properly: Heart failure occurs when the myocardium cannot generate sufficient force to maintain adequate blood flow to meet the body’s metabolic demands. This can involve systolic dysfunction (impaired contraction), diastolic dysfunction (impaired filling), or a combination, leading to fluid retention, congestion, and reduced tissue perfusion.
Correct Answer is C
Explanation
A. The time during which the heart is at rest: This describes diastole, a single phase of the cardiac cycle when the heart muscles relax and chambers fill with blood. While diastole is part of the cardiac cycle, it does not encompass the entire sequence of events from one heartbeat to the next.
B. The flow of blood through the body: This refers to systemic and pulmonary circulation, which are physiological processes involving blood movement but do not specifically define the cardiac cycle. The cardiac cycle is the sequence of mechanical and pressure events that drives this flow rather than the flow itself.
C. The series of pressure changes that occur within the heart from the beginning of one heartbeat to the beginning of the next: The cardiac cycle includes atrial and ventricular systole and diastole, with corresponding pressure and volume changes in all four chambers. These events coordinate valve opening and closing, blood ejection, and filling, ensuring efficient cardiac output with each heartbeat.
D. The electrical impulses of the heart: Electrical impulses, generated by the sinoatrial node and conducted through the conduction system, trigger the mechanical events of the cardiac cycle. While essential for initiating contraction, they represent only the electrical component and do not define the full sequence of pressure and volume changes that constitute the cardiac cycle.
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