A physician-validated, board-style question from the Active Transport QBank. Try it, then check the reasoning for every option.
A scientist observes a myocyte beating in cell culture. Which step is the most direct necessary component of relaxation for this cell?
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A
Influx of sodium ionsIncorrect. Sodium influx initiates depolarization (phase 0) and triggers contraction-related events, not relaxation.
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B
Influx of calcium ions from the sacroplasmic reticulumIncorrect. Calcium release from the sarcoplasmic reticulum (CICR) raises cytosolic calcium and initiates contraction, not relaxation.
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C
Influx of calcium ions from outside the myocyteIncorrect. Calcium influx from outside the cell (through L-type Ca channels) triggers CICR and contraction, not relaxation.
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D
Efflux of calcium ionsCorrect. relaxation requires removal of cytosolic calcium via SERCA (back into the SR) and Na+/Ca2+ exchanger and PMCA (out of the cell), allowing troponin C to release and cross-bridges to detach.
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E
Influx of potassium ions through delayed-rectifier channelsIncorrect. Potassium efflux through delayed-rectifier channels drives repolarization during phase 3 of the action potential and is part of electrical recovery, not the direct calcium-removal step required for myofilament relaxation; relaxation depends specifically on calcium efflux from the cytosol via SERCA, NCX, and PMCA.
↑ Tap an answer to reveal the reasoning
Answer: D. Cardiac myocyte contraction is initiated by calcium influx through L-type voltage-gated calcium channels (during the action potential plateau), which triggers calcium-induced calcium release from the sarcoplasmic reticulum (via ryanodine receptors). The cytosolic calcium binds troponin C, allowing cross-bridge cycling between actin and myosin.
Relaxation requires removal of cytosolic calcium so that troponin C releases tropomyosin's block on actin and cross-bridges detach. The most direct necessary component of relaxation is efflux of calcium from the cytosol. This occurs via two main routes: (1) SERCA (sarco/endoplasmic reticulum Ca2+-ATPase) pumps calcium back into the sarcoplasmic reticulum, and (2) the sarcolemmal Na+/Ca2+ exchanger (NCX) and a plasma membrane Ca2+ ATPase extrude calcium across the cell membrane.
The distractors describe steps of excitation/contraction rather than relaxation: sodium influx initiates depolarization (phase 0); calcium influx from the sarcoplasmic reticulum and from outside the myocyte initiates contraction (phase 2 plateau and CICR), the opposite of relaxation. Pearl: cardiac myocyte relaxation = cytosolic calcium removed by SERCA (back into SR) and NCX/PMCA (out of cell); SERCA function is the rate-limiting step and is regulated by phospholamban.
**Why each option:**
**A.** Sodium influx initiates depolarization (phase 0) and triggers contraction-related events, not relaxation.
**B.** Calcium release from the sarcoplasmic reticulum (CICR) raises cytosolic calcium and initiates contraction, not relaxation.
**C.** Calcium influx from outside the cell (through L-type Ca channels) triggers CICR and contraction, not relaxation.
**D.** Correct: relaxation requires removal of cytosolic calcium via SERCA (back into the SR) and Na+/Ca2+ exchanger and PMCA (out of the cell), allowing troponin C to release and cross-bridges to detach.