How is cytosolic Ca2+ removed after contraction in skeletal muscle?

• A. Only diffusion to extracellular space.
• B. Calcium remains in cytosol.
• C. SERCA pumps re-sequester Ca2+ into SR; Ca2+-ATPases extrude some Ca2+ from the cell; extrusion via transporters.
• D. Myosin heads degrade Ca2+

Answer: (C)

The key idea is how the muscle returns calcium to resting levels to end contraction. After contraction, calcium must be removed from the cytosol so the contractile machinery can relax. The primary process is active re-sequestration of Ca2+ into the sarcoplasmic reticulum by SERCA pumps, which use ATP to move Ca2+ back into the SR lumen. This rapid drop in cytosolic Ca2+ stops the activation of the contractile apparatus, allowing troponin-tropomyosin to cover the actin binding sites and relaxation to occur. In addition to this core mechanism, some Ca2+ is pumped out of the cell by Ca2+-ATPases and transported out via other membrane transporters, providing extra clearance to restore baseline Ca2+ levels. The idea that Ca2+ simply diffuses out or that myosin heads degrade Ca2+ isn’t how relaxation is achieved, and Ca2+ does not stay in the cytosol.