Final answer:
The Ca2+ ATPase pump removes Ca2+ from the cytosol of muscle cells after contraction by transporting ions back into the SR, using ATP energy. This causes muscle relaxation by preventing actin and myosin interaction. A low concentration of Ca2+ remains to maintain muscle tone.
Step-by-step explanation:
The Ca2+ ATPase pump plays a critical role in the relaxation of muscle fibers after contraction. During contraction, calcium ions (Ca2+) are released from the sarcoplasmic reticulum (SR) into the sarcoplasm, which allows the actin and myosin in muscle cells to interact and generate contraction. After contraction, it's essential to remove the Ca2+ from the cytosol to allow the muscles to relax. The Ca2+ ATPase pump actively transports Ca2+ ions back into the SR, utilizing energy from ATP hydrolysis.
This reduction in cytosolic Ca2+ concentration allows tropomyosin to reshield the actin-binding sites, thus preventing the myosin heads from attaching to actin, which stops the muscle contraction. However, a low concentration of Ca2+ remains, maintaining muscle tone and keeping muscles slightly contracted, important for certain physiological functions. This process is crucial for the regulation of muscle contraction and relaxation cycles.