During muscle contraction, what happens after calcium is released from the terminal cisternae?

During muscle contraction, once calcium is released from the terminal cisternae of the sarcoplasmic reticulum, it plays a crucial role by binding to troponin. This binding of calcium to troponin induces a conformational change in the troponin complex, which is part of the thin filament of the muscle fiber. As a result, this change alters the position of tropomyosin, another regulatory protein, allowing the myosin heads on the thick filaments to interact with actin, which is essential for muscle contraction to occur. This process is key in enabling the cross-bridge cycle, leading to muscle shortening and contraction.

In understanding this process, it’s important to note that myosin heads do not directly bind calcium; rather, their action is mediated through the changes initiated by calcium binding to troponin. Additionally, once contraction is complete and calcium levels need to return to baseline, calcium is actively pumped back into the sarcoplasmic reticulum or out of the cell, rather than remaining unchanged or moving into the extracellular space immediately after its release.