Actin and myosin can only bind when exposed to which ion?

Actin and myosin are the key proteins involved in muscle contraction. For these two proteins to interact and form cross-bridges, which is essential for muscle contraction to occur, the presence of calcium ions is crucial. When calcium is released within muscle fibers, it binds to troponin, a regulatory protein associated with actin. This binding causes a conformational change that moves tropomyosin away from the binding sites on actin filaments, allowing myosin heads to attach to the actin.

Without calcium, the actin binding sites remain blocked by tropomyosin, preventing myosin from binding to actin and thus inhibiting muscle contraction. Therefore, in the context of muscle physiology and the role of calcium in initiating contraction, it is clear why calcium is the essential ion needed for actin and myosin to bind. This mechanism is fundamental in the process of muscle contraction in both cardiac and skeletal muscles.