Final answer:
Myosin binds tightly to actin when it has bound ADP, during cross-bridge formation and power stroke in muscle contraction. ATP binds to myosin to release the bond post-power stroke, allowing the muscle to contract and relax in cyclic sequences.
Step-by-step explanation:
Myosin tightly interacts with actin in the muscle fiber when it has bound a molecule of ADP. This interaction occurs during the cross-bridge formation when the myosin head attaches to the actin with ADP and inorganic phosphate (Pi) still attached. Following the power stroke, ADP is released, creating a strong bond between the actin and myosin, and then another ATP molecule binds to myosin to release this bond, allowing the muscle to enter another cycle of contraction and relaxation.
The role of ATP is crucial as it provides energy for the muscle contraction process. Initially, ATP binds to myosin and is hydrolyzed to ADP and Pi, energizing the myosin head. The energized myosin head then binds to actin, forming a cross-bridge and is considered 'cocked' in a high-energy state. As the power stroke occurs, the myosin head pivots, pulling the actin filament towards the M line, which causes muscle contraction. Following the power stroke, ADP is released from the myosin, although the myosin-actin complex is still intact until a new ATP molecule binds to myosin, leading to the dissociation of the myosin from actin, and this releases the energy stored, priming the muscle for the next contraction.