What is the first step of the contraction cycle involving myosin?

The initial step of the contraction cycle involving myosin indeed begins with the binding of ATP to the myosin head. When ATP binds to the myosin head, it causes a conformational change that leads to the release of myosin from the actin filaments. This is crucial because it allows the myosin head to reset and prepare for the next cycle of muscle contraction.

Once ATP is bound, it is hydrolyzed to ADP and inorganic phosphate. This process energizes the myosin head and puts it in a position to bind to actin. Without the initial binding of ATP, the myosin head would remain tightly bound to actin, which can lead to muscle stiffness or rigor mortis due to the lack of a fresh ATP supply to allow for muscle relaxation and subsequent contractions.

The other options all represent steps that follow the binding of ATP. For instance, the release of ADP corresponds to the power stroke, where the myosin head pulls actin towards the center of the sarcomere, contributing to muscle contraction. The cycle is highly sequential, with each step depending on the previous one functioning properly, starting from the crucial ATP binding to the myosin head.