Which structures attach, rotate, detach, and reattach during muscle contraction?

During muscle contraction, the myosin crossbridges play a crucial role in the process known as the sliding filament theory. These crossbridges are the projections of the myosin filaments and they attach to specific sites on the actin filaments during contraction.

When a muscle is stimulated to contract, calcium ions are released within the muscle fibers, allowing myosin heads to bind to actin, forming crossbridges. This attachment is critical as it initiates the power stroke, where the myosin pulls the actin filaments toward the center of the sarcomere. Following this, ATP is used to detach the myosin heads from the actin, and the cycle can repeat. Each cycle involves attachment, rotation (due to the power stroke), detachment, and reattachment as long as calcium ions and ATP are present.

In this context, the correct answer highlights the specific mechanism of myosin crossbridges in facilitating muscle contraction, differentiating them from other structures like actin filaments, which primarily serve as the substrate that myosin interacts with, muscle fibers that encompass the whole muscle, and neurons that are responsible for sending the signals to initiate the contraction but do not participate directly in the mechanical process of contraction itself.