Final answer:
Tropomyosin is the regulatory protein that blocks myosin cross-bridges from binding to actin in a resting skeletal muscle, preventing contraction until calcium ions trigger a shift in position.
Step-by-step explanation:
When skeletal muscle is in its resting state, the regulatory protein that prevents myosin cross-bridges from binding to actin molecules is tropomyosin. This protein blocks the myosin binding sites on the actin filament, ensuring that the muscle does not contract without the necessary signal from the nervous system. The troponin complex helps to properly position the tropomyosin on actin and also binds calcium ions, which are essential in the regulation of muscle contraction.
During muscle contraction, calcium ions released into the sarcoplasm bind to troponin, which triggers a conformational change that causes tropomyosin to shift away from the myosin binding sites on actin. This exposes the sites and enables myosin cross-bridges to attach to actin, forming a cross-bridge and consequently leading to muscle contraction.