Final answer:
Muscle contraction is initiated when calcium ions bind to troponin, leading to the exposure of myosin-binding sites on actin and allowing cross-bridge formation. Troponin-tropomyosin complex movement and ATP are essential for this process.
Step-by-step explanation:
To initiate muscle contraction, the troponin-tropomyosin complex must be moved to expose the myosin-binding sites on the actin filaments. This is achieved when calcium ions (Ca2+) bind to troponin, which causes a conformational change in the troponin-tropomyosin complex. Subsequently, tropomyosin slides away from the binding sites, allowing the myosin heads to bind to the actin and form cross-bridges. These cross-bridges are crucial for the contraction cycle, as this enables the myosin heads to pull the actin filaments and shorten the muscle cell. It is important to note that ATP is also needed for muscle contraction; after the power stroke, ATP attaches to the myosin head, causing it to detach from actin and allowing for a new cycle of contraction.