Final answer:
Calcium ions bind to troponin and allow crossbridges to form in milliseconds, facilitating the muscle contraction process.
Step-by-step explanation:
The time it takes for calcium ions (Ca++) to bind to troponin and allow cross bridges to form between myosin and actin in muscle contraction is on the order of milliseconds. Once calcium binds to troponin, it induces a conformational change in tropomyosin, revealing the myosin binding sites on actin and enabling the myosin heads to attach and initiate the power stroke of muscle contraction. It is a rapid process that is essential for muscle function, including the heart muscle where timing is critical for proper cardiac function. This allows the myosin heads to bind to the actin and form cross bridges.
Therefore, the correct answer is option a. Milliseconds.