How is the sliding filament model activated?

• A. With Calcium (Ca2+) ions only
• B. With ATP and Calcium (Ca2+) ions
• C. With ATP energy alone
• D. With Magnesium (Mg2+) and ATP

Answer: (B)

Activation of the sliding filament mechanism requires both calcium ions and ATP. Calcium ions released into the cytosol bind to troponin C, causing tropomyosin to move off the active sites on actin. With those sites exposed, myosin heads can attach to actin and form cross-bridges. ATP provides the energy for the cross-bridge cycle: its hydrolysis energizes the myosin head for the power stroke and its later binding allows detachment and re-cocking for another cycle. As long as Ca2+ stays elevated and ATP is available, cross-bridge cycling continues and the filaments slide past one another. Without calcium, the binding sites remain blocked and contraction can’t start; without ATP, the cycle cannot complete because the myosin heads cannot detach and reset.