What triggers exposure of actin binding sites in muscle contraction?

• A. Na+ influx triggers depolarization
• B. ATP binding to myosin head
• C. Ca2+ binds troponin C, causing tropomyosin to move and expose actin binding sites
• D. K+ efflux returns to resting

Answer: (C)

Calcium ions being present in the cytosol directly regulate the exposure of the actin-binding sites. When Ca2+ binds to troponin C, it causes a conformational shift in the troponin-tropomyosin complex on the actin filament, moving tropomyosin away from the myosin-binding sites on actin. That unblocks the sites and allows energized myosin heads to attach and form cross-bridges, initiating contraction. Other steps, like Na+ entering to depolarize the membrane or K+ leaving to repolarize, start the excitation process or help reset the cell, but they don’t directly reveal the actin binding sites. ATP binding to the myosin head is needed for detachment and re-cocking of the head during the cycle, not for exposing the sites.