The cross bridge cycle is a series of molecular events that occur after excitation of the sarcolemma. what is a cross bridge?
- atp bound to a myosin head
- calcium bound to troponin
- a myosin head bound to actin
- troponin bound to tropomyosin

The correct answer is C: a myosin head bound to actin.

Explanation:

The cross bridge cycle is a series of events that occur during muscle contraction. When muscle contracts, a myosin head (projected from a thick myosin filament) attaches to a thin actin filament (this is the so-called cross bridge). The thin filament also contains tropomyosin and troponin.

Then, a second myosin head binds to ATP and catalyzes it in presence of calcium. This step gives the muscle the energy to contract.

Without calcium, tropomyosin and troponin avoid myosin and actin to interact.

correct answer is a. and d.