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Step 3:
Then there is cross bridge detachment. This is when ATP attached to myosin head causing the cross bridge to detach

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Step 4:
The next step is clocking of myosin head. This is where energy from hydrolysis of ATP clocks myosin head into an energized state.

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step 2:
The power stroke is when the myosin head pivots and pulls actin filament toward the M line

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step 1:
The cross bridge formation is high energy myosin heads attaching to actin myofilaments forming a cross bridge cycle.

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Step 3 and 4:
Calcium combines with troponin and removes the blocking of tropomyosin. It's does this by changing the shape of actin so the myosin binding site are exposed. It then binds to actin and forms the cross bridge cycle and contraction starts.

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Step 2:
This causes the adjacent sarcoplasmic reticulum to release calcium which flows into the cytosol.

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Step 1:
Action potential moves down the sarcolemma down the t tubule.

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The brain sends a nerve impulse also known as action potential through the motor unit. The nerve impulse causes the voltage-gated calcium ion channels to enter. The increase in the calcium stimulates releasing the neurotransmitter, acetycholine (Ach). Ach is then let go into the cleft. Which then activates the voltage-gated Na+ ion channel. This channel pumps more Na+ ions in than K+ ions out which makes the cell depolarized. When the voltage-gated NA+ channel opens it eventually initiates and action potential that moves along the sarcolemma.The Na+ concentrations caused by depolarization stimulate a change by opening voltage-gated K+ ion channels which allows the K+ to diffuse. This is called repolarization.