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Step 4: As ATP is hydrolyzed to ADP and Pi, the myosin head returns to its prestroke, high-energy position.

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Step 3: After ATP attaches to myosin, the link between myosin and actin weakens, and the myosin head detaches.

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step 2: ADP and Pi are released and the myosin head pivots and bends, changing to its bent, low-energy state. As a result, it pulls the actin filament to the M line.

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step 1: An energized myosin head attaches to an actin myofilament, forming a cross bridge.

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Step 3 and 4: Ca2+ combines with troponin and removes the blocking action of tropomyosin by changing the shape of the actin so that the myosin binding sites are exposed. The myosin then binds to action, forms a cross-bridge, and contraction begins.

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Step 2: transmission of the action potential along the  T-tubules cause the adjacent sarcoplasmic reticulum to release Ca2+ which flows into the cytosol

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Step 1: the action potential moves down the sarcolemma and down the T-tubules

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The brain sends an action potential through the motor unit down to a neuromuscular junction.
This impulse causes voltage-gated Ca+ channels to open and allows calcium ions to flood into the neuron. This then stimulates the release of acetylcholine into the cleft.
Acetylcholine then binds to a receptor site and activates voltage-gated Na+ channels which pumps Na+ ions in and K+ ions out.
Voltage-gated sodium channels  open and initiate action potential that moves along the lengths of the sarcolemma.
Na+ concentrations caused by depolarization stimulate a change in membrane permeability by opening voltage-gated K+ pumps.
This allows K+ to diffuse across the sarcolemma, also known as repolarization.