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step 2: ADP and Pi are released and the myosin head pivots, changing to its low energy bent state. This pulls the actin filament toward the M line.

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

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

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step 1: The energized myosin head attaches to an actin microfilament, forming the cross bridge.

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Step 3 and 4: Calcium combines with troponin which removes the blocking protein of tropomyosin. This exposes the myosin binding site. The myosin binds to actin forming a cross bridge. Contractions will now start.

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Step 2: The transmission of the AP in the T tubules cause the adjacent sarcoplasmic reticulum to release calcium into the cytosol.

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Step 1:  The AP will move down the sarcolemma and down the T tubules which is a deep protrusion in the muscle cell.

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An action potential is sent to a neuromuscular junction. This causes the calcium ion channels to open and calcium ions will enter the neuron. This stimulates the release of the neurotransmitter acetylcholine, which leave the axon terminal. The voltage gated sodium channel open pumping Na+ in and K+ out. The cell interior in now depolarized. The depolarization moves down the sarcolemma. The sodium concentrations cause the opening of voltage gated K+ pumps. K+ diffuse across the sarcolemma which is called repolarization.