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Step 3: New ATP attaches to myosin head and cross bridge detaches.

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Step 1:  The action potential moves down the sarcolemma and down the t-tubules.

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Step 4: New ATP is split into ADP and P, cocking of myosin head occurs.

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step 2: The working stroke - Myosin head pivots and bends as it pulls on the actin filament, sliding it toward the M

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step 1: The myosin cross bridge attaches to the actin myofilament.

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Step 3 and 4: Ca2+ combines with troponin (protein located on actin) and removes the blocking action of tropomyosin by changing the shape of the troponin so that the myosin binding sites are exposed. The myosin then binds to the actin and forms cross bridges and contraction begins.

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Step 2: Transmission of the action potential along the t-tubules causes the adjacent sarcoplasmic reticulum to release Ca2+, which flows into the cytosol.

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The brain sends a nerve impulse through the motor unit down to a neuromuscular junction (axon terminal). The nerve impulse causes voltage gated calcium ion channels to opens which allows calcium ions to enter the neuron. This increase in calcium stimulates the release of acetylcholine which is released into the cleft. Acetylcholine binds to a receptor site and activates voltage gated Na+ ion channel which pumps Na+ ions out and K+ ions in. More Na is pumped out causing the cell interior to become depolarized. The depolarization then spreads to adjacent areas, opening voltage gated Na+ channels and eventually initiating an action potential that moves along the lengths of the sarcolemma. Lastly, Na concentrations caused by depolarization stimulate a change in membrane permeability by opening voltage gated K+ pumps which allows K to diffuse across the sarcolemma out of the muscle fiber, this is repolarization.