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step 2: ADP & P are released causing the myosin head to pivot. The change in shape results in the actin filament to be pulled towards the M line.

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Step 3: The ATP attaches to myosin weakening the bond between the myosin and actin. The myosin head then detaches

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Step 4: ATP is hydrolyzed to the ADP & P. Myosin head returns to the prestroke state or cocked position.

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Step 3 and 4: Ca2+ then combines with troponin & removes the blocking action of tropomyosin by changing the shape of the actin.  The myosin binding sites are exposed. Myosin the binds to actin to form 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+. This then flows into the cytosol.

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step 1: The myosin head attaches to an actin myofilament which creates the cross bridge.

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

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A nerve impulse is sent down the axon terminal to the neuromuscular junction.  The nerve impulse causes voltage gated calcium ion channels to open, allowing calcium ions to enter the neuron. Acetycholine is then released into the cleft.  ACh then binds to a receptor site and activates the voltage gated sodium channel. This pumps Na+ ions in and K+ ions out which causes it to depolarize. Depolarization spreads which causes voltage gated sodium channels to open which will then initiate action potential that travels down the sarcolemma.  A change in the membrane permeability is caused by Na+ concentrations.  This opens the voltage gated K+ ion channels which will allow K+ ions to diffuse across the sarcolemma.  This process is repolarization.