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

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Step 3: As new ATP attaches to the myosin head, the cross bridge detaches

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step 2: Working stroke- the myosin head pivots and bends as it pulled on the actin filament, sliding it towards the M line.

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Step 1: Myosin cross bridge attaches to the actin myofilament.

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Step 3: 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.
Step 4: Myosin binds to actin and forms cross bridges 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 potentl moves down the sarcolemma and down the T tubules.

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Step 1: The brain sends a nerve impulse through the motote unit down to a neuromuscular junction.
Step 2: Nerve impulse causes voltage gated calcium ion channels to open which allow calcium ions to enter the neuron.This increase in calcium stimulates the release of acetylcholine, which is released into the cleft.
Step 3: ACh binds to a ACh receptor site and activates voltage gated sodium channels which pump Na+ in and K+ out. More Na+ is pumped into the muscle cell then K+ is pumped out causing the cell interior to become less negative, or depolarized.
Step 4: Initial depolarization spreads to adjacent areas, opening voltage gated sodium channels and eventually initiating an action potential the moves along the lengths of the sarcolemma.
Step 5: Na+ concentrations caused by depolarizations, stimulates a change in membrane permeability by opening voltage gated K+ pumps which allow K+ to diffuse across the sarcolemma out of the muscle fiber. This is called repolarization.