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Actin is covered in troponin and tropomyosin proteins, preventing myosin heads from binding normally.
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When the action potential arrives at the sarcomere, calcium ions are released from the sarcoplasmic reticulum.
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These then bind to the troponin, which changes the shape (non-competitive inhibition ) and moves the tropomyosin away from the binding sites.
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Now myosin heads are able to form cross-bridges.
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The myosin head then bends, bringing the actin with it. This shortens the sarcomere (which is contraction).
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Since this is now a stable structure, ATP attaches to the myosin head to remove the cross-bridge and so return it to the original position (hydrolysing the ATP into ADP + P).
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If Calcium ions are still present, the myosin head can attach further along and contract again.
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The contraction is (apparently) called a Power Stroke.
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