A nerve impulse in the sarcoplasmic reticulum causes calcium ions to move out of it into the muscle cells where it binds to tropmyosin/troponin causing it to move and reveal the actin filament. The myosin can then bind to the actin. The myosin head bends pulling the actin along with it via the hydrolysis of ATP attached to the head as it releases energy. Calcium aids with the attachment of ATP to the myosin head causing it to detach so it can bind again.This process takes place as long calcium ions are present due to the innervation by an action potential in the SR; when it stops the sliding filament theory stops.
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