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Here is what happens in detail. The process of a muscle contracting can be divided into 5 sections:
A nervous impulse arrives at the neuromuscular junction, which causes a release of a chemical called Acetylcholine. The presence of Acetylcholine causes the depolarisation of the motor end plate which travels throughout the muscle by the transverse tubules, causing Calcium (Ca++) to be released from the sarcoplasmic reticulum.
In the presence of high concentrations of Ca++ the Ca++ binds to Troponin, changing its shape and so moving Tropomyosin from the active site of the Actin. The Myosin filaments can now attach to the Actin, forming a cross-bridge.
The breakdown of ATP releases energy which enables the Myosin to pull the Actin filaments inwards and so shortening the muscle. This occurs along the entire length of every myofibril in the muscle cell.
The Myosin detaches from the Actin and the cross-bridge is broken when an ATP molecule binds to the Myosin head. When the ATP is then broken down the Myosin head can again attach to an Actin binding site further along the Actin filament and repeat the 'power stroke'. This repeated pulling of the Actin over the myosin is often known as the ratchet mechanism.
This process of muscular contraction can last for as long as there is adequate ATP and Ca++ stores. Once the impulse stops the Ca++ is pumped back to the Sarcoplasmic Reticulum and the Actin returns to its resting position causing the muscle to lengthen and relax.
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