How do skeletal muscles contract ?

A motor neuron stimulates the sarcoplasmic reticulum, causing it to release Calcium ions. The calcium ions binds to the toponin that is blocking the myosin binding site on the acting filament, the binding of the calcium causes toponin to change its shape and it becomes tropomyosin, this exposes the myosin binding site on the actin. Once the site is exposed the myosin head binds to the actin forming a cross bridge. ATP binds to the myosin head causing the cross bridge to break. ATP hydrolysis causes the myosin heads to change position and swivel, moving them towards the next actin binding site, this allows the myosin head to bind to other regions on the actin. The myosin drags the actin towards the centre causing the sacromere to shorten. This sliding or the two filaments allows the muscle to contact.