Describe how muscle cells contract

An action potential depolarises the sarcoma and depolarisation travels down the T-tubules to the sarcoplasmic reticulum, which releases Ca²⁺ (calcium ions) to the sarcoplasm. Ca²⁺ cause tropomyosin to change shape so that the binding site of actin is exposed. Myosin head binds to actin forming an actin-myosin cross bridge. Ca²⁺ promotes the hydrolysis of ATP which releases energy that causes the myosin head to bend, pulling the actin filament along in a rowing action. Another ATP molecule provides the energy to break down the cross bridge and the myosin head attaches to a different actin binding site further along the actin filament. Many cross bridges form and break very rapidly, pulling actin along. This shortens the sarcomere, which causes the muscle cells to contract.