Explain the processes that lead to the contraction of a single muscle

A nervous impulse travels to the neuromuscular junction and causes the release of acetylcholine into the synaptic cleft. Depolarisation of the motor end plate spreads through the muscle via transverse tubules which are invaginations of the sarcolemma. Once the wave of depolarisation has reached the sarcoplasmic reticulum calcium ions are released into the sarcoplasm as calcium ion protein channels are opened. These ions bind to troponin, a protein bound to tropomyosin which blocks the attachment site for the myosin heads. This binding action causes troponin to change shape and thus tropomyosin detaches from the attachment site located on the actin filament. The myosin heads are then free to attach to this site on the actin and an actin-myosin cross bridge forms. The breakdown of ATP into ADP and Pi provides the energy required for the myosin heads to pull the actin filaments inwards which shortens the length of the muscle as the Z lines begin to move closer together. ATP is then reformed and attaches to the myosin head causing it to the detach from the actin attachment site. The actin filaments then return to their original position and then contraction has ended. When the contraction stops and there is no stimulus, calcium ions are actively transported back into the sarcoplasmic reticulum.