The depolarisation of the sarcolemma is spread throughout the muscle fibre by the T-tubules which are in contact with the sarcoplasmic reticulum (SR). The SR actively absorbs calcium ions from the sarcoplasm.Depolarisation of the SR stimulates calcium ion channels to open. Calcium ions diffuse out of the SR and into the sarcoplasm down their concentration gradient. The calcium ions bind to the troponin complex causing it to change shape and expose the myosin binding sites on the actin filaments.