(i) Sarcoplasmic reticulum: It releases calcium ions into the sarcoplasm that binds to troponin causing a change in its shape and position and thereby remove the masking of active sites for myosin.
(ii) Myosin head: The myosin head is an active ATPase enzyme and has binding sites for ATP and active sites for actin. It binds to the exposed active sites on actin to form a cross bridge.
(iii) F-actin: The actin sites on the F-actin are specific to the mysoin head for cross-bridge formation.