The role of Ca2+ ions in muscle contraction are :

i) Stored calcium ions are released by the sarcoplasmic reticulum which binds into the specific sites of the troponin component of the thin filament.

ii) This binding causes a change in conformation of the troponin molecule and the active sites which are present on the F-actin molecules become exposed.

iii) The active sites of F-actin are specific to myosin head which have ATPase activity dependent on Mg2+ .

iv) Ca2+ is pumped back to the sarcoplasmic reticulum during the relaxation of the muscles.

v) This causes the masking of active sites of myosin head by the troponin component.

vi) The muscles get relaxed due to the breaking of cross-bridge and thin filaments occupying their normal position.

The above figure represents the role of calcium ions in the process of muscle contraction and muscle relaxation.