released from the sarcoplasmic reticulum bind to troponin to trigger exposure of binding sites on actin and begin the contraction cycle. supplies energy to detach the myosin head from actin.

Explanation:

1. Calcium ions are released in the response of a stimuli from the sarcoplasmic reticulum ( a special type of ER found only in muscle cells) which then bind to troponin.

2.  Binding of calcium ions to troponin to trigger exposure of binding sites on actin and in the rsult it begin the contraction cycle of muscle.

3. Actin and myosis are the main components of  muscle contraction.

4.  ATP  supplies energy to detach the myosin head from the actin molecule.