Answer:
Calcium ions, ATP
Step-by-step 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.