The answer is step 3. Nerve impulse on a muscle cell induces release of calcium ions
Steps of a muscle contraction are:
1. Nerve impulse travels down the axon and causes a release of acetylcholine.
2. Acetylcholine binds to the receptor of a muscle cell.
3. Nerve impulse on a muscle cell induces a release of calcium ions.
4. Calcium ions induce exposing of myosin to actin.
5. Sliding of actin filaments past myosin filaments.
6. The muscle contract.