What are the steps leading to a calcium ion entering a muscle cell?

Myosin is involved. Actin might too.

See this image: http://legacy.owensboro.kctcs.edu/gcaplan/anat/images/Image331.gif

1. A depolarization in the motor neuron starts an action potential which travels along the axon to the pre-synaptic terminal of the neuromuscular synapse.
2. The action potential causes acetylcholine filled vesicles to bond the membrane and release the Ach into the synaptic cleft.
3. The Ach diffuse and bind to receptors on the sarcolemma of the muscle cell.
4. The binding of the Ach to the receptors causes Na+ to enter the muscle cell and starts an action potential within the muscle cell.
5. The action potential travels to the SR causing it to release Ca++ ions into the cell.