Electric stimulation of the musculocutaneous nerve causes a rapid twitch of the biceps muscle. Electron microscopy shows numerous vesicles within the musculocutaneous nerve terminals. In order for these vesicles to release their contents into the synaptic cleft, which of the following substances is required?
1) Sodium
2) Calcium
3) cAMP
4) IP3
5) E. DAG
6) F. cGMP