It had been hypothesized that the release of four calcium ions is necessary for the release of one acetylcholine packet (which is necessary for the neuromuscular communication taking place at the neuromuscular junction), and thus that calcium has a cooperative impact on neuromuscular transmissions (Dodge & Rahamimoff, 1967). Specifically, these researchers noted a logarithmic scale that described the relationship between calcium ion concentration and the level of neuromuscular activity taking place during a given activation cycle that suggests the fourth power of calcium concentration is equal to action at the junction (Dodge & Rahamimoff, 1967). Later research into the same basic phenomenon has yielded still greater understanding of the mechanism by which calcium achieves its purposed ends in neuromuscular transmission. Augustine and Charlton (1986) have shown that the cooperative...

Despite ongoing research into calcium's role in neural transmission, the exact mechanisms of neural transmissions remain a matter of some guesswork.
References

Augustine, G. & Charlton, M. (1986). Calcium dependence of presynaptic calcium current and post-synaptic response at the squid giant synapse. The Journal of Physiology 381: 619-40.

Dodge, F. & Rahamimoff, R. (1967). Co-operative action of calcium ions in transmitter release at the neuromuscular junction. The Journal of Physiology 193: 419-32.

Katz, B. & Miledi, R. (1968). The…