What is the role of calcium in synaptic transmission?

Calcium is found as an ion, Ca2+ in the extracellular space of the pre-synaptic neuron. The ions enter the neuron through voltage -gated channels after the depolarisation of the pre-synaptic neuron. This causes an influx of calcium ions which help sacs filled with neurotransmitters move to the membrane facing the synaptic cleft. These sacs fuse with the plasma membrane and exocytosis takes place, by which a chemical message is sent to the post-synaptic neuron. The calcium ions are then actively transported out of the neuron to restore the chemical gradient in time for the next depolarisation.