In the nervous system, calcium ions crossing the gap between these cause a release of acetylcholine
[DOC] [TLE] Synaptic Transmission - Biology Encyclopedia - cells, body, function ... [PAR] Synapses are junctional complexes between presynaptic membranes ... Action 
potentials open calcium channels in the membrane of the synaptic knob, which 
causes an ... Calcium ions trigger the release of neurotransmitter from synaptic 
vesicles ... Electrical synapses, although rare in vertebrate nervous systems, do 
exist.