Why does opening of the Ca-channels trigger the release of neurotransmitters? I understand that the opening of the Calcium channels trigger the release of stored neurotransmitter in granules, but what is it about the flow of positive ions that makes the vesicles fuse with the cell membrane? Does the vesicle itself carry a charge or concentration gradient?

Synaptotagmin is a protein that is part of the synaptic release machinery. It binds calcium specifically, which leads to a conformational change that encourages vesicle fusion with the membrane, therefore the ultimate release mechanism is not via voltage or charge but by calcium itself.