4.2 when the action potential reaches the presynaptic terminal causes an action potential depolarization by Ca ++ channel enables the presynaptic terminal and open-Ca ++ will flow around them is believed to be the Ca ++ with protein at the surface membrane of the inside of a synaptic terminal where neurotransmitter is released causing neurotransmitter vesicle membrane that contains a link to the island and includes the same content as the fuse and then opens out into the outside of the neurotransmitter is released on synaptic cleft.