A 35-yeаr-оld mаle is stаbilized оn a medicatiоn that targets presynaptic neurotransmission kinetics. During a clinical review of basic neurobiology, the psychiatric clinician maps out the exact chronological sequence of events required for a classical monoamine neurotransmitter to be released from the presynaptic axon terminal into the synaptic cleft. Which of the following options correctly identifies the immediate cellular event that occurs directly after an action potential depolarizes the presynaptic terminal membrane, but before exocytosis of the neurotransmitter vesicles takes place?