Presynaptic active zone (site of vesicle fusion)
Figure 5.11  "A receptor's eye view of neurotransmitter release
(a)   Extracellular surface of the active zone at the frog neuromuscular junction.  The particles are believed to be calcium channels. (b) The presynaptic terminal has been stimulated to release neurotransmitter. The exocytotic fusion pores are where synaptic vesicles have fused with the presynaptic membrane and released their contents. (Heuser and Reese, 1973)

Schematic figure of active zone from the neuromuscular junction