Fig. 7. Hypothetical vesicle cycle diagram showing possible roles of the proteins synapsin II and Rab3a in the preparatory steps for the release process.

A. Cartoon showing the organization of docked vesicles around bands of active zone material (AZM) observed by electron tomography (Nagwaney et al., 2009). The diagram shows “primary” vesicles (gray) enclosed between the bands of AZM and “secondary” vesicles observed at the periphery of AZM. Most likely, primary vesicles would have higher release probabilities being in the immediate vicinity of Ca²⁺ channel clusters.

B. A hypothetical model proposing that upon interacting with AZM, a vesicle can be either docked as “primary”, or as “secondary”, or fused with the plasma membrane releasing neurotransmitter. Rab3a promotes vesicle targeting to AZM and docking. Synapsin II positively regulates the pathway that diverts the vesicles from other pathways to become docked as “secondary”.