Figure 1.

Functional reconstruction of synaptic boutons of Drosophila in silico. (A) Confocal image of a Drosophila larval neuromuscular junction (muscle 13) showing the synaptic terminals of two motor neurons that form type Is or Ib boutons, respectively. Release sites (green) were labeled with an antibody recognizing the active zone protein Bruchpilot (Wagh et al., 2006). The axonal surface (red) was labeled by anti-HRP immunoreactivity. Right panel: magnification of representative Is and Ib boutons (arrow and arrowhead) displaying an average diameter of 2 and 3 μm, respectively. Scale bars: 5 μm (left) 1.5 μm (right). (B–D) Release sites (green) and the vesicular diffusion area (red) are represented in silico by tetrahedral volume grids (Bastian et al., 2000). Release sites are positioned according to nearest neighborhood laws derived from ultrastructural data (Meinertzhagen et al., 1998; Sigrist et al., 2003). The bouton center is free of vesicles (Denker et al., 2009). (E,F) Simulation of vesicle dynamics in a Ib bouton (3 μm) during 30 Hz or 60 Hz stimulation (P_o = 7%) with initial vesicle density of 275 μm⁻³. Encoded in color is the spatially resolved vesicle density (in vesicles/μm³) at the indicated stimulus number (599–601). Note that 60 Hz-stimulation rapidly depletes most release sites and their immediate surroundings of releasable vesicles (white) while more distant areas maintain high vesicle densities (red).