Fig. 5.

Depiction of 26 active zones, as viewed from within the bouton, reconstructed from serial sections. The black bars correspond to the active zone of a single section (thus the thickness of the bar equals 60 nm), and theshaded circles indicate the position of docked vesicles. Note that because the section is approximately twice as thick as the vesicle diameter, the location of the vesicle within the section is unknown; all vesicles have been placed in the center of the section for illustration, but the actual vesicles would not be arranged in the neat rows as shown here. The point of representing the vesicles here is to indicate the extent to which areas of the active zone seem to be unoccupied. Many active zones could be described as round or oval, although distortions of these idealized forms are common. Two active zones were perforated. The occupancy of active zones by docked vesicles varies greatly.