Immunolabels were categorized according to their position, relative to the PSD at asymmetric synaptic junctions. Those PEG particles occurring within the cleft were categorized as being in Zone I (“at cleft” in Fig. 3). The position of the immunolabel was categorized as Zone II (“at PSD” in Fig. 3), if the immunolabel was directly over the PSD and as Zone III (“near PSD” in Fig. 3), if the immunolabel was within a distance equal to the thickness of the PSD in its plane of section (<60 nm from the postsynaptic membrane surface) but not directly over the PSD. Zones I, II, and III were combined and considered “postsynaptic.” Labeling that occurred removed from the postsynaptic membrane surface by >60 nm but still within the cytoplasm or along the plasma membrane of the spine head or neck was categorized as Zone IV and considered to be nonsynaptic. PEG particles occurring across Zones I–IV of spines were considered “total” spinous labeling, determined by counting PEG particles at “synaptic” plus “nonsynaptic” zones. Figure 3 shows our schematic of the zones used for PEG
