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Figure 1. Foveal S-coneterminals express more than 20 active zones and contact more than 30 S-cone ON bipolar cell dendrites provided by four or five bipolar cells. In these stereo reconstructions of the synaptic surfaces of two foveal S cones, each cluster of yellow spheres indicate the site where a synaptic ribbon anchored to the presynaptic membrane. Thus, each cluster marks an active zone (numbered) where synaptic vesicles were released. Each active zone was presynaptic to a triad of postsynaptic processes, but the regions occupied by horizontal cell spines were excised (see Materials and Methods). Here, we show only the central processes (i.e., the ON bipolar cell dendrites) as colored patches. A, Terminal 35 contained 24 active zones that were presynaptic to 31 central elements from four different S-cone ON bipolar cells. Each color codes for a different bipolar cell, with each bipolar cell providing the number of central elements indicated in Figure 2 (see also Herr et al., 2003, their Table 2A). B, Terminal 52 (modified from Herr et al., 2003, their Fig. 9B) contained 25 active zones that were presynaptic to 43 central elements from five different S-cone ON bipolar cells, with numbers provided in Figure 2. Of five (orange) triad-associated contacts, 1, 2, 1, and 1 are provided by the bipolar cells colored green, blue, yellow, and cyan, respectively; two of these contacts are visible in this view.
