Figure 3.

Electron micrographs of synapses between bipolar cell dendrites and cone pedicles (CP). (A) An invaginating synapse (IMB-1 in red) of an ON MB cell under the ribbon (arrowhead) flanked by two horizontal processes (H), “TA” synapses of cells FMB-1 and DB3a-2 (in yellow), and two “middle NTA” synapses of cells FMB-1 and DB3b-3 (in green) are demarcated. (B) ON DB cells form two invaginating synapses (OnD-2 and OnD-3 in red), adjacent to which are two “TA” synapses of cells DB1-2 and DB3a-2 (in yellow). Nearby and to the left is one “middle NTA” synapse of cell DB2-2 (in green). (C) Two rectangular areas are enlarged in (D,E). (D) An ON DB cell (OnD-1 in red contour) projects an invaginating dendrite immediately underneath a ribbon, just adjacent to which two dendrites (of cells DB2-3 and 3a-2 in yellow) form “TA” synapses. Four dendrites (of cells DB2-3, 2-2, 3b-3, and 2-3 in green) form “middle NTA” synapses, which are separated by two or more other processes from the ribbon but are still less than 1.5 μm from it. (E) A dendrite of cell DB2-2 (in violet) forms one “marginal NTA” synaptic contact with the marginal extension of CP32. This contact is located about 3.5 μm from the nearest ribbon of CP32 but only about 2 μm from the nearest ribbon of CP28 (arrowhead). (F) The marginal extension of CP20 is located under CP26, where two basal contacts with CP20 are much closer to the nearest ribbon area (^*) of CP26 than that of CP20. The rectangular area is enlarged in (G). (G) “Marginal NTA” synapse of cells FMB-6 and DB3b-1 with CP20. S-353 stands for section 353 etc. Bipolar letterings are abbreviated without “B.”