Fig. 5. Schematic of a rod photoreceptor terminal in the mammalian retina. The left drawing is perpendicular to the ribbon, the right drawing parallel to the ribbon. Presynaptically, a long active zone docks about 130 vesicles (yellow) and the extensive ribbon tethers about 770 vesicles. Postsynaptically, four processes (two horizontal cell and two bipolar cell [b]) occupy the invagination; the horizontal cell receptors (white speckling) arch parallel to the active zone and thus always lie near (16 nm) the docking sites; the bipolar cell receptors (white speckling distributed over entire surface) lie far (130 to 640 nm) from the docking sites. The mouth of the invagination is exaggerated because parts of the rod were cut away for clarity. (Drawing and description from Rao-Mirotznik R, Harkins A, Buchsbaum G, Sterling P: Mammalian rod terminal: architecture of a binary synapse. Neuron 14:561, 1995, with permission of the authors and Cell Press)