THE SYNAPTIC COMPLEX OF CONES IN THE FOVEA AND IN THE PERIPHERY OF THE MACAQUE MONKEY RETINA

Parallel pathways for visual information processing start at the first synapse of the retina, at the cone pedicle. At least eight different types of bipolar cells receive direct synaptic input from an individua l cone. The present study explores whether enough synaptic sites are a vailable at the cone pedicle to supply all these bipolar cells. Monkey retinae were optimally fixed for electron microscopy. Serial horizont al sections were cut through the cone pedicle layer in a piece close t o the fovea (eccentricity: 0.75 mm) and in a peripheral piece (eccentr icity: 5 - 6 mm). The ribbon synapses (triads) at the cone pedicle bas e were analysed. The average number of synaptic ribbons per cone pedic le increased from 21.4 +/- 1.6 (n = 26) in central retina to 41.8 +/- 3 (n = 14) in peripheral retina. Five central and five peripheral pedi cles were reconstructed and the invaginating bipolar cell dendrites fo rming the central elements of the triads were characterized. Close to the fovea an average of 18 invaginating bipolar cell dendrites was fou nd, in peripheral retina the average was 90. Pedicles of foveal cones have one invaginating central process per ribbon, pedicles of peripher al cones have two. It is possible that midget bipolar cell dendrites o ccupy the majority of triads in the fovea, while in peripheral retina both midget and diffuse bipolar cell dendrites share the triads. Copyr ight (C) 1996 Elsevier Science Ltd.