The Neuronal Organization of the Outer Plexiform Layer of the Primate Retina

In the primate retina at the level of the first synapse in the visual system, the outer plexiform layer, processes from 15 different types of neurons have so far been described. These are the synaptic spherules of rods, the pedicles of three spectral types of cones, dendrites and axons of two types of horizontal cell, dendrites of seven types of bipolar cell, processes of interplexiform cells, and the outwardly coursing dendritic extensions of biplexiform ganglion cells. The interconnections of these neurons as studied by electron microscopy and Golgi-EM are presented in a summary diagram (Fig. 27). Basal processes from cone pedicles contact the cone pedicles, and rod spherules forming gap junctions. The dendrites of both types of horizontal cell (hI and hII) connect only to cone pedicles and form lateral elements of triads at the ribbon synaptic complex. The HI axon terminals end as lateral elements at rod spherules while the axons of HII horizontal cells connect with cones in a manner similar to their dendrites. Interplexiform cells (ipc) do not contact either rod or cone synaptic endings. Rod bipolar cell (rb) dendrites end as central elements at the ribbon synaptic complex of rod spherules. The dendrites of flat midget (fm), flat top (fb), and giant bistratified bipolar (gb) cells all form basal junctions with cone pedicles. Ending as central elements of triads at cone pedicles are the dendrites of invaginating midget (im), diffuse invaginating cone (ib), and blue-cone (bb) bipolar cells. Biplexiform ganglion cells (bgc) connect to rods as central elements opposite the synaptic ribbon in the spherules. As compared to an earlier summary diagram of the outer plexiform layer (Kolb, 1970), the primate retina is now known not to be as simply organized as was once thought. Although our knowledge of the types of neurons contributing processes to this first synaptic layer, and the nature of their connections with other neurons has been broadened, especially within the past few years, this summary diagram is not intended to represent the complete or final "picture." Undoubtedly, future investigations along the lines of research outlined here will provide additional details to this wiring diagram so that we may better understand the processing of visual information by neurons in the retina.