Directional summation in non-direction selective retinal ganglion cells

Abstract

There is a severe gap in knowledge about how individual excitatory post synaptic potentials (EPSPs) are summed at the level of the retinal ganglion cell (RGC) dendrites. The terminals of bipolar cells, each with a distinct pattern of activity, contact both proximal and distal sites of RGC dendrites. The distal EPSPs could be at a disadvantage since their magnitude is reduced at the cell body. The attenuation of distal inputs could present a problem for RGCs, since they must encode inputs along their dendrites in an accurate and reliable manner. However, there may be mechanisms in RGCs which compensate for dendritic filtering.;The sequence of inputs along dendrites could also dictate how multiple EPSPs sum. In the retina, starburst amacrine cells (SACs) and some direction selective ganglion cells (DSGCs) show enhanced excitability in response to the centrifugal sequence of inputs. It is not known whether non-direction selective RGCs also exhibit directional summation.;In this study, I examined the integration properties of non-direction selective retinal ganglion cells. In quiescent RGCs, multiple dendritic inputs arriving close in time lead to an enhanced, supralinear EPSP summation. The RGCs sum multiple dendritic inputs independent of the distance of inputs from the soma. Their dendrites also possess a filtering mechanism that helps represent the strength of individual input uniformly across distal and proximal dendrites. When the sequence of dendritic inputs is directed away from the soma, the EPSP summation is enhanced. The actively firing RGCs are also sensitive to the timing of an additional subthreshold input. An input arriving close to a previous spike leads to the greatest increase in the instantaneous firing frequency of the firing cycle. Overall, the results from this study establish that due to their intrinsic properties, RGCS are well poised for the encoding of closely timed inputs and the detection of stimulus movement along their dendrites.