Form Meets Function: Cellular and Functional Properties of Retinal Ganglion Cells

Retinal ganglion cells are the output cells of the retina that communicate visual information to the brain. Each cell's representation of visual space is dependent upon both the synaptic input it receives from other cells in the retina and on the cell's biophysical properties. This study is comprised of two sections that examine these factors in mouse alpha-type retinal ganglion cells. The first section addresses the dynamics of calcium regulation in the proximal dendrites of these cells. Calcium is a ubiquitous neuronal second messenger but its role in ganglion cells is unknown. Fluorescent calcium indicators and two-photon imaging were employed to determine the endogenous calcium binding ratio and the extrusion rate constant of individual cells using the "added buffer" method. These properties are variable over the population of cells studied, yet the amplitude and time constant of evoked calcium influxes are similar. The results suggest that there may be a homeostatic mechanism that regulates calcium influx, buffering and extrusion. The second section of the dissertation addresses how synaptic inputs and the dendritic arbor of a cell influence its receptive field. Spatial maps were made of the excitatory and inhibitory synaptic inputs and the spike output of ON-alpha cells by recording their responses to small spots of light flashed at various locations. A commensurable map was made of dendrite density by imaging the cell's morphology. Comparisons of these maps reveal that the location, size and relative density of the dendritic arbor of a cell are correlated with the location, size and spatial profile of its receptive field. The receptive field is also dependent on how a light-evoked signal spreads as it propagates through the retina. This spread is evident in the excitatory synaptic input to the ganglion cell. Inhibitory synaptic inputs are spatially confined and delayed in time, but their influence on the receptive field could not be determined.