Detailed compartmental models of starburst amacrine cells in the rabbit retina

Purpose. To evaluate the integrative properties of dendrites in starburst amacrine cells of the rabbit retina and the limitations which soma-applied, single electrode voltage-clamping places on this analysis. Methods. A starburst amacrine cell was stained with an intracellular injection of HRP, subsequently traced with the Eutectic Neuronal Tracing System (ENTS), and imported into the NEURON simulation package (Hines, M., 1993). To provide a biologically relevant range, the dendritic diameters were varied from 0.1 - 0.3 μm and the membrane resistance (R_m) was varied from 4,000 to 100,000 Ω cm². Both passive and active membrane models were developed with the latter consisting of five non-linear ion channels required to support low repetitive firing. Results. Over a large range of membrane resistance values and dendritic tree diameters, conductance changes delivered to peripheral dendritic branches caused a high degree of polarization of the injected dendritic tree, but much greater attenuation into neighboring dendrites which emerged from the soma. The presence of impulse activity generated in dendritic branches resulted in a rectifying flow of impulse activity. Impulses readily propagated toward the periphery, but typically failed as they propagated toward the soma. Voltage-clamping from the soma was unsuccessful for most regions of the dendritic structure and could not control impulse generation even when the impulse was fairly close to the soma. Conclusions. When high values for R_m are used to model starburst amacrine cells, the dendritic branches are electrotonically compact. Despite these short electrotonic distances, activity generated in one dendrite does not effectively spread to neighboring dendrites because of impedance mismatching that is evident in the soma.