Passive electrical cable properties and synaptic excitation of tiger salamander retinal ganglion cells

The passive electrical properties of 17 ON-OFF retinal ganglion cells were derived from electrophysiological recordings. The parameters for each cells' equivalent model were obtained from the transient current responses to small step changes in clamp potential. Thirteen of the cells could be adequately approximated by a spherical soma connected to an equivalent dendritic cable. Estimates for the cell input conductance (G_N), membrane time constant (τ_m), the dendritic-to-soma conductance ratio (ρ), and the normalized electrotonic length (L) were obtained (mean ± standard deviation, n = 13): G_N = 580 ± 530 pS, τ_m = 91 ± 72 ms, ρ = 2.8 ± 2.8, and L = 0.34 ± 0.13. Series resistance averaged 32 ± 11 MΩ. The mean of the derived soma diameters was 18 ± 6 μm and the mean diameter and length of the equivalent cables were 1.4 ± 0.6 and 470 ± 90 μm, respectively. The average of the specific membrane conductances, 1.67 ± 1.08 S/cm², corresponded to a membrane resistivity of 60 kΩ·cm². Computer simulations of synaptic inputs were performed on a representative model, with an electrode at the soma and using the worst-case configuration, in which all synaptic inputs were confined to the tips of the dendrites. We draw three conclusions from the modeling: (1) Under voltage clamp, fast, spontaneous EPSCs would be significantly attenuated and slowed while the time course of the slower, light-evoked non-NMDA and NMDA EPSCs would be minimally distorted by dendritic filtering. (2) Excitatory synaptic reversal potentials can be accurately determined under voltage clamp. (3) In the absence of GABAergic and glycinergic inhibition, the efficacy at the soma of excitatory conductance changes is essentially independent of their dendritic location. The specific membrane resistivity appears to represent a good compromise between having a small membrane time constant and minimal EPSP attenuation.