DENDRITIC INTEGRATION IN GANGLION-CELLS OF THE MUDPUPPY RETINA

Computer simulations were carried out to evaluate the influence of var ying the membrane resistance (R(m)) on the dendritic integration capac ity of three classes of ganglion cells in the mudpuppy (Necturus macul osus) retina. Three broadly different morphological classes of ganglio n cells were selected for this study and represent the range of dendri tic tree sizes found in the ganglion cell population of this species. Simulations were conducted on anatomical data obtained from cells stai ned with horseradish peroxidase; each cell was traced, using a compute r as an entry device and later converted to a compartmental (electrica l) representation of the cell. Computer-simulation analysis used a tim e-variant conductance change which was similar in waveform to light-ac tivated bipolar cell input. The simulated membrane resistance for each cell varied between 5000 and 100,000 Omega cm(2), and conductance cha nges were introduced into different regions of the soma-dendritic tree to evaluate dendritic integration efficiency. When higher values of R (m) are used, even the largest cells become electrotonically compact a nd attenuation of voltage responses is minimized from distal to soma r egions. Responses were less attenuated from proximal to distal regions of the cell because of the favorable impedance matching, and because less current is required to polarize small ''sealed'' dendritic termin ations. Steady-state responses integrate more effectively than transie nt responses, particularly when R(m) is high, since transient response s were more attenuated by the membrane capacitance. The possibility th at R(m) is a dynamic property of retinal ganglion cells is discussed i n view of the functional organization of dendritic integration efficie ncy as R(m) fluctuates from low to high values.