THE INTRINSIC ELECTROPHYSIOLOGICAL CHARACTERISTICS OF FLY LOBULA PLATE TANGENTIAL CELLS .1. PASSIVE MEMBRANE-PROPERTIES

The passive membrane properties of the tangential cells in the fly lob ula plate (CH, HS, and VS cells, Fig. 1) were determined by combining compartmental modeling and current injection experiments. As a prerequ isite, we built a digital base of the cells by 3D-reconstructing indiv idual tangential cells from cobalt-stained material including both CH cells (VCH and DCH cells), all three HS cells (HSN, HSE, and HSS cells ) and most members of the VS cell family (Figs. 2, 3). In a first seri es of experiments, hyperpolarizing and depolarizing currents were inje cted to determine steady-state I-V curves (Fig. 4). At potentials more negative than resting, a linear relationship holds, whereas at potent ials more positive than resting, an outward rectification is observed. Therefore, in all subsequent experiments, when a sinusoidal current o f variable frequency was injected, a negative DC current was superimpo sed to keep the neurons in a hyperpolarized state. The resulting ampli tude and phase spectra revealed an average steady-state input resistan ce of 4 to 5 M Omega and a cut-off frequency between 40 and 80 Hz (Fig . 5). To determine the passive membrane parameters R(m) (specific memb rane resistance), R(i) (specific internal resistivity), and C-m (speci fic membrane capacitance), the experiments were repeated in computer s imulations on compartmental models of the cells (Fig. 6). Good fits be tween experimental and simulation data were obtained for the following values: R(m)=2.5 k Omega cm(2), R(i)=60 Omega cm, and C-m=1.5 mu F/cm (2) for CH cells; R(m)=2.0 k Omega cm(2) R(i)=40 Omega cm, and C-m=0.9 mu F/cm(2) for HS cells; R(m)=2.0 k Omega cm(2), R(i)=40 Omega cm, an d C-m=0.8 mu F/cm(2) for VS cells. An error analysis of the fitting pr ocedure revealed an area of confidence in the R(m)-R(i) plane within w hich the R(m)-R(i) value pairs are still compatible with the experimen tal data given the statistical fluctuations inherent in the experiment s (Figs. 7, 8). We also investigated whether there exist characteristi c differences between different members of the same cell class and how much the exact placement of the electrode (within+/-100 mu m along th e axon) influences the result of the simulation (Fig. 9). The membrane parameters were further examined by injection of a hyperpolarizing cu rrent pulse (Fig. 10). The resulting compartmental models (Fig. 11) ba sed on the passive membrane parameters determined in this way form the basis of forthcoming studies on dendritic integration and signal prop agation in the fly tangential cells (Haag et al., 1997; Haag and Borst , 1997).