CABLE PROPERTIES OF DENDRITES IN HIPPOCAMPAL-NEURONS OF THE RAT MAPPED BY A VOLTAGE-SENSITIVE DYE

Dendrites of pyramidal neurons from embryonic rat hippocampus are inve stigated in culture using a voltage-sensitive fluorescent dye. The ele ctrical response to somatic stimulation is observed as a time-resolved map with a resolution of 0.9 mu m at a time constant of 0.4 ms withou t signal averaging. The data are interpreted in terms of a tapering ca ble with Hodgkin-Huxley parametrization. The spread of short hyperpola rizing transients is damped by capacitive shunting. The invasion of an action potential is boosted by voltage-gated conductances of a low de nsity. No irregularity is observed at a bifurcation. The passive cable parameters of internal resistance and membrane resistance at resting voltage are R-i = 300 Omega cm and R-m = 40 k Omega cm(2) respectively , at a maximum sodium conductance of similar to 4.4 mS/cm(2). The elec trotonic length constant and the dynamic length constant at 1 kHz are 580 and 90 mu m respectively. These results are compatible with electr ophysiological data of dendrites in slices of adult hippocampus and wi th optical data of narrow processes of leech neurons in culture. The f unctional implications of boosting an action potential by voltage-gate d channels of low density are considered.