ANALYSIS OF VOLTAGE-DEPENDENT MEMBRANE CURRENTS IN SPATIALLY EXTENDED NEURONS FROM POINT-CLAMP DATA

1. Numerical methods were used to evaluate voltage space-clamp performance in the investigation of a voltage-dependent inward current similar to the noninactivating Ca current. In addition, the cell is equipped with a repolarizing system, represented by leak and outwardly rectifying outward conductances. The electrotonically compact model cell is represented by a cable with an electrotonic length of 1 space constant under control conditions, but that becomes effectively only 0.33 space constants during a 90% reduction of the leak and outward conductance. The cable is perfectly voltage clamped at one end. 2. The apparent voltage dependence, activation, and inactivation of the clamp current depend on the distribution of the membrane slope conductance along the cable; this depends on 1) the distribution of the inward current along the cable and 2) the amplitude of the inward current relative to the amplitudes of the leak and voltage-dependent outward currents. 3. Under control conditions, the membrane voltage decays steeply with distance from the command voltage at the clamp site to almost resting potential for most of the rest of the cable. This is because the leak and outward current are dominant over the inward current. The inward current is activated primarily at the clamped part of the cable. Clamp currents are activated instantaneously. The clamp-current current-voltage (I-V) relation is less steep with depolarization because the membrane potential for locations away from the clamp site lags behind the clamp potential. 4. When the conductances for leak and outward current are reduced by 90%, these conductances lose their dominance. The membrane slope conductance now has a range with negative values. In consequence, a large part of the membrane depolarizes beyond the command potential, resulting in a slower apparent activation of membrane and clamp currents. The clamp-current I-V relation is steeper than the inward current I-V. Depolarization beyond the potential of maximal inward current flow results in an apparent current inactivation. 5. With a more distal localization of the inward conductance, the membrane depolarization and current flow becomes maximal around the conductance site, and apparent activation of the clamp current is further delayed. 6. Quantitative reduction of all membrane currents allows for control of membrane slope conductance and improves space-clamp performance. The effects on the clamp currents give information on the localization of the inward current. 7. We conclude that when the Na current is blocked, space-clamp errors are considerable in compact cables but not obvious in current recordings. Our simulations demonstrate the range of possible interpretations of voltage-clamp studies on nonspherical cells including localization of inhomogenously distributed membrane currents. The validity and the useful range of voltage-clamp application can be extended through quantitative experimental control of membrane conductances.