Measurement of nonuniform current densities and current kinetics in Aplysia neurons using a large patch method

A large patch electrode was used to measure local currents from the cell bodies of Aplysia neurons that were voltage-clamped by a two-microelectrode method. Patch currents recorded at the soma cap, antipodal to the origin of the axon, and whole-cell currents were recorded simultaneously and normalized to membrane capacitance. The patch electrode could be reused and moved to different locations which allowed currents from adjacent patches on a single cell to be compared. The results show that the current density at the soma cap is smaller than the average current density in the cell body for three components of membrane current: the inward Na current (INa), the delayed outward current (Iout), and the transient outward current (IA). Of these three classes of ionic currents, IA is found to reach the highest relative density at the soma cap. Current density varies between adjacent patches on the same cell, suggesting that ion channels occur in clusters. The kinetics of Iout, and on rare occasions IA, were also found to vary between patches. Possible sources of error inherent to this combination of voltage clamp techniques were identified and the maximum amplitudes of the errors estimated. Procedures necessary to reduce errors to acceptable levels are described in an appendix.