Detection of inhomogeneities in membrane ohmic resistance in geometricallycomplex systems

DC field-evoked transients in arbitrary shaped neurons and syncytia were an alyzed theoretically. In systems with homogeneous passive membrane properti es the transients develop much faster than membrane discharges. The conduct ion of the proximal membrane could be larger due to the injury imposed by s harp electrode impalement. In this case, the transients have overshoot and undershoot when the field is switched on and off. The overshoot and undersh oot decay with the time-constant of the response to the current injection. If conductance of the distal membrane is larger, then the fast transients d evelop only partially and have slow tails that decay according to the time- constant of the response to current injection. We recorded DC field-evoked potentials in motoneurons in turtle spinal cord slices by sharp electrodes and in the whole-cell mode. All three theoretically predicted types of resp onses were observed. We have found that sharp electrodes do not impose shun t in 60% of recorded cells. Detection of various membrane inhomogeneities i n 1D-syncythium is discussed. We suppose also that it is possible to detect the inhomogeneities in intercellular resistance of the syncytium and intra cellular resistance of a neuron when the membrane passive properties are ho mogeneous.