Detection of inhomogeneities in membrane ohmic resistance in geometrically complex systems

DC field-evoked transients in arbitrarily shaped neurons and syncytia were analyzed theoretically. In systems with homogeneous passive membrane properties, the transients develop much faster than the membrane discharges. Conductance of the proximal membrane could be larger due to the injury imposed by sharp electrode impalement. In this case, the transients have an overshoot and an undershoot when the field is switched on and off. The overshoot and undershoot decay with the time constant of the response to current injection. If the conductance of the distal membrane is larger, the fast transients develop only partially and have slow tails which 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 responses were observed. The sharp electrodes were found not to impose a shunt in 60% of recorded cells. Detection of various membrane inhomogeneities in 1D-syncytium is discussed. We also suggest that it is possible to detect the inhomogeneities in intercellular resistance of the syncytium and intracellular resistance of a neuron when the membrane passive properties are homogeneous.