Relative Electrical Conductivity Measurements Of The Rat Frontal Cortex And The Influence On Current Source Density

The relative electrical conductivity gradient with depth was estimated in the frontal cortex of anaesthetized rats. Current source density (CSD) approximations of field potentials evoked by ventromedial thalamic stimulations with an assumed homogeneous electrical conductivity of the neocortical tissue were compared to those with correction for the estimated conductivity gradient. In spite of the cellular heterogeneity the electrical conductivity of the frontal cortical tissue was found to be fairly homogeneous inside the superficial (layers I through IV) or deep layers (V- VI). The relative conductivity increased twofold at the transition between superficial and deep layers. Regardless of this changes CSD analysis of the field potentials evoked by ventromedial thalamic stimulation revealed negligible differences between estimations ignoring the conductivity and those taking the conductivity into account. No sinks or sources appeared or disappeared. Both CSD approximations revealed: 1) a strong sink in layer I representing most likely summed monosynaptic EPSPs of the ventromedial thalamic afferents; 2) a strong sink in layer VI, probably representing summed disynaptic EPSPs on dendrites of layer VI pyramidal cells, generated by axons of upper layer pyramidal cells; and 3) a sink in lower layer V representing probably threesynaptic summed EPSPs on dendrites of layer V pyramidal cells.