Fig. 4.

Schematic illustration of electrical behavior of a cell-free and cell-covered ECIS working electrode.a Schematic side view of a cell-free electrode and respective equivalent circuit presentation as resistor and capacitor in series. b Schematic presentation of electric compression of electric field lines (blue arrows) at the small opening of the working electrode. c Scheme of electrical double layer at a the electrode-solution interface. d Schematic side view of a cell-covered electrode, and respective equivalent circuit presentation which includes an additional impedance Z_cl that arises from the cell layer. e Schematic presentation of AC current flow at the cell layer-electrode junction. Depending on AC frequencies, two different limiting cases of current flow across a cell layer consisting of cells with the radius r_c in distance d above the electrode surface can be distinguished. At low frequencies the current takes the paracellular route (blue arrow) and takes up individual resistance contributions from the subcellular cleft (R_cleft) and the intercellular junctions (R_j). TJ: Tight junctions, AJ: Adherens junctions. At high frequencies the current couples capacitively across the membrane (green arrow). The impedance is dominated by the capacitance of the cell membrane (C_m). At intermediate frequencies current can take a varying combination of paracellular and transcellular routes (red arrows).