Specific impedance of rabbit's cortical tissue

¹ Kerckhoff Laboratories of the Biological Sciences, California Institute of Technology, Pasadena, California

The impedance of a tissue slab consisting of both cortical gray and underlying white matter was measured at 1,000 cycles/sec between a surface electrode and a deep electrode placed in the lateral ventricle or in the underlying tissue. By calibrating the electrode assembly with a KCl solution of known conductivity the mean specific impedance of the slab could be computed. Through a hole in the center of the surface electrode a probe was lowered stepwise, and the potential between its tip and the deep electrode was determined after each change in position. In a graph of the potential plotted against the position of the tip, the ratios of the specific impedances of the various layers of tissues in the slab and the thicknesses of these layers could be determined. From these data and the mean specific impedance of the tissue slab the specific impedance of the cortex was computed. The mean value determined in 14 experiments was 208 ± 6 ohms cm. The specific impedance of the white matter was materially greater. Taking into consideration the conductivity of the blood in the vessels the specific impedance of the cortical tissue proper was estimated as 220 ohms cm.

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