Specific impedance of cerebral cortex during spreading depression, and an analysis of neuronal, neuroglial, and interstitial contributions

Abstract The specific impedance of cerebral cortex of rabbits anesthetized with urethane was measured during spreading cortical depression (SD) at frequencies from 5 to 50,000 cycle/sec. During SD the amplitude of impedance increased at all frequencies, the maximum occurring progressively later at lower frequencies for those less than 5,000 cycle/sec. The phase angle increased at 5,000 and 50,000 cycle/sec with the same time course as the changes in amplitude of impedance. At 50 cycle/sec the phase angle initially decreased, sometimes to zero, and then increased to greater than the pre-SD state, with the maximum occurring later than the maximum in amplitude of impedance at this frequency. At 500 cycle/sec the phase angle initially decreased or did not change. It then increased, the maximum occurring later than the maximum of the amplitude of impedance at 500 cycle/sec, but earlier than the maximum phase angle at 50 cycle/sec. These data are interpreted in terms of a previously published analysis of specific impedance of cerebral cortex. Three processes must occur during SD. Early in SD the size of the interstitial space of cortex must decrease and the membrane resistance of neurons must decrease. Later the membrane resistance of neuroglia must increase. The significance of these processes is discussed with relevance to SD and cortical processes in general. It is suggested that the membrane change of neuroglia is similar to that of “anomalous rectification” of skeletal muscle.