Electrical Conductivity of Brain Cortex Slices in Seizing and Non-seizing States

The electrical conductivity of thin living slices of mouse cerebral cortex is measured. Two only out of fifteen different attempting ways were effective. I have successfully measured the electrical conductivity of mouse brain cortex in seizing and non-seizing conditions. The first successful approach is called the van der Pauw method, where four silver-silver chloride cylindrical wire electrodes were immersed in full length at the corners of the sample. The second is a one-dimensional technique where two flat electrodes were placed on either face of the 400 micrometer thick samples. In both methods the electrodes were connected to an Agilent E4980A impedance monitor. The conductivity at 10 kHz of each sample was calculated based on measurements of injected current and potential difference between electrodes. Both approaches were validated by measuring electrical conductivities of known solutions. There were two main challenges: the small size of the sample and keeping it alive. I overcame these challenges by suitable electrodes and fast measuring equipment (Agilent E4980A LCR meter). For the one-dimensional technique I also measured the conductivity across the frequency range 20 Hz to 2 MHz. The results consistently show the mean conductivity of seizing brain tissue is significantly lower than that of non-seizing tissue at 10 kHz. Also, the conductivity of seizing slices is lower than the conductivity of non-seizing slices over the frequency range 20 Hz to 2 MHz. These results suggest a link between electrical conductivity and seizure activity. I have not investigated the causes of these differences but explanations consistent with the literature are a change in chemical environment during seizure or a reduction in gap junction connectivity

Measuring the electrical impedance of mouse brain tissue

We report on an experimental method to measure conductivity of cortical tissue. We use a pair of 5mm diameter Ag/AgCl electrodes in a Perspex sandwich device that can be brought to a distance of 400 microns apart. The apparatus is brought to uniform temperature before use. Electrical impedance of a sample is measured across the frequency range 20 Hz-2.0 MHz with an Agilent 4980A four-point impedance monitor in a shielded room. The equipment has been used to measure the conductivity of mature mouse brain cortex in vitro. Slices 400 microns in thickness are prepared on a vibratome. Slices are bathed in artificial cerebrospinal fluid (ACSF) to keep them alive. Slices are removed from the ACSF and sections of cortical tissue approximately 2 mm times 2 mm are cut with a razor blade. The sections are photographed through a calibrated microscope to allow identification of their cross-sectional areas. Excess ACSF is removed from the sample and the sections places between the electrodes. The impedance is measured across the frequency range and electrical conductivity calculated. Results show two regions of dispersion. A low frequency region is evident below approximately 10 kHz, and a high frequency dispersion above this. Results at the higher frequencies show a good fit to the Cole-Cole model of impedance of biological tissue; this model consists of resistive and non-linear capacitive elements. Physically, these elements are likely to arise due to membrane polarization and migration of ions both intra- and extra-cellularly.http://www.iupab2014.org/assets/IUPAB/NewFolder/iupab-abstracts.pd