How the active and diffusional nature of brain tissues can generate   monopole signals at micrometer sized measures
  We investigate mechanisms which could generate transient monopole signals in measuring current source density (CSD), as it had been indicated to occur in recent small volume experiments. A simple model is defined for this purpose. It is emphasized that the active nature of the neural biological activity, with its ability to generate ionic density imbalances, might be able to induce appreciable monopole signals in CSD detectors at micrometer scales. Thus, it follows that when both diffusive and ohmic transport are considered to be present in neural tissues, potential measures in micrometer regions can include appreciable electric monopole signals, for sufficiently small values of the ratio (\sigma a^{2})/(\epsilon D), where "\sigma" is the conductivity, "\epsilon" is the dielectric constant, "D" is the diffusion constant and "a" is the linear dimension of the ionic charge densities generated by the neural processes. Ranges of possible magnitudes for these parameters in the considered experimental studies are estimated. The analysis indicates values for the ratio between the dipolar and monopole signals which are close to the ones measured in Pyramidal cells in recent experiments. The measured results for Spiny Stellate cells are also qualitatively described by the model by predicting a finite monopole signal in combination with vanishing dipolar and quadrupole ones. 
Alejandro Cabo and Jorge J. Riera
physics.bio-ph