  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.
