Contribution of ionic currents to magnetoencephalography (MEG) and electroencephalography (EEG) signals generated by guinea-pig CA3 slices

A mathematical model was used to analyse the contributions of different types of ionic currents in the pyramidal cells of longitudinal CA3 slices to the magnetic fields and field potentials generated by this preparation. Murakami et al. recently showed that a model based on the work of Traub et al. provides a quantitatively accurate account of the basic features of three types of empirical data (magnetic fields outside the slice, extracellular field potentials within the slice and intracellular potentials within the pyramidal neurons) elicited by stimulations of the soma and apical dendrites. This model was used in the present study to compute the net current dipole moment (Q) due to each of the different voltage- and ligand-gated channels in the cells in the presence of fast GABA(A) inhibition. These values of Q are proportional to the magnetic field and electrical potential far away from the slice. The intrinsic conductances were found to be more important than the synaptic conductances in determining the shape and magnitude of Q. Among the intrinsic conductances, the sodium (g(Na)) and delayed-rectifier potassium (g(K(DR))) channels were found to produce sharp spikes. The high-threshold calcium channel (g(Ca)) and C-type potassium channel (g,(c)) primarily determined the overall current waveforms. The roles Of g(Ca) and g(K(C)) were independent of small perturbations in these channel densities in the apical and basal dendrites. A combination of g(Na), g(K(DR)), g(Ca), and g(K(C)) accounted for most of the evoked responses, except for later slow components, which were primarily due to synaptic channels.