GENESIS OF MEG SIGNALS IN A MAMMALIAN CNS STRUCTURE

Neuromagnetic signals of guinea pig hippocampal slices were characteri zed and compared with the extracellular field potential to elucidate t he genesis of magnetoencephalographic signals in a mammalian CNS struc ture. The spatial distribution of magnetic evoked field (MEF) directed normal to bath surface was similar for transverse CA1, longitudinal C A1 and longitudinal CA3 slices in the presence of 0.1 mM picrotoxin (P TX) which blocks inhibitory synaptic transmission; Their MEFs were pro duced by currents along the longitudinal axis of the pyramidal cells. Comparisons of the MEF with the laminar potential profile revealed tha t the MEF was generated by intracellular longitudinal currents. The di polar component of the intracellular currents was the dominant factor generating the MEF even at a distance of 2 mm from the slice. The MEF from a slice in Ringer's solution without PTX became similar in tempor al waveform with time to the MEF in the presence of PTX, indicating th e predominance of excitatory connections in generating the MEF and the existence of highly synchronous population activities across the slic e even in PTX-free Ringer's solution. The presence of such highly sync hronous population activities underlying the MEFs was verified directl y with field potentials recorded across the slice. A systematic Variat ion of the stimulation site revealed a characteristic waveform for eac h site. The variation of the waveform with stimulation site suggested the contribution of many factors, both synaptic and voltage-sensitive conductances, to the overall waveform of the MEF. (C) 1997 Elsevier Sc ience Ireland Ltd.