Modification of Focal Cortical Epileptogenic Discharge by Afferent Influences

Studies of epileptogenic spike triggering in cortical spike foci produced by penicillin were done using macro‐ and microelectrode recording techniques in order to analyze the cellular abnormalities in such foci and the mechanisms of precipitation and control of interictal spike discharge. Surface spikes in this model system are the summation of slow intracellular depolarizations termed “depolarization shifts” (DS's). These slow depolarizations are most likely graded responses of cell membranes to synaptic inputs. The extent to which basic abnormalities in the function of cell membranes vs. alterations of synaptic drive onto the involved neurons contribute to DS generation is not clear. Alterations in the latency of DS triggering during closely spaced stimulation suggest that “presynaptic” factors (in terms of the epileptic neuronal aggregate) are of importance. Limitation of the frequency of interictal surface spiking was not usually due to cathodal or anodal block in the population of neurons studied. Neurons within the area of the focus were involved to varying degrees in DS generation. Cells which are inhibited during DS generation may be involved in limiting the frequency and spread of interictal spike discharge.