Single pyramidal-tract fiber analysis of neocortical propagated seizures with reference to inactivation responses

Abstract

Intracellular records from pericruciate pyramidal-tract cells and extracellular records from ipsilateral medullary pyramid pyramidal-tract axons were obtained during major propagated neocortical seizures initiated by repetitive electrical stimulation of the homotopic contralateral pericruciate cortex. Comparisons of the soma-dendritic with the axonal responses during the ictal event was possible because of the close temporal correspondence between these and cortical surface EEG. During seizures, the majority of pyramidal-tract cells responded with “inactivation responses” when the soma-dendritic membrane depolarized by 25–30 mv above the pre-ictal steady state value. However, the axonal output of the pyramidal-tract cells consisted of high-frequency bursts of spikes during all phases of the ictal event. It is suggested that axonal recordings more accurately reflect the neuronal output during major neocortical seizures and that inactivation responses of neocortical neurons are due, in large part, to the trauma of microelectrode penetration.