Single-unit analysis of propagated seizures in neocortex

Abstract

Cortical-surface, extracellular, and intracellular recording was carried out in the pericruciate cortex anesthetized and unanesthetized cats during propagated seizures produced by surface repetitive stimulation of the opposite homotopic neocortex. Well developed trans-synaptically initiated seizures could be easily elicited in the unanesthetized animals. Almost all cells observed were found to be overtly involved in the seizure sequence and responded in a relatively uniform manner. Neurons were classified on the basis of their behavior as types I and II (active) and type III (passive). In active neurons, changes in the membrane polarization and spike discharges were observed which characterized the various phases of the seizure. The afterhyperpolarization following either spontaneous or antidromically evoked action potentials was lost and replaced by an after depolarization at the onset of the seizure. The tonic phase was characterized by excessive membrane depolarization and spike discharge with resultant inactivation of A and B spike generators. The clonic phase corresponded to slow membrane repolarization, recovery of the A followed by the B spike generators, and large depolarizing waves exhibiting afterdepolarizations. Hyperpolarization characterized the postictal electrical silence. In passive neurons, recurring depolarizing waves with high-frequency bursts of spikes occurred throughout the tonic and clonic phoses without inactivation of the spike generator followed by a slight postictal hyperpolarization. Light barbiturate anesthesia profoundly suppressed the trans-synaptic elicitation of seizures. Intense tetanizing current produced weak unit and surface seizure activity. The majority of cells simply ceased firing or were not overtly involved in the seizure sequence recorded from the cortical surface.