Synaptic processes in pericruciate cortical neurons evoked by pyramidal tract stimulation in cats

In cats anesthetized with chloralose and pentobarbital and immobilized with D-tubocurarine activity of 423 pericruciate cortical neurons was recorded (342 extra- and 81 intracellularly); 78 neurons had spontaneous activity. Stimulation of the pyramidal tract evoked antidromic action potentials in the pyramidal neurons with a latent period of 0.5-16.0 msec. Recurrent and lateral PSPs also developed both in pyramidal and in unidentified neurons in all layers of the cortex; IPSPs were recorded in 46.7% of neurons, EPSPs in 21.0%, mixed responses in 26.0%, and no visible changes were found in 6.3%. The latent period of the IPSPs was 1.5-14.0 msec, ther amplitude 1.3-17.0 mV, their rise time from 4 to 18 msec, and their duration 18-120 msec (sometimes up to 250-500 msec). In 30% of cases in which IPSPs appeared, their course was divided into two phases: fast (duration 10-20 msec) and slow. EPSPs developed after a latent period of 2.6-29.0 msec; their amplitude was 1.0-7.8 mV and their duration from 10.0 to 50.0 msec. In 51.2% of spontaneously active neurons the antidromic volley inhibited their activity in the course of 200-400 msec, in 19.5% it stimulated their activity, in 7.4% it had a mixed effect, and in 21.9% no visible change took place in their activity. The role and participation of axon collaterals of pyramidal neurons and of the interneuronal system in the formation of these processes are discussed.