[Effects of a cortical after-discharge on lateral geniculate neurones. Electrophysiological study in the chronic cat...

(1) A chronic preparation is described in which it is possible to record juxtacellularly from identified thalamo-cortical relay (TCR) neurones in lateral geniculate body (LGB) of normally respiring, nonanaesthetized cats during natural sleep. (2) Cellular events were analyzed during and after focal electrical stimulation of homotopic visual cortex. (3) Projection of cortical epileptiform after-discharge (EAD) was strictly limited to functionally related areas of LGB. Of the 47 neurones tested only 30% were activated. (4) During the 2 sec, 50 c/sec tetanus the cortically evoked IPSPs in TCR cells were replaced by facilitation of cortico-thalamic transmission as demonstrated by the occurrence of a burst of action potentials (AP) following the antidromic AP. (5) During the ensuing EAD, high frequency AP discharges occurred in the cortico-thalamic axons during each cortical wave. This period was accompanied by prolonged (50-300 msec) low amplitude rhythmic depolarisations leading to temporary spike inactivation of TCR neurones. Comparable inactivating responses were recorded during paradoxical sleep. (6) Between paroxysmal bursts facilitation of synaptic transmission to the optic tract stimulation was observed. (7) Persistence of a positive collision test after a spontaneous AP indicates that AP are orthodromically propagated during the EAD.