Dendritic spikes induced in fast pyramidal tract neurons by thalamic stimulation

In deeply anesthetized cats the synaptic events induced in fast pyramidal tract cells (Pt cells) by ventrolateral (VL) stimulation were analyzed with intracellular recordings. In 40% of the fastest conducting Pt neurons it was found that VL stimulation induced fast depolarizing potentials (FPPs) with or without underlying excitatory postsynaptic potentials (EPSPs). These FPPs were all or non fast rising events lasting about 2 ms. They could be induced by brachium conjunctivum stimulation or they could also occur spontaneously. Those occurring spontaneously had amplitude and time course similar to those evoked by VL stimulation suggesting that they were of thalamic origin. On the basis of their amplitude and lack of collision with antidromic action potentials FPPs were neither axonal nor IS events. Passage of hyperpolarizing currents could block them in an all or none manner. It is concluded that FPPs represent electronically attenuated dendritic spikes generated in small side branches (oblique ascending dendrites) or fast Pt neurons where VL terminals most probably establish their synaptic contacts.