PROPERTIES OF CONVERGENT THALAMOCORTICAL AND INTRACORTICAL SYNAPTIC POTENTIALS IN SINGLE NEURONS OF NEOCORTEX

We explored differences in the properties of convergent afferent input s to single neurons in the barrel area of the neocortex. Thalamocortic al slices were prepared from mature mice. Recordings were made from ne urons in layer V, and either thalamocortical afferents or horizontal i ntracortical axons were stimulated. Monosynaptic EPSPs from both sourc es had latencies shorter than 1.8 msec and low shape variance. Disynap tic thalamocortical IPSPs had latencies longer than 1.8 msec. All neur onal types, as defined by intrinsic firing patterns, received both tha lamocortical and intracortical monosynaptic input. The shape parameter s (rate of rise and half-width) of monosynaptic EPSPs from the two inp uts did not differ significantly The rate of rise of EPSPs varied cons iderably across cells, but the rates of rise of thalamocortical and in tracortical EPSPs onto single cells were strongly correlated. The rela tive thresholds for activation of synaptic excitation and inhibition w ere strikingly different between the two tracts: thalamocortical stimu lation induced GABA(A)-dependent IPSPs at stimulus intensities equal t o or less than those required for evoking EPSPs in 35% (24 of 68) of t he cells. In contrast, the threshold response to intracortical stimula tion was always an EPSP, and only stronger stimuli could generate di- or polysynaptic IPSPs. We suggest that postsynaptic factors may tend t o equalize the waveforms of EPSPs from thalamocortical and intracortic al synapses onto single neurons. A major difference between the two co nvergent tracts is that the thalamocortical pathway much more effectiv ely activates feedforward inhibitory circuits than does the horizontal intracortical pathway.