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Volume 16, Number 20, Issue of October 15, 1996 pp. 6567-6578
Copyright ©1996 Society for NeuroscienceProperties of Convergent Thalamocortical and Intracortical Synaptic Potentials in Single Neurons of Neocortex
Received May 21, 1996; revised July 8, 1996; accepted July 22, 1996.
Ziv Gil and Yael Amitai Department of Physiology, Zlotowski Center for Neuroscience, Faculty of Health Sciences, Ben-Gurion University, Beer-Sheva, Israel 84105
We explored differences in the properties of convergent afferent inputs to single neurons in the barrel area of the neocortex. Thalamocortical slices were prepared from mature mice. Recordings were made from neurons in layer V, and either thalamocortical afferents or horizontal intracortical axons were stimulated. Monosynaptic EPSPs from both sources had latencies shorter than 1.8 msec and low shape variance. Disynaptic thalamocortical IPSPs had latencies longer than 1.8 msec. All neuronal types, as defined by intrinsic firing patterns, received both thalamocortical and intracortical monosynaptic input. The shape parameters (rate of rise and half-width) of monosynaptic EPSPs from the two inputs did not differ significantly. The rate of rise of EPSPs varied considerably across cells, but the rates of rise of thalamocortical and intracortical EPSPs onto single cells were strongly correlated.
The relative thresholds for activation of synaptic excitation and inhibition were strikingly different between the two tracts: thalamocortical stimulation induced GABAA-dependent IPSPs at stimulus intensities equal to or less than those required for evoking EPSPs in 35% (24 of 68) of the cells. In contrast, the threshold response to intracortical stimulation was always an EPSP, and only stronger stimuli could generate di- or polysynaptic IPSPs.
We suggest that postsynaptic factors may tend to equalize the waveforms of EPSPs from thalamocortical and intracortical synapses onto single neurons. A major difference between the two convergent tracts is that the thalamocortical pathway much more effectively activates feedforward inhibitory circuits than does the horizontal intracortical pathway.
Key words: barrel cortex; thalamocortical synapses; intracortical synapses; local cortical circuit; feedforward inhibition; EPSPs; slices
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