Thalamic inputs and subcortical targets of cortical neurons in areas 5 and 7 of cat

Subcortical projection structures and thalamic inputs of single neurons in cortical areas 5 and 7 of cat were extracellularly investigated in locally anesthetized, encéphale isolé preparations and chronically implanted, behaving animals. Testing stimulations were applied to the lateralis intermedius (LI), lateralis posterior (LP), ventralis anterior (VA), and centrum medianum (CM) thalamic nuclei, the mesencephalic reticular formation, and the pontine nuclei. Output cells ( N = 101) were identified by antidromic invasion, selectively elicited in 91 cells from PN, CM, LI, or LP; the remaining 10 neurons had branching axons to two of the above targets. Two cortical-subcortical circuits involve areas 5 and 7. In one of them, afferents of LI and/or LP origin mainly reach cortico-CM cells and, less often, neurons giving rise to a reciprocal cortico-LI or cortico-LP projection, Within the other circuit, corticopontine cells are synaptically activated from the VA nucleus or convergently modulated from both the VA and LI nuclei. The unexpectedly high proportion of antidromically identified cells suggest that many functions of the parietal “association” cortex depend on downstream projections to the thalamic integrative nuclei and to the pontocerebellar system. Forty-two nonoutput cells were modulated synaptically, especially from the LI nucleus, with high-frequency spike barrages similar to activities described in putative interneurons recorded from other cerebral structures. The thalamically elicited excitation of nonoutput cells occurred immediately after the early discharges induced in output cells, in close time relation with secondary depth-negative waves and with the onset of a long-lasting slow positive shift associated with silenced firing in output cells.