The afferents to 735 cells of the dorsomedian nucleus (MD) of the cat's thalamus from various subcortical structures, septum (Spt), amygdala (Amg), mesencephalic reticular formation (MRF), posterior hypothalamus (Hpt) and centromedian nucleus (CM) were studied in unanesthetized curarized acute preparations carefully infiltrated with procaine at the sites indicated. Some animals were also chloralosed. The extra-cellular unit activity was recorded with steel microelectrodes, and short duration square pulses were delivered through concentric macroelectrodes. Of the MD cells, 50.4% responded to stimulation of the CM, 34.7% to the Spt, 20.7% to the MRF, 20.8% to the Amg and 16.7% to the Hpt. The responses were of an excitatory type or considered of a transient arrest of the spontaneous discharge. A combination of both types of response was also observed. The latencies ranged between 2 and 2000 msec. Numerous responses with latencies greater than 500 msec were evoked from Amg, Spt and CM, whereas latencies above 400 msec were never recorded following stimulation of the MRF. Seventy-two cells responded to stimulation of more than one structure. The convergence between the rhinencephalic nuclei (Amg, Spt) on the on ehand, and between the CM and RMF on the other, were conspicuous. The latencies following stimualtion of the CM were greater than those after MRF stimulation, a finding that would be in favor of the concept that these two afferent systems to MD are independent. © 1971.
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