Pyramidal tract control over cutaneous and kinesthetic sensory transmission in the cat thalamus

In the thalamic ventrobasal complex (VB) of the cat, effects of electrical stimulation of the pyramidal tract (PT) upon activities of 112 relay cells and 18 internuncial cells were examined. Single PT shocks to the cerebral peduncle elicited short-latency discharges in 31 relay cells (mean latency, 1.4 +/- 0.5 msec). When weak PT stimuli were employed as conditioning shocks, facilitatory effects upon responses to medial lemniscal (ML) stimulation were observed. It was revealed that VB relay cells were excited monosynaptically via collaterals of the fast PT fibers. Among 31 PT-excited cells 22 were fired by movements of joints (joint-movement units) and they made up 88% of all the joint-movement units. A majority of the relay cells responding to stimulation of hairs (hair units) did not receive excitatory effects from PT, except some special ones which represented long hairs at the distal or proximal end of the forearm-forepaw. In 44 relay cells repetitive PT shocks suppressed both evoked responses to ML stimulation and spontaneous discharges for 70--100 msec. Of these, 34 were hair units. The PT-induced inhibition in the hair units increased as their receptive fields shifted from the trunk towards the digits. Some intracellular recordings showed that the PT-induced inhibition was due to IPSPs generated disynaptically. Among 18 interneurons presumed to be inhibitory 10 responded with short latencies to PT stimulation. These were mostly the interneurons which presumably subserve the recurrent collateral inhibition in VB.