Effect of electrical stimulation of the centre median nucleus on neuronal activity of the thalamic ventrobasal complex in the anesthetized cat.

Abstract

Involvement of the centre median nucleus (CM) of the thalamus in the central mechanisms of somatosensory system including pain sensation was investigated in the anesthetized cat. Extracellular multi-unitary discharges were recorded simultaneously from the thalamic ventrobasal complex (VB) using the multimicroelectrode system. The effects of CM stimuli with single shocks and pulse train on VB neuronal activities were examined. The VB neurons were characterized according to their responsiveness to mechanical stimuli, and then classified into nociceptive and non-nociceptive neurons. The effects of pulse train stimulation were more pronounced than those of single shocks. Of 86 units tested, the effects of pulse train CM stimulation were observed in 45 units including 31 nociceptive and 14 non-nociceptive units. The VB neurons were divided into four classes according to the response types to pulse train CM stimulation. Of the 86 units, 20 were classified as class 1 which exhibited inhibition of spontaneous discharges followed by excitation. Thirteen units were class 2 which exhibited only excitation, and 12 were class 3 which showed only inhibition of spontaneous discharges. The remaining 41 units were class 4 which did not respond to CM stimulation. Differences in response to CM stimulation were observed among simultaneously recorded units even when the receptive fields and recording sites were in close proximity. Response to single shocks with a short latency of approximately 1.5 ms was observed in several VB neurons. Further inhibitory effects of pulse train CM stimulation on discharges evoked by stimulation of the receptive field were also observed.Thus, the CM appears to modify neuronal activities in the VB. However, the CM exerts not only an inhibitory effect but also excitatory on the VB neuronal activities. The effects are also observed on non-nociceptive units in the VB.