Unitary responses in ventrolateral thalamus upon intranuclear stimulation.

Abstract

Unitary responses to close intranuclear single shocks, twin pulses and high frequency stimulation were studied in the ventrolateral thalamus of anesthetized and paralyzed cats. Two clear-cut groups of cells were disclosed on the basis of the responses. In one group were cells which fired spontaneously at rates below 100/sec (LF [low frequency] cells) and usually responded to shocks by single spikes at latency 0.5-3 msec. Critical stimulus interval for twin pulses usually ranged from 2 to 7 msec. High frequency stimulation (100-300/sec) could drive these cells during the first 3-4 pulses only. Their spontaneous activity was usually arrested by single shocks for periods up to 200 msec. Another group consisted of cells responding to single shocks with long (up to 500 msec) barrages at 200-550/sec. The latency of first spike ranged from 2 to 20 msec. These repetitive responses resembled the discharge of Renshaw cells and were considered to be interneurons. Evidence was provided demonstrating that thalamic LF cells and interneurons are in close proximity and functionally interdependent. Interneurons specifically influence LF cells by increasing their membrane potential and thereby modulating their spike activity. The circuit for this functional interaction is probably similar to that of spinal recurrent collateral inhibition via Renshaw cells.