Effects upon Thalamic Somatosensory Transmission of Reticular Formation Stimulation

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Abstract
Effects of high frequency electrical stimulation of the mesencephalic reticular formation upon impulse transmission of the thalamic somatosensory system were studied in unanesthetized dogs. As indices of impulse transmission, evoked potentials or single unit activities were recorded from the thalamic ventrobasal complex (VBC) in response to single shock stimulation of the medial lemniscus. The VBC evoked potential recorded focally was a positive-negative diphasic wave of short duration. By examining the recovery cycle and effects of asphyxia, anesthesia and cortical conditioning stimulation, it was demonstrated that the positive component of the evoked potential represented ascending impulses of the medial lemniscus and the negative one indicated the postsynaptic activity. The negative component of the VBC evoked potential was increased in amplitude for variable lengths of time after high frequency reticular stimulation while the positive component was not affected. Adequate arousal stimulation applied peripherally showed about the same effect as reticular stimulation. The enhancement of the negative component of the VBC evoked potential was obtained at about the same threshold of reticular stimulation as for eeg arousal. Concerning the VBC unit activity, effects of the reticular stimulation were judged by using the following three criteria latency of evoked spikes, firing index and extent of latency variability. It was observed that half of the VBC units examined were affected more or less by reticular stimulation, two thirds of them being facilitated and one third inhibited. Facilitation and inhibition were manifested most frequently decreased and increased spike latency, respectively.