Pain and Temperature Encoding in the Human Thalamic Somatic Sensory Nucleus (Ventral caudal): Inhibition-Related Bursting Evoked by Somatic Stimuli

Abstract

Stimulus-evoked inhibitory events have not been demonstrated in thalamic spike trains encoding of pain and temperature stimuli. We have now tested the hypothesis that the human thalamic response to mechanical and thermal stimuli is characterized by low-threshold calcium spike (LTS)-associated bursts of high-frequency action potentials preceded by prolonged inhibition. The results included 57 neurons recorded in the human thalamic principal somatic sensory nucleus (ventral caudal, Vc) of 24 patients during awake surgery. Neurons were classified by the grading of their response with stimulus intensity into the painful range (graded or nongraded) and the stimulus response (to mechanical, cold, or heat stimuli). Firing rates were analyzed by the response to all stimuli combined (stimuli overall) and to the stimulus characteristic of the stimulus response type (optimal stimulus), e.g., cold stimuli for neurons of the cold stimulus response type. All neuronal categories had clear stimulus-evoked LTS bursting as identified by the criteria for selecting bursts in the spike train, by significant preburst inhibition, and by preburst inter-spike interval not significantly <100ms. Stimulus-evoked LTS burst rates were significantly higher for neurons in the cold stimulus response type independent of the firing rate between bursts. The parameters of preburst inhibition were largely independent of the neuronal category and the stimuli included in the analysis, which suggests inhibitory mechanisms are similar across neuronal types. Therefore LTS bursting is a substantial, nonlinear component of the spontaneous and stimulus-evoked activity of thalamic neurons in awake humans.