The effects of stimulation in the medial brain stem on the responses of 48 spinothalamic tract (STT) cells were examined in anesthetized monkeys. Trains of stimuli delivered in the nucleus raphe magnus (NRM) were found to be more effective in depressing STT neuron responses to volleys in C-fibers than responses to volleys in A-fibers of the sural nerve. Spinothalamic tract neuron responses to both innocuous and noxious cutaneous mechanical stimulation and to noxious heat were also inhibited by stimulation in the NRM. This was found to be true for 34 of the 38 wide dynamic range (WDR) spinothalamic tract cells studied. The responses to noxious mechanical and thermal stimulation were inhibited in all 10 high-threshold (HT) cells investigated. No low-threshold (LT) spinothalamic tract neurons were found in this study. Four WDR spinothalamic tract cells were examined in which the responses to noxious cutaneous stimuli were inhibited, while responses to innocuous brushing or indentation of the skin were minimally affected. Ten neurons in the nucleus gracilis were also investigated. The responses of these cells to innocuous cutaneous mechanical stimuli were minimally inhibited by NRM stimulation. The possible roles of cells of the dorsal column nuclei and of differentially inhibited spinothalamic tract cells in behavioral responses during NRM stimulation are considered. The inhibitory effects of stimulation in the medial reticular formation on STT cell responses were examined and found to be qualitatively and quantitatively similar to those caused by NRM stimulation.
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