Neuronal activity in medullary dorsal horn of awake monkeys trained in a thermal discrimination task. I. Responses to innocuous and noxious thermal stimuli.

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Abstract
The activity of 65 neurons located in the medullary dorsal horn (trigeminal nucleus caudalis) was recorded during performance of a behavioral task in which monkeys [M. mulatta] were trained to discriminate between noxious (45-49 C) and innocuous (37-43.degree. C) thermal stimuli applied to the maxillary region of the face. Similar to medullary dorsal horn neurons in anesthetized monkeys, nociceptive neurons in the awake monkey were classified as wide dynamic range (WDR) or nociceptive specific (NS), based on their responses to passive, experimenter-presented mechanical stimulation of the face. Receptive-field sizes and the locations of neurons in the dorsal horn were similar in awake and anesthetized monkeys. WDR and NS neurons responded to thermal stimuli with thresholds of 39-49.degree. C. Stimulus-response functions were monotonic from threshold to 49.degree. C and WDR neurons were most sensitive when stimuli were applied to the central portion of their receptive fields. WDR neurons showed steeper stimulus response functions than did NS neurons. The magnitude of the neuronal response to an innocuous or noxious thermal stimulus usually was reduced by the preceding stimulus at interstimulus intervals of 20 s or less. This effect was greatest following noxious (45-49.degree. C) trials. Responses to stimuli preceded by innocuous (37-43.degree. C) trials were suppressed when compared to responses following 35.degree. C trials (no change from baseline temperature). These effects are attributed partly to partial inactivation of peripheral receptors following innocuous and noxious thermal stimuli. WDR and NS neurons responded to noxious stimuli before the shortest discrimination latencies of the monkeys. For 9 of 17 neurons the magnitude of the neuronal response to noxious stimuli was inversely related to discrimination latency. Both the magnitude and latency of neuronal responses suggest that activity in thermally sensitive WDR and NS neurons provides signal related to the monkey''s ability to discriminate noxious heat stimuli from innocuous thermal stimuli. Both WDR and NS neurons apparently code sensory-discriminative information about noxious heat stimulation.