Thalamic mechanisms that process a temporal pulse code for pain

The sequential ordering of spikes emitted by single thalamic neurons which respond to noxious stimulation was studied using rats anesthetized with a mixture of alpha-chloralose and urethane. An electrical stimulus applied to the sciatic nerve contralateral to the thalamic recording site fired single thalamic SII neurons with a short latency spike burst and with long latency spikes which occurred at relatively fixed intervals. On repetition of stimulation, the short latency spike burst formed a high amplitude peak on sequential spike density histograms (I of Fig. 2B); the long latency spikes formed peaks of relatively low amplitude (M1, M2, M3 of Fig. 2B). Histograms of touch and light pressure relay neurons found within the thalamic SII differed conspicuously from that of Fig. 2B. Further experiments revealed that the I peak contained frequency coded information about the stimulus intensity, whereas the M peaks with their temporal relationship to the I peak coded information pertaining to a particular sensory modality. The M peaks are formed by timed firing in a positive feedback loop found between the thalamic SII and the nucleus centrum medianum-nucleus parafascicularis (CM-Pf) neurons. Consequently, the M peaks can be abolished without losing the I peak by a lesion placed in a portion of the CM-Pf complex or by the administration or morphine which is able to disorganize the timing mechanism of the feedback loop. Therefore, it is reasonably certain that the modality coded by the M peaks is pain.