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The somatotopic organization, mechanoreceptive field (RF) and response properties of nociceptive neurons in the ventral posteromedial thalamic nucleus (VPM) were studied in urethane/α-chloralose anesthetized rats. Microelectrode recordings were made of the activity of single neurons within VPM, and neurons were identified as either nociceptive [wide-dynamic-range (WDR), nociceptive-specific (NS)] or non-nociceptive [low-threshold mechanoreceptive (LTM)]. Of the 350 neurons tested, 317 responded to orofacial mechanical stimulation; all the RFs were contralateral. The incidence of NS neurons was significantly higher than that of WDR neurons (21.2% versus 4.2%) but LTM neurons were most common (62.9%). WDR and NS neurons were scattered among the LTM neurons in the VPM. NS neurons were significantly denser in rostral and caudal portions of VPM; most of the WDR neurons (87%) were located in the core of VPM. Neurons with an ophthalmic division RF were located most dorsally and those with a mandibular division RF most ventrally. The RF of most neurons (77%) was located in the maxillary region, with the RF of nociceptive neurons mainly in the intraoral region and that of most LTM neurons in the perioral region. The majority of LTM, WDR and NS neurons tested had a low spontaneous activity and there was a significant difference in spontaneous firing between WDR and NS neurons, and between WDR and LTM neurons. The mean mechanical activation threshold of NS neurons (195 ± 11.2 g) was significantly higher than that of WDR neurons (4.7 ± 1.3 g). Response magnitude and peak firing frequency to graded mechanical stimuli were significantly higher in the WDR neurons. These findings indicate significant differences in the spontaneous activity, RF and response properties between WDR and NS neurons in VPM. However, they also indicate that both types of nociceptive neurons can encode the spatial properties and intensity of a noxious orofacial stimulus and, thus, might contribute to the sensory-discriminative component of pain.
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