The upper cervical spinal cord contains over one-third of the cells of the spinothalamic tract (STT). This study investigated response properties of contralaterally projecting STT neurons in C2 of the cat by the use of single-unit, microelectrode recordings. Standard antidromic stimulation and collision techniques were used to identify STT units projecting to the contralateral thalamus. Once an STT unit was found, its receptive field (RF) and responses to cutaneous stimuli such as touch, pressure, deep muscle squeeze, tap, noxious pinch, and heat were characterized. C2 units that were not activated from the contralateral thalamus (non-STT units) were also characterized. The locations of thalamic stimulation electrodes and spinal recording sites were reconstructed from electrolytic lesions. A total of 48 STT and 68 non-STT units were well characterized. RF sizes were classified as small, intermediate, large, or whole body. Each unit was also classified as having one of two possible response types: simple units were those with homogeneous responses within the RF and were classified as low threshold (LT), high threshold (HT), wide dynamic range (WDR), deep, or tap. Complex units were those that responded differently in different regions of the RF. The average depth of non-STT units subdivided by RF size was 2.1 ± 0.6 (SD) mm for cells with small RFs, 2.4 ± 0.8 mm for cells with intermediate RFs, 2.8 ± 0.3 mm for cells with large RFs, and 2.7 ± 0.5 mm for cells with whole-body RFs. The average depth of non-STT units based on response type was 2.0 ± 0.5 mm for LT, 2.3 ± 0.7 mm for HT, 2.1 ± 0.7 mm for WDR, 2.6 ± 0.9 mm for deep, 2.6 ± 0.5 mm for tap, and 2.4 ± 0.2 mm for complex. A somatotopic organization along the rostrocaudal length of C2 and upper C3 was observed for non-STT units with small- and intermediate-size RFs. The average distance of the recording sites from the rostralmost dorsal rootlet of C2 was 3.8 ± 2.1 mm for units with RFs on the face, 7.1 ± 4.3 mm for units with RFs on the neck, and 11.9 ± 5.1 mm for units with RFs on the forelimb. The average threshold for antidromic activation of STT units was 175 ± 120 μA. Most C2 STT units were activated from the ventroposterior region of the thalamus. Within the ventroposterior region, significantly more STT units were activated from electrodes within AP levels of 7.5-8.5 than from more anterior electrodes. Moreover, units with simple responses and small or intermediate RFs were usually activated from thalamic electrodes located posteriorly and laterally, whereas those with whole-body RFs tended to be activated from electrodes located either anteriorly or medially. Forty-eight percent of STT units with cutaneous RFs were classified as having large or whole-body RFs. In comparison, only 9% of non-STT units had large or whole-body RFs. Bilateral RFs were found in 55% of STT units, but in only 10% of non-STT units. Complex type responses were found in 29% of STT units compared with only 4% of non-STT units. Responses to noxious stimuli were seen in 45-56% of STT units and 34-38% of non-STT units. No cutaneous RF could be demonstrated for 17 of the STT units (these unresponsive STT units are not included in the above calculations). Of the 64 non-STT units with simple responses, 50% were LT, 11% were HT, 19% were WDR, 9% were deep, and 11% were tap. Of the 22 STT units with simple responses, 18% were LT, 27% were WDR, 27% were deep, and 27% were tap. The results of this study demonstrate a striking difference between non-STT and contralaterally projecting STT units within C2. It therefore seems likely that STT neurons in C2 serve a function beyond processing and relaying local, segmental information from the spinal cord. Rather, upper cervical STT neurons appear to process and relay information from widespread areas of the body to the thalamus. It is also likely that C2 STT and non-STT neurons play important, although distinct, roles in processing nociceptive information.
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