Response properties of neurons of the lateral cervical nucleus in the rat

Pursuing a recent anatomical study which revealed the presence of a lateral cervical nucleus (LCN) in the rat, the present investigation sought to characterize electrophysiologically various properties of 98 neurons lying within the dorsolateral funiculus of C2 in urethane anesthetized rats. Cells were examined for their responsiveness to hair movement, touch, pressure, and noxious pinch. All LCN neurons gave rapidly adapting bursts of activity to hair movement on at least a portion of the ipsilateral body surface. Such cells were often driven by movements of as little as 1 mm of what appeared to be a single hair. Firing was also evoked by hair movement within contralateral receptive fields in 40% of LCN neurons. Receptive fields frequently included two or more body quadrants and in some case covered 90% of the available body surface, including the entire head. Seventy‐three percent of LCN neurons responded maximally to innocuous cutaneous stimuli. The remaining 27% also responded to such stimuli but responded more vigorously and tonically to noxious pinching of the skin in a manner related to stimulus intensity. Thirty‐four cells were also tested for responsiveness to increased skin temperature within their receptive fields. Twenty‐two of 23 LCN neurons responsive only to hair movement were unaffected by temperature increases up to 49°C. In contrast, 10 of 11 cells activated by noxious mechanical stimuli were powerfully activated by increases in skin temperature into the noxious range (>43°C). Three thermosensitive LCN neurons also responded clearly to innocuous skin temperature increases. Twenty‐two LCN neurons were tested for the presence of an input from the viscera using intraperitoneal injections of a hypertonic saline solution. Again, LCN neurons responsive only to hair movement and/or touch were unaffected by this stimulus. Four LCN neurons activated by pinch on regions of the body other than the abdomen were also unresponsive to such injections. On the other hand, five of six neurons responsive to pinch within receptive fields including a portion of the abdomen were powerfully activated by hypertonic saline for up to one minute, paralleling the duration of nociceptive responding caused by the same injection in the awake rat. Equal volumes of physiological saline were without effect on these cells. Forty‐five LCN neurons were tested for a projection to the contralateral thalamus using antidromic activation techniques. The 43 cells found to contribute to the cervico‐thalamic tract had a mean conduction velocity of 6.93 m/sec. This surprisingly slow conduction velocity in the rat suggests that this pathway may serve a different function than the cervico‐thalamic tract in the cat. To assess the effects of urethane anesthesia on responding of LCN neurons to innocuous and particularly to noxious stimuli, the LCN was explored in six spinalized, decerebrate, unanesthetized rats. In these animals, 89% of LCN neurons responded only to hair movement. The remaining 11% responded differentially to noxious pinch. These percentages do not significantly differ from those found in the anesthetized animal. These findings confirm and extend our earlier anatomical demonstration of a lateral cervical nucleus in the rat and indicate that a comparatively large population of LCN neurons is capable of coding nociceptive information in this species. In addition, these data indicate that, unlike the cat, the LCN of the rat is not somatotopically organized. Finally, the high degree of spatial convergence seen to characterize LCN cells would seem to preclude a major role for this nucleus in the accurate localization of somatic stimuli.