Somatotopical organization of fos-like immunoreactivity in rat cervical spinal cord following noxious stimulation of the forelimb.

In the present study c-fos expression has been used as a marker of neuronal activation following noxious stimuli applied to one of three different sites on the forelimb in rats. In three treatment groups (n=4 animals in each group) rats were anaesthetized with barbiturate and a mechanical pinch was applied to either (i) the most medial digit, (ii) the most lateral digit, or (iii) the shoulder area of one forelimb. An additional control group (n=4) received no pinch. The presence of Fos-like immunoreactivity was used to chart the distribution of cervical spinal cord neurons activated by the stimulus. No significant difference was found in the number of labelled cells between the contralateral side of each treatment group and either side of the control group. By contrast, there was a significant increase in labelled cells between the ipsilateral and contralateral sides within each treatment group. Labelled cells were present mainly in the dorsal horn of the ipsilateral cervical spinal cord where they were clustered in laminae I and II. Clear topographical differences were also evident between treatment groups in the distribution of labelled cells. The most medial digit was represented rostromedially compared to the most lateral digit (cell peak at segmental levels C5/C6 and C7, respectively), while the shoulder stimulus produced a more widespread distribution of labelled cells which was centred rostrolaterally (peak at segmental levels C4/C5).Overall, the findings suggest that forelimb inputs to the cervical cord are organized somatotopically in a similar fashion to hindlimb inputs to the lumbar cord, although the representation of individual forelimb digits may be more extensive in the rostrocaudal axis. This difference could reflect the use of the rats' forepaws in more complex sensorimotor tasks such as grasping and exploring objects.