Somatotopy of spinal nociceptive processing.

Relatively little is known about the spatial organization of spinal nociceptive processing. This study has employed the expression of c-fos-like protein as a marker for neuronal activity and has analyzed the patterns of immunoreactivity seen within the rodent spinal cord following noxious mechanical stimulation of various portions of one hindlimb. The results indicate that noxious mechanical stimulation induces distinct, somatotopic patterns of immunolabeling in laminae I-IV. Individual digits of a foot are represented medially in the dorsal horn over a short rostrocaudal distance, with the most lateral digit represented approximately one segment caudal to the most medial digit. Representation of the hip region is more lateral, is centered at L2, and extends rostrocaudally over many segments. The patterns of neuronal excitation seen in laminae V-IX following noxious peripheral stimulation were similar to those noted in laminae I-IV but were less tightly organized. C-fos-like immunoreactivity was noted both medially and laterally in the deeper laminae following stimulation of any portion of the hindlimb, but stimulation of different areas produced different columns of labeled cells extending from the superficial dorsal horn into lamina VII. In the rostrocaudal direction, immunolabeling in lamina V-IX was maximal at the same segmental level as in laminae I-IV, but the more ventral laminae exhibited increases in c-fos-like immunoreactivity over longer rostrocaudal distances. Experiments in spinally transected animals indicated that long, descending pathways contributed little or nothing to the pattern of immunolabeling. The results of this study imply that spinal nociceptive processing is spatially organized not only in laminae I-IV, but also in more ventral regions of the spinal cord.