1. Somatosensory representation of the forelimb in the dorsal horn of spinal segments C5-T2 was examined in 13 North American raccoons anesthetized with pentobarbital sodium. Single- or multiple-unit reponses to light mechanical stimulation were recorded at a total of 504 loci, which were subsequently reconstructed from stained, transverse sections. From these, dorsal view maps of forelimb representation in Rexed's laminae III and IV were synthesized.
2. There was a shifting, serial overlap of representations of different forelimb regions in both the rostrocaudal and mediolateral axes of the dorsal horn. The rostrocaudal progression of receptive fields was from preaxial forelimb to forepaw to postaxial forelimb, whereas the mediolateral progression was from the volar glabrous forepaw toward the trunk.
3. Representations of the glabrous surfaces of the digits and palm pads showed considerable overlap, with the digital representations extending more laterally, but the palmar representations extending more rostrally and caudally. One-third of all recording loci were devoted exclusively to glabrous skin representation.
4. Comparison with results of earlier studies in raccoons indicates that representation of a given digit or palm pad is more restricted in rostrocaudal extent in the dorsal horn that in the dorsal root, and that, compared with various nuclear regions of dorsal column-medial lemniscal system, the glabrous surfaces of the forepaw are underrepresented in the dorsal horn. The results suggest that there is a dorsoventral modular organization of forelimb representation in the dorsal horn. These wedge-shaped modules form larger aggregates which represent major body subdivisions and which course sinuously in the rostrocaudal dimension.