Dorsal root ganglion neurones with dichotomising axons are present in several species and are considered to play a role in referred pain. Clinically, patients with lesions in the lower lumbar discs occasionally complain of pain in the groin. We investigated the existence of dichotomising afferent neurones projecting axons both to the lumbar disc and to the groin skin, using the double fluorescent-labelling technique in rats.

We observed neurones labelled with a tracer applied at the ventral portion of the L5-L6 disc and another tracer placed on the groin skin in L1 and L2 dorsal root ganglia. Our results showed that the double-labelled neurones had peripheral axons which dichotomised into both the L5-L6 disc and the groin skin, indicating the convergence of afferent sensory information from the disc and groin skin. Our findings provide a possible neuroanatomical mechanism for referred groin pain in patients with disc lesions.

Based on a study using a retrograde neurotracer, we have previously found that the dorsal portion of the L5/6 disc in the rat is multisegmentally innervated by dorsal root ganglia (DRG) from the level of T13 to L6, and that sensory nerve fibres from DRG of T13, L1 and L2 pass through the paravertebral sympathetic trunks. In this study in newborn rats, we injected crystals of 1,1′-dioctadecyl-3,3,3′,3′-tetramethylinedocarbocyanine perchlorate (DiI) into the DRG of T13, L1 and L2 and showed DiI-labelled sensory nerve fibres in the dorsal portion of the discs from the level of T13/L1 to L5/6. Our results show that the dorsal portion of the lumbar discs is innervated by the DRG from levels T13 to L2.