Monosynaptic convergence of C- and Aδ-afferent fibres from different segmental dorsal roots on to single substantia gelatinosa neurones in the rat spinal cord

Although it is known that each spinal cord segment receives thin-fibre inputs from several segmental dorsal roots, it remains unclear how these inputs converge at the cellular level. To study whether C- and A delta-afferents from different roots can converge monosynaptically on to a single substantia gelatinosa (SG) neurone, we performed tight-seal recordings from SG neurones in the entire lumbar enlargement of the rat spinal cord with all six segmental (L1-L6) dorsal roots attached. The neurones in the spinal cord were visualized using our recently developed oblique LED illumination technique. Individual SG neurones from the spinal segment L4 or L3 were voltage clamped to record the monosynaptic EPSCs evoked by stimulating ipsilateral L1-L6 dorsal roots. We found that one-third of the SG neurones receive simultaneous monosynaptic inputs from two to four different segmental dorsal roots. For the SG neurones from segment L4, the major monosynaptic input was from the L4-L6 roots, whereas for those located in segment L3 the input pattern was shifted to the L2-L5 roots. Based on these data, we propose a new model of primary afferent organization where several C- or A delta-fibres innervating one cutaneous region (peripheral convergence) and ascending together in a common peripheral nerve may first diverge at the level of spinal nerves and enter the spinal cord through different segmental dorsal roots, but finally re-converge monosynaptically on to a single SG neurone. This organization would allow formation of precise and robust neural maps of the body surface at the spinal cord level.