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

doi:10.1113/jphysiol.2008.154898

NEUROSCIENCE

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

Vitor Pinto¹^,2, Peter Szûcs¹, Victor A. Derkach³ and Boris V. Safronov¹^,2

¹ Instituto de Biologia Molecular e Celular, Universidade do Porto, Rua do Campo Alegre 823, 4150-180 Oporto, Portugal
² Laboratório de Biologia Celular e Molecular, Faculdade de Medicina, Universidade do Porto, Alameda Professor Hernâni Monteiro, 4200-319 Oporto, Portugal
³ Vollum Institute, Oregon Health and Science University, 3181 SW Sam Jackson Park Road, Portland, OR 97239, USA

Although it is known that each spinal cord segment receivesthin-fibre inputs from several segmental dorsal roots, it remainsunclear how these inputs converge at the cellular level. Tostudy whether C- and A ${delta}$ -afferents from different roots can convergemonosynaptically on to a single substantia gelatinosa (SG) neurone,we performed tight-seal recordings from SG neurones in the entirelumbar enlargement of the rat spinal cord with all six segmental(L1–L6) dorsal roots attached. The neurones in the spinalcord were visualized using our recently developed oblique LEDillumination technique. Individual SG neurones from the spinalsegment L4 or L3 were voltage clamped to record the monosynapticEPSCs evoked by stimulating ipsilateral L1–L6 dorsal roots.We found that one-third of the SG neurones receive simultaneousmonosynaptic inputs from two to four different segmental dorsalroots. For the SG neurones from segment L4, the major monosynapticinput was from the L4–L6 roots, whereas for those locatedin segment L3 the input pattern was shifted to the L2–L5roots. Based on these data, we propose a new model of primaryafferent organization where several C- or A ${delta}$ -fibres innervatingone cutaneous region (peripheral convergence) and ascendingtogether in a common peripheral nerve may first diverge at thelevel of spinal nerves and enter the spinal cord through differentsegmental dorsal roots, but finally re-converge monosynapticallyon to a single SG neurone. This organization would allow formationof precise and robust neural maps of the body surface at thespinal cord level.