Supraspinal connections of neurones in the thoracic spinal cord of the cat: ascending projections and effects of descending impulses.

Single unit electrical activity has been recorded extracellularly from 103 neurones in the thoracic spinal cord of decerebrate cats. The responses of these neurons to electrical stimulation of cutaneous and visceral afferent fibres, their projection through ascending sensory pathways and the effects of descending impulses on the neurones have been studied. Of the 103 neurones recorded, 45 (43.7%) responded only to activation of cutaneous afferent fibres ('Somatic' neurones). Their recording sites were located mainly in laminae II, III and IV of the dorsal horn. The remaining 58 neurones (56.3%) responded to stimulation of cutaneous and visceral afferent fibres ('Viscero-somatic' neurones). Their recording sites were located in laminae I, V, VII and VIII of the grey matter. Sixteen neurones had axons projecting through ascending pathways: 6 were post-synaptic dorsal column cells (PSDC), 2 were spino-cervical tract cells (SCT), 5 projected through the contralateral ventro-lateral funiculus (VLQ) and 3 through the ipsilateral dorso-lateral funiculus (DLF). All PSDC cells were somatic and all VLQ neurones were viscero-somatic. Reversible spinalization of the animals by cold block resulted in a selective increase of the responses of viscero-somatic neurones to cutaneous and visceral C-fibre input. In some viscero-somatic neurones, cold block induced a reduction or abolition of the visceral input suggesting its mediation via supraspinal loops. Electrical stimulation of the ipsilateral DLF evoked non-specific inhibitions of all inputs to viscero-somatic neurones. These results are discussed in relation with the mechanisms of visceral sensation.