THE EFFECTS OF ELECTRICAL-STIMULATION OF A-VISCERAL AND C-VISCERAL AFFERENT-FIBERS ON THE EXCITABILITY OF VISCEROSOMATIC NEURONS IN THE THORACIC SPINAL-CORD OF THE CAT

Single unit electrical activity has been recorded from viscerosomatic neurones in the lower thoracic spinal cord of decerebrate spinalized cats. The responses of the cells to electrical stimulation of afferent fibres in the splanchnic (SPLN) nerve and the effects of repetitive stimulation of somatic and visceral afferent C-fibres have been studied. Four groups of viscerosomatic neurones could be distinguished according to the type of visceral afferent input of the cells: (1) A-only cells (32.9%), driven only by stimulation of Aδ afferent fibres in the SPLN nerve; (2) C-only cells (3%), driven only by stimulation of C afferent fibres in the SPLN nerve; (3) A+C cells (45.7%), driven by both Aδ and C afferent fibres in the SPLN nerve; and (4) A+C? cells (18.6%), driven by Aδ visceral afferents and showing signs of responsiveness to C-fibres though lacking a distinct response volley to visceral C-fibre activation. Two cells of the A+C group and located in lamina I of the dorsal horn responded to SPLN nerve stimulation in a manner consistent with the afferent fibre composition of the nerve, that is, showed evidence of strong monosynaptic links with SPLN afferent C-fibres and weaker responses to SPLN Aδ afferents. Excitability changes of viscerosomatic neurones ('wind up', 'wind down' and changes in background activity) were also observed in the majority of neurones following electrical stimulation of somatic and of visceral afferent C-fibres. It is concluded that the functional relay of visceral afferent fibres in the spinal cord does not merely follow a representation of the afferent fibre composition of visceral nerves. Evidence from this study suggests a strong and highly divergent action of visceral Aδ afferents in the spinal cord and a more diffuse effect of visceral C-fibres. © 1990.