Effects of activity in non-myelinated afferent fibres on the spinocervical tract

Microelectrode recordings were made from axons of the spinocervical tract (SCT) in unanaesthetized decerebrate-spinal cats. Pure volleys in non-myelinated (C) cutaneous nerve fibres were obtained by DC-polarization block of conduction in the myelinated (A) fibres, and SCT cells were classified according to whether or not they were excited by C fibres. The effects of conditioning C responses in the SCT with A and C fibre input were examined, as were the effects of conditioning A fibre responses in the tract with C fibre inputs. The effects of stimulating descending systems in the cervical spinal cord on the C-fibre-evoked responses of SCT cells were determined. SCT units with a slowly-adapting pressure-sensitive component in their receptive fields responded to cutaneous C fibres in addition to A fibres. Units which responded only to hair movement when the receptive field was stimulated mechanically were not excited by C fibres. Responses in SCT neurones produced by C fibres were profoundly inhibited by conditioning volleys in A fibres of both ipsilateral and contralateral cutaneous nerves and by activity in several descending systems. Conditioning with C fibres failed to affect the responses of SCT cells to either A or C fibres. It is concluded that cutaneous C fibres excite some SCT cells but have no other effect on transmission through this system. It is suggested that there are common inhibitory interneurones in the paths from descending systems and cutaneous A and C fibres to the excitatory input to SCT cells.