Characteristics of spinal neurones responding to cutaneous myelinated and unmyelinated fibres.

1. Spike discharges were recorded from neurones in the lumbar spinal cord in cats anaesthetized by barbiturate.2. The neurones were examined systematically for various physiological parameters and for their location. Especially the neurones situated in the dorsal horn were classified for the following parameters: mono- or polysynaptic linkage to myelinated afferents; type of natural stimuli which excited the neurones; depth from the cord surface; number of impulses discharged upon a cutaneous A fibre stimulus; steady-state discharge in the absence of intentional stimulation.3. All neurones were also tested as to whether or not they responded to volleys in cutaneous C fibres. Of 111 units which were activated by the A fibres in nerves from the hairy skin, 57 (= 51%) responded to C volleys in those nerves too.4. By blocking conduction in the A fibres using polarizing currents it was shown that the responses to C fibre volleys were partially or totally suppressed by a preceding discharge of the neurone in response to an A volley. Using search stimuli which were suprathreshold for C fibres one cell out of 36 could be found which responded only to afferent volleys in C fibres.5. About half of all neurones were shown to be connected monosynaptically to cutaneous A fibres, as was judged from the synaptic delay. The other half were polysynaptically linked to the A fibres. Both mono- and polysynaptic neurones were found in all layers of the dorsal horn. About 15% of the cells had additional input from muscle Group II and/or III fibres via polysynaptic pathways.6. Subdividing the A and A+C responsive neurones according to their mono- (M) or polysynaptic (P) connexions yielded the following sub-samples: MC, 39%; PC, 15%; MA, 13%; PA, 33%. Most MC neurones had, and most PA units had not, a spontaneous discharge. About half of the PA cells could not be driven by natural skin stimulation. The majority of MC units responded specifically to movement of hairs.7. A model was proposed hypothesizing two pathways in the dorsal horn, one showing convergence of A and C fibres and the other not. Some relations concerning other observations on C fibre effects were discussed.