1. The patterns of facilitation and inhibition of the Sa and Sb components of the post-ganglionic compound action potential after a single conditioning stimulus were different and dependent on stimulus parameters.2. With submaximal conditioning and test stimuli, the Sa component showed a phase of early facilitation (40-75 msec after the conditioning stimulus) followed by a prolonged tail of facilitation. With maximal stimuli, early facilitation and late facilitation (700-2000 msec after the conditioning stimulus) were separated by a phase of inhibition or relative inhibition, most pronounced 100-300 msec after the conditioning stimulus.3. During early facilitation, a submaximal Sa response was facilitated by 33·1 ± 3·9%, while a maximal Sa response was facilitated by 14·5 ± 2·9%.4. Providing preganglionic C fibres were excited, facilitation of the Sb component remained relatively constant for 40-500 msec after the conditioning stimulus, with no phase of inhibition.5. Early facilitation of submaximal Sa responses was greatest when the conditioning stimulus excited about 50% of the preganglionic B fibres, but that of maximal responses was greatest when the conditioning stimulus excited all the B fibres. The preganglionic C fibres modulated facilitation of the Sa component. Maximal facilitation of this component was associated with depression of the Sb component.6. Submaximal Sa responses are more strongly inhibited than maximal Sa responses 200 msec after a conditioning stimulus. The C fibre pathway seems able to modulate the degree of inhibition of the Sa ganglion cells.7. A neuronal model with divergent and convergent preganglionic B and C fibres supplying Sa ganglion cells is consistent with the results. The preganglionic input is able to vary the size of the subliminal fringe. The Sb component is in part due to the Sa ganglion cells firing to their C fibre input.
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