Nociceptive reflexes evoked by TTX-resistant C-fibre afferents and their sensitivity to opioids in the cat.

The contribution of nociceptive Adelta-fibres and C-fibres of the central pad of the foot to nociceptive spinal flexor reflex pathways (FRA-type) and to nociceptive excitatory reflex pathways to foot extensors (non-FRA type) was investigated in high spinal cats by blocking A-fibres completely by TTX; effects persisting after TTX were attributed to nociceptive C-fibres. The results revealed that both Adelta- and C-fibre afferents contributed to nociceptive reflexes of an FRA pattern and of a non-FRA pattern, the effects of Adelta-fibres being evoked with a distinctly shorter delay than those of C-fibres. Partly Adelta-fibres exerted a significant inhibitory influence on the C-fibre action in FRA pathways. A distinct part of the opioid action on nociceptive reflex pathways of the FRA-type and of the non-FRA-type was evidently exerted via C-fibres.