Parallel nociceptive reflex pathways with negative and positive feedback functions to foot extensors in the cat

1. Nociceptive reflex pathways to foot extensors were investigated with par ticular attention given to those not following a flexor reflex (FRA) or wit hdrawal pattern. 2. In anaemically decapitated, high spinal paralysed cats nociceptive affer ents of the foot pad were activated by noxious radiant heat (48-60 degreesC ), while for comparison non-nociceptive afferents were activated by weak me chanical stimulation of the skin or graded electrical nerve stimulation. Th e reflex action of the afferents on hindlimb motoneurones, innervating plan taris and intrinsic foot extensors (tibial nerve), was investigated by intr acellular recording, by monosynaptic reflex testing and by recording of neu rograms during fictive locomotion. A possible descending control of the noc iceptive and non-nociceptive pathways was tested by application of opioider gic and monoaminergic compounds. 3. Beside the typical FRA pattern evoked in the majority of hindlimb motone urone pools by nociceptive afferents from different skin areas of the foot, the results revealed parallel excitatory and inhibitory nociceptive reflex pathways from the central pad and partly from the toe pads to foot extenso rs. The excitatory pathways, which did not follow the FRA pattern, were pre dominantly to plantaris and intrinsic foot extensors. They were distinctly less depressed by opioids and monoaminergic compounds than FRA pathways. 4. While the nociceptive FRA pathways have a general nocifensive withdrawal function, the nociceptive excitatory non-FRA pathway to the foot extensors causes a movement of the affected area towards the stimulus or at least a resistance against the stimulus, i.e. it mediates a positive feedback.