SPINAL REFLEX PATTERN TO FOOT NOCICEPTIVE SIMULATION IN STANDING HUMANS

Ipsi-and contralateral patterns of lower limb nociceptive reflex respo nses were studied in 6 normal subjects in free standing position. Once the position was stabilized, only ankle extensor muscles showed consi stent tonic activity while ankle flexors and knee extensors and flexor s were virtually silent. Reflex responses, elicited by painful electri cal stimuli to the skin of the plantar and dorsal aspect of the foot, were recorded from ipsi-and contralateral quadriceps (Q), biceps femor is (Bic), tibialis anterior (TA) and soleus (Sol) muscles. Plantar foo t stimulation evoked a large excitatory response in the ipsilateral TA at about 80 ms and a smaller responses in Bic and Q at 70 ms and 110 ms, respectively. Ipsilateral excitatory effects after dorsal fool sti mulation consisted of a Bic response at about 75 ms. In addition to ex citatory effects, both plantar and dorsal foot stimulation evoked long -lasting suppression of ipsilateral Sol background activity starting a t about 60 ms. Contralaterally, the only nociceptive effects after pla ntar or dorsal foot stimulation were a small excitatory response of So l at about 85 ms. Evidence is provided that only excitatory responses were contingent upon nociceptive volley. The main mechanical effects s een after plantar stimulation were dorsiflexion of the foot without lo ss of heel contact with the floor, no withdrawal response of the foot followed nociceptive dorsal stimulation. Our main conclusion is that o nly reflex nociceptive responses serving to avoid the stimulus without conflicting with limb support function are expressed. The mechanisms reconciling nociceptive action and postural function of the lower limb s are discussed. (C) 1997 Elsevier Science Ireland Ltd.