CHANGES IN IB HETERONYMOUS INHIBITION TO SOLEUS MOTONEURONS DURING CUTANEOUS AND MUSCLE NOCICEPTIVE STIMULATION IN HUMANS

The effects of selective tonic cutaneous and muscle pain stimulation o n heteronymous Ib pathways from the gastrocnemius medialis to the sole us motoneurones were studied in five healthy human subjects. Tonic pai n stimulation, monitored by a psychophysical method, was obtained by l ocal injection of 60 mg levo-ascorbic acid (L-AS) in a volume of 0.3 m i. Nociceptive cutaneous and muscle stimulation of the dorsal foot pro duced opposite, long-lasting changes in heteronymous Ib pathways to so leus motoneurones: Ib facilitation during cutaneous stimulation was re versed by disinhibition during muscle stimulation. The time-course of these Ib changes strictly paralleled that of subjective pain sensation . On the contrary, when the same nociceptive stimuli were applied to t he leg (at pretibial level), the differences between cutaneous and mus cle pain disappeared and similar short-lasting phases of facilitation and inhibition of Ib activity were observed. It is concluded that toni c discharge of cutaneous and muscle nociceptive afferents arising from the foot have specific and opposite effects on Ib inhibitory pathways to ankle extensor motoneurones. These interactions between muscle and cutaneous nociceptive and Ib pathways may be used to change muscle sy nergies, thus contributing to the establishment of appropriate adaptiv e locomotor strategies during pain. (C) 1997 Elsevier Science B.V.