Presynaptic excitability changes of group Ia fibres to muscle nociceptive stimulation in humans

Tonic muscle nociceptive discharge evoked chemically from the foot extensor digitorum brevis muscle in man produces a depression of Ia excitation and Ib inhibition of the soleus (Sol) motoneurones (Mns). The possibility that both these changes partly result from presynaptic inhibition of Ia fibres p rojecting to Sol Mns and to interneurones mediating group I non-reciprocal inhibition is tested. Convergence of Ia fibres on these interneurones was d educed from evidence that reducing the excitatory effect of the extensor la fibres (by potentiation of their presynaptic inhibition) resulted in Tb di sinhibition. Nociceptive-induced potentiation of Ia presynaptic inhibition was deduced From the following congruent results obtained by two independen t methods: (1) depression of heteronymous Ia monosynaptic facilitation of t he quadriceps muscle to Sol Mns: (2) potentiation of presynaptic inhibition of Sol Ia fibres evoked by mechanical activation of the tibialis anterior primary spindle afferents. it is concluded that nociceptive volleys arising from dorsal foot muscles facilitate the activity of interneurones intercal ated in pathways responsible for presynaptic inhibition of Sol Ia fibres. I t is also proposed that the same la presynaptic inhibition depresses the ex citability of interneurones mediating group I non-reciprocal inhibition, th us resulting in Ib disinhibition of Sol Mns. (C) 1999 Elsevier Science B.V. All rights reserved.