Organization of recurrent inhibition and facilitation in motor nuclei innervating ankle muscles of the cat.

The distribution of recurrent inhibition and facilitation to motor nuclei of muscles that act at the cat ankle joint was compared with the locomotor activity and mechanical action of those muscles described in published studies. Emphasis was placed on motor nuclei whose muscles have a principal action about the abduction-adduction axis and the pretibial flexors: tibialis posterior (TP), peroneus longus (PerL), peroneus brevis (PerB), the anterior part of tibialis anterior (TA) and extensor digitorum longus (EDL). Most intracellular recordings in spinalized, unanesthetized decerebrate cats showed only inhibitory or excitatory responses to antidromic stimulation of peripheral nerves, but mixed effects were also seen. Recurrent effects among motor nuclei of ankle abductors and adductors were not distributed uniformly. TP motoneurons received recurrent inhibition from most other nuclei active in stance and stimulation of the TP nerve inhibited these motor nuclei. Although PerB motoneurons are also active during stance, they received primarily facilitation from most motor nuclei. PerL received mixtures of inhibition and facilitation from all sources. Stimulation of the nerves to PerL, PerB, and peroneus tertius (PerT) produced weak recurrent inhibition and facilitation, even in homonymous motoneurons and motoneurons of Ia synergists. The ankle flexors TA and EDL displayed different patterns of recurrent inhibition and facilitation. TA motoneurons received prominent homonymous inhibition and inhibition from semitendinosus (St). EDL, whose activity profile differs from TA and which also acts at the digits, did not receive strong recurrent inhibition from either TA or St, nor did stimulation of the EDL nerve produce much inhibition. The distribution of recurrent inhibition and facilitation is correlated with the pattern of locomotor activity, but with exceptions that suggest an influence of mechanical action, particularly in the antagonistic interactions between TP and PerB. The extended pattern of recurrent inhibition, the reduction or absence of inhibition produced by motor nuclei with individualized functions or digit function and the prevalence of facilitation suggest that the recurrent Renshaw system is organized into inhibitory and disinhibitory projections that participate in the control of sets of motor nuclei engaged in rhythmic and stereotyped movements.