Pattern of ‘non‐reciprocal’ inhibition of motoneurones by impulses in group Ia muscle spindle afferents in the cat

Abstract

Inhibitory post-synaptic potentials (IPSP) evoked by adequate stimulation of group Ia muscle spindle afferents of triceps surae and plantaris and by near-threshold electrical stimulation of quadriceps and hamstring nerves were recorded in a number of motoneuron species. The aim of the study was to compare the pattern of non-reciprocal Ia inhibitory actions on hind-limb motoneurons with the pattern of inhibition evoked from group Ib tendon organ afferents. In all the motoneuron species analyzed in which IPSP were evoked by electrical stimulation maximal for both group Ia and Ib afferents of triceps surae and plantaris, they were also evoked when these muscles were stretched and the amplitude of the stretch (10-35 .mu.m) was below threshold for Ib afferents; 70-100% of motoneurons with Ib IPSP showed stretch-evoked IPSP. The stretch-evoked IPSP appeared with latencies compatible with di- and trisynaptic linkage. Since these latencies were too short to allow their mediation by group II afferents the IPSP are attributed to a selective action of Ia afferents. The IPSP did not appear after the nerves to triceps surae and plantaris had been cut. Electrical stimulation of quadriceps and hamstring nerves which was near threshold for Ia afferents and well below threshold for either the Ib component of the incoming volley or group II afferents, similarly evoked non-reciprocal IPSP. They were found in those motoneurons in which inhibition was evoked by stimulation maximal for group I afferents. Such Ia IPSP were evoked both in homonymous motoneurons and in motoneurons of 4 other hind-limb muscles. Their latencies corresponded to di- and trisynaptic coupling. In some motoneurons of the pretibial flexors (anterior tibial, extensor digitorum longus and peroneus longus), disynaptic IPSP evoked from triceps surae and/or plantaris which were depressed by a conditioning ventral root stimulation (i.e., Ia reciprocal IPSP) were followed by trisynaptic IPSP which were not depressed in this way (Ia non-reciprocal IPSP). It thus appears that the same motoneurons may be inhibited by impulses in group Ia afferents via different spinal pathways. The non-reciprocal inhibition from group Ia muscle spindle afferents operates in parallel with the inhibition from group Ib tendon organ afferents in all motoneuron species tested.