Autogenetic inhibition of motoneurones by impulses in group Ia muscle spindle afferents.

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Abstract
Inhibitory post-synaptic potentials [IPSP] evoked by adequate stimulation of group Ia muscle spindle afferents of homonymous and synergistic muscles and by selective electrical stimulation of tendon organ afferents were analyzed in motoneurons of triceps surae and plantaris. Selective activation of Ia afferents occurred with brief stretches of triceps surae and plantaris 35 .mu.m or less in amplitude with an initial muscle tension of 5 N; stretches of 30-35 .mu.m were estimated to activate 80-90% of Ia afferents in these muscles. Stretches .ltoreq. 30 .mu.m evoked IPSP in 80% of triceps surae and plantaris motoneurons; lowest thresholds for evoking i.p.s.p. were 10 .mu.m or less. Such low thresholds for stretch-evoked IPSPs, lower than the thresholds for activation of Ib afferents, were found mainly in spinalized, unanesthetized (after decerebration) or lightly anesthetized animals. The latencies of these IPSP indicated disynaptic and trisynaptic coupling between Ia afferents and motoneurons. The IPSP were evoked from the homonymous and synergistic muscles stretched together, from the homonymous muscles alone and from the synergistic muscles alone. Control experiments showed that IPSP could be evoked by stretches subthreshold for discharging motoneurons, thus showing that those IPSP were not mediated by Renshaw cells. The stretch-evoked IPSP disappeared after sectioning the nerves from the corresponding muscles, further excluding their mediation by afferents other than group Ia afferents from the stretched muscle. To selectively activate tendon organ afferents, thresholds for excitation of Ia afferents by electrical stimuli were increased to a level above the threshold for Ib afferents by prolonged muscle vibration (Coppin, et al., 1970). Amplitudes of Ia IPSP evoked by muscle stretches activating about 80% of muscle spindle afferents were compared with amplitudes of Ib IPSP due to less than 50% of tendon organ afferents of the same muscles. Inhibition of motoneurons may be evoked from Ia muscle spindle afferents from homonymous and synergistic muscles and from Ib tendon organ afferents. In view of much stronger inhibitory effects of Ib afferents alone, the Ia inhibition may operate primarily to support inhibition evoked from tendon organs and in conjunction with the latter.