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Istituto di Fisiologia Umana II, Università degli Studi di Milano, Italy.

1. The possibility that the Ia afferent fibres from the gastrocnemius medialis muscle could be responsible for a decrease in excitability of the soleus motor pool was investigated. 2. The soleus H reflex, evoked by tibial nerve stimulation in the popliteal fossa, was conditioned by a single stimulus to the gastrocnemius medialis nerve at various stimulus intensities and conditioning-test intervals. Care was taken to avoid spread of current from the conditioning stimulus to the tibial nerve, and the results obtained by surface stimulation were compared with those obtained by stimulation through a needle whose tip was positioned closer to the nerve. 3. Stimulation of the gastrocnemius medialis nerve induced two short-lasting periods of inhibition in the soleus H reflex, peaking at about 0 and 5 ms of conditioning-test delay. The early inhibition could begin at a stimulus strength as low as 0.5 x MTh (the Motor Threshold). The later inhibition appeared on greater stimulus strength than the earlier. 4. Prolonged vibration of the Achilles tendon abolished the capability of the conditioning stimulus to induce the short-latency inhibition of the soleus H reflex. 5. By stimulating the gastrocnemius medialis nerve at two points separated by a known distance, the conduction velocity of the fibres responsible for the early inhibition was estimated, and found to be around 100 m s-1. 6. Isometric leg flexion, accomplished by tonic activation of gastrocnemius medialis and lateralis but not soleus, was able to induce an inhibition of the soleus H reflex even at very low levels of gastrocnemius electromyographic activity. 7. These findings strongly suggest the existence of an inhibitory effect of primary spindle afferent fibres from the gastrocnemius medialis muscle onto the soleus motor pool. This is not unexpected, since the gastrocnemius medialis muscle can be either agonist or antagonist to the soleus muscle in the performance of different movements.

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