This study tested whether the recurrent inhibition of soleus and wrist flexor motoneurones could be modified by transcranial magnetic stimulation in human subjects. 2. Magnetic stimulation was given through a circular coil centred at the vertex. The intensity of the magnetic stimulus was subthreshold for evoking a motor response in the active soleus and wrist flexor muscles. The recurrent inhibition brought about by a conditioning H1 reflex discharge was estimated by a test H' reflex. The modifications of the recurrent inhibition after cortical stimulation were distinguished from the motoneuronal changes by comparing H' to a reference H reflex. 3. In the soleus motoneurones, the reference H reflex was inhibited at a minimum conditioning-test interval of -2 ms (H reflex stimulus before magnetic stimulation). In contrast, the H' reflex was facilitated at minimum conditioning-test intervals of +1 ms. In the wrist flexor motoneurones, both H' and reference H reflexes were facilitated. However, at lower cortical stimulus intensities, only the H' reflex was facilitated at minimum conditioning-test intervals of +1 ms. 4. In both motoneurone pools, H' facilitation started 3-4 ms later than the earliest changes in the reference H reflex. Also, the threshold of H' facilitation was lower than that of reference H reflex. 5. It is concluded that facilitation of the H' reflex is produced by corticospinal inhibition of Renshaw cells via a short interneuronal chain in both the upper and lower limb.
