Evidence favouring different descending pathways to soleus motoneurones activated by magnetic brain stimulation in man.

1. In resting subjects low-intensity magnetic stimulation of the brain evoked an inhibition of the soleus H reflex at short latency (conditioning-test interval, -2 to +1 ms) followed approximately 10 ms later by a period of facilitation. During voluntary dynamic or tonic plantar flexion the same stimulus evoked a facilitation with a shorter latency than the inhibition (conditioning-test interval, -5 to -1 ms). 2. At the onset of ramp-and-hold plantar flexion the short-latency facilitation was seen at lower intensities of stimulation than the long-latency facilitation in six of seven subjects. At rest and/or during tonic plantar flexion the opposite was observed in four of the subjects, whereas the two facilitations had approximately the same threshold in the remaining subjects. 3. The short-latency facilitation decreased approximately 100 ms after the onset of ramp-and-hold plantar flexion in all of eight subjects. The long-latency facilitation, in contrast, either had the same size throughout the ramp phase or even increased around the end of the ramp phase. 4. The short-latency facilitation of the reflex was significantly larger at the onset of a fast ramp-and-hold plantar flexion (10 N m (150 ms)-1) than at the onset of a slow contraction (10 N m (600 ms)-1), whereas the opposite was the case for the long-latency facilitation. 5. As the short- and long-latency facilitations had different thresholds and were differently regulated during voluntary movement, it is suggested that they are caused by activation of different descending pathways by the magnetic stimulus.