Corticospinal control of soleus motoneurons in man

Electrical or magnetic stimulation of the human motor cortex causes a strong, short latency facilitation of tibialis anterior (TA) motoneurons but only weak, longer latency changes in the excitability of soleus (SOL) motoneurons. The facilitation of TA motoneurons has been attributed to the monosynaptic action of the "fast" corticospinal pathway. The present study further investigates the cortical control of soleus motoneurons in man. In tests of reaction time to auditory stimuli, normal subjects took significantly longer to activate soleus motoneurons than tibialis anterior motoneurons. Thus we could not demonstrate the existence of a "fast" pathway from the brain to SOL motoneurons that, for some reason, is not activated by magnetic stimulation. The hypothesis that the cortex might control soleus motoneurons indirectly by modulation of the Ia input from muscle spindles was tested. Magnetic stimulation of the cortex was used to condition the facilitation of soleus motoneurons resulting from the stimulation of group I fibres in the tibial nerve. There were no consistent changes in Ia facilitation. We conclude (i) that there is no evidence so far that SOL motoneurons are excited by a direct pathway from the cortex (similar to that projecting to TA motoneurons) and (ii) that the observed changes in firing probability of soleus motoneurons produced by magnetic stimulation over the motor cortex do not result from modulation of presynaptic inhibition of Ia afferents.