Corticospinal projections to upper and lower limb spinal motoneurons in man

The pattern of muscle activation produced by magnetic stimulation over the human motor cortex has been examined in normal subjects. Magnetic stimulation elicited short latency compound muscle action potentials (CMAPs) from upper limb muscles contralateral to the stimulus and from lower limb muscles bilaterally. In the upper limb, the hand and forearm muscles had the lowest thresholds for activation and the largest CMAPs. There was little or no activation of the more proximal muscles, biceps and tricepts. In the lower limb the extensor digitorum brevis and tibialis anterior had low thresholds and large CMAPs. There was little or no short latency activation of soleus. This pattern of muscle activation remained the same even when the stimulating coil was moved to various locations within a radius of 3 cm from the vertex. Recordings from individual tibialis anterior and soleus motor units indicated that the differential activation of these muscles reflected the relative amplitudes of the short latency postsynaptic potentials produced by magnetic stimulation in their motoneurons. The 'jitter' in the activation of single muscle fibers by magnetic stimulation suggests that these projections are monosynaptic. It is argued that the pattern of muscle activation reflects the projections from the cortex to motoneurons rather than the projections of afferents onto cortical neurons. We conclude that magnetic stimulation can be used to identify the projections of the corticospinal pathways in man.