tromagnetic stimulation of the brain was used to examine the corticospinal projections to the motoneurons of upper and lower limb muscles in healthy subjects and in patients with neurological disorders. On the basis of preliminary studies designed to explore the currents induced by magnetic stimulation, the coil was applied flat on the scalp overlying the motor cortex in order to activate a large population of motor cortical neurons. Recordings from limb muscles were made with surface electrodes and information about the postsynaptic potentials (PSPs) in single spinal motoneurons was derived from peristimulus time histograms (PSTHs) of single voluntarily activated motor units.

Magnetic stimulation resulted in short latency activation of contralateral muscles in a distinct pattern which reflected the magnitude of the estimated underlying excitatory PSPs generated in motoneurons. The conduction velocity of the responsible pathway was calculated to be 45-67 m/s. The durations of the PSTH peaks were short, the "jitter" low and the pattern of motoneuron facilitation similar to that of the projections of the corticospinal tract in nonhuman primates. It is concluded that magnetic stimulation applied in this manner indirectly activates the fast conducting corticospinal tract which makes monosynaptic connections to spinal motoneurons.

In subjects with cerebral palsy (CP), the normal pattern of facilitation to lower limb motoneurons (strong facilitation of tibialis anterior and little or no activation of soleus) was altered. The motoneurons of both of these muscles were facilitated equally. This pattern appeared to be unique to CP as it did not occur in patients with adult onset cerebral lesions or incomplete spinal cord injuries. It is postulated that damage to the immature brain results in altered corticospinal projections. These aberrant projections may help to explain the impairment of voluntary movements in CP which are often characterized by co-contraction of agonist and antagonist muscles.