Responses of single spinal motoneurons to magnetic brain stimulation in healthy subjects and patients with multiple sclerosis

Magnetic stimuli delivered over the scalp can cause single motor units to discharge in intrinsic hand muscles. The discharge characteristics of 26 tonically active, low threshold single motor units in the first dorsal interosseous muscle of 13 healthy subjects and of 21 motor units in 9 patients with multiple sclerosis (MS) were studied. Up to 500 transcranial magnetic stimuli were delivered at the vertex and were given randomly with respect to, or at a fixed delay after, the previous voluntary discharge. Peristimulus time histograms (PSTHs) of motor unit discharges were constructed. In healthy subjects, two periods of increased firing probability were seen at onset latencies of 20-31 ms and 56-90 ms after the stimulus. These periods have been termed the primary peak and the secondary peak. The primary peak had a mean duration of 4.6 ms and was found to be multimodal in 17 motor units. Subpeaks had intermodal intervals of between 0.6 and 2.4 ms. Subpeaks probably result from a sequence of excitatory postsynaptic potentials (EPSPs) induced at the motoneuron by corticospinal impulses. In patients with MS, the primary peak could be absent, delayed in onset, of increased duration without discernible subpeaks, or showing increased intermodal intervals between subpeaks. In 3 motor units from patients with MS, PSTHs with normal features were found. It is postulated from this study of the corticospinal inputs to single motoneurons that motor impairment in MS can be due to a number of mechanisms including slowed conduction in corticospinal fibres, dispersion of arrival times of corticospinal impulses at spinal motoneurons and conduction block in corticospinal fibres, which may be frequency dependent.