The firing probability of single motor units following transcranial magnetic stimulation in healthy subjects and patients with neurological disease.

Magnetic stimuli applied to the scalp can cause single motor units (MUs) to discharge in intrinsic hand or more proximal arm muscles. MUs are much more likely to be fired by stimuli if they are being activated voluntarily at the same time. The changes in firing probability induced by magnetic stimuli in such tonically active MUs have been studied in healthy subjects and patients using peri-stimulus time histograms (PeSTHs). The mean firing level in the 250 msec before the stimulus, was used to define 2 peaks of increased firing after the stimulus. The first, termed the primary peak (PP), was always present, had an onset latency in first dorsal interosseous (FDI) muscle of 20-31 msec and was short in duration (mean 4.9 msec). A second increase in firing probability, the secondary peak (SP), was present in about half the FDI MUs studied, had a mean peak latency of 81.5 msec and had a duration up to about 40 msec. PP often consisted of a number of sub-peaks with intermodal intervals of around 1.5 msec. It is believed that PP is the result of activation of corticospinal (CS) fibres making monosynaptic connections and that sub-peaks within PP are due to the arrival at the motoneuron of successive CS impulses produced by the stimulus. SP is probably due to operation of other excitatory inputs to the motoneuron. In patients, a number of different abnormalities have been identified. PP has been found to be normal, absent, delayed and dispersed or having abnormally separated sub-peaks. Clearly the technique is a powerful tool for dissecting abnormalities in CS projections at the synaptic level.