VARIABILITY OF MOTOR EVOKED-POTENTIALS - COMPARISON OF 3 SMALL HAND MUSCLES

The size of motor evoked potentials (MEP) produced by transcranial mag netic stimulation (TCMS) varies considerably even with constant stimul us conditions. In experiment 1 we investigated whether this variabilit y occurs in several muscles synchronously. In 20 healthy subjects we d elivered 42 consecutive stimuli with a figure-8-coil centered over the hand area of the motor cortex. Stimulus intensity was 110% of the ind ividually determined motor threshold. The stimuli were delivered at a constant frequency of 5/min. We recorded simultaneously from small han d muscles (APE; FDI; ADM) with the subject being at rest. For each mus cle we calculated the change of MEP-amplitude from one stimulus to the next and correlated these changes between the respective muscles. We almost exclusively found positive correlations, the value of which, ho wever, showed considerable inter- and intraindividual variability. The mean value of the correlation coefficients was +0.4, but they varied between -0.2 and +0.88. As the positive correlation shows, the muscles had the tendency to react similarly, but they also reacted independen tly of each other. We suggest that the fluctuations in the excitabilit y level of motoneurones are caused by synchronizing and desynchronizin g influences. In a second experiment we examined the variability of th e amplitudes with differing pre-innervation and stimulus intensity. In 5 healthy volunteers we delivered 3 series of 42 stimuli and recorded the responses from the first dorsal interosseus muscle (FDI). In the first series stimulus intensity was 110% of the motor threshold, deter mined with the relaxed muscle, recordings were obtained from the relax ed FDI. In the second series stimulus intensity was the same, but reco rdings were now obtained from the preinnervated muscle. This was the s ame in the third series, but stimulus intensity was now 110% of the th reshold obtained from the preinnervated muscle. The responses varied s ignificantly more in the first than in the second series. Variability in the third series was lower than in the first, but higher than in th e second series, the differences were weakly significant. Preinnervati on renders the input that neurones supplying the muscle receive more c onstant. Variable input decreases and so does the variability of the m uscle responses. Preinnervation has a stronger influence on the excita bility of neurones than does increasing the intensity of the magnetic stimuli.