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The influence of different mechanical stimuli on the excitability of spinal motoneurones was investigated in 5 different experiments using transcranial magnetic brain stimulation. A servo controlled moving coil was used to deliver rectangular mechanical stimuli or vibration to the right abductor digiti minimi muscle (ADM) thus exciting spindle primaries. With the ADM relaxed, vibration produced an early enhancement of compound responses (CMAPs) after transcranial magnetic brain stimuli. This is thought to be due to simultaneous arrival of the descending corticospinal volley and of the afferent la volley at the anterior horn cell. With long lasting vibration an enhancement of responses to brain stimuli continued up to 5 seconds after the onset of vibration. Voluntary muscle contraction increased the CMAP. CMAPs measured with voluntary muscle contraction plus mechanical muscle stimulation did not differ from those with muscle contraction alone. Therefore an excitatory influence of mechanical stimuli cannot be seen with ADM contracted.

Using concentric needle electrode recording from hand muscles (abductor digiti minimi, first dorsal interosseus, or abductor pollicis brevis muscle) the latencies of single motor unit potentials in response to threshold magnetic brain stimuli were studied under different conditions. It has been shown that the motor units activated by threshold brain stimuli had the lowest threshold for voluntary activation (A = vol. activated, B = brain stim.). Onset latencies of 23 motor unit potentials from different sites in the relaxed muscles of four healthy subjects ranged from 22.4 to 32.4 ms (average: 26.4 ms; SD: 2.80 ms) but proved to be relatively stable when stimulating and recording conditions were kept constant (variation less than 0.5 ms, see poststimulus time histogram in Fig. 2). With concurrent contraction of an ipsilateral neighbouring or of the contralateral homologous muscle the motor unit potentials from the relaxed target muscle jumped to an earlier latency by 1.2 to 1.7 ms (C in Fig. 3 and B-2 in Fig. 4). On one occasion another motor unit of higher threshold was alternatively activated by this procedure (B-3 in Fig. 4). Since the same procedures are known to enhance the compound muscle action potentials as recorded from the relaxed muscle with surface electrodes, the shorter latencies of single motor units are considered to be caused by a facilitatory influence on the motoneurones. A possible explanation for these latency shifts would be that the motor units discharge later in response to the brain stimulus induced repetitive cortico-spinal impulses when there is no facilitation, whereas during facilitation the firing level is reached earlier.(ABSTRACT TRUNCATED AT 250 WORDS)