[Interaction of transcranial magnetic stimulation and mechanical stimuli]

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.