Repetitive spinal motor neuron discharges following single transcranial magnetic stimulation: relation to dexterity

Transcranial magnetic stimulation allows to study the properties of the human corticospinal tract non-invasively. After a single transcranial magnetic stimulus, spinal motor neurons (MNs) sometimes fire not just once, but repetitively. The biological significance of such repetitive MN discharges (repMNDs) is unknown. To study the relation of repMNDs to other measures of cortico-muscular excitability and to physiological measures of the skill for finely tuned precision movements, we used a previously described quadruple stimulation (QuadS) technique (Z'Graggen et al. 2005) to quantify the amount of repMNDs in abductor digiti minimi muscles (ADMs) on both sides of 20 right-handed healthy subjects. Skillfulness for finger precision movements of both hands was assessed using a finger tapping task. In 16 subjects, a follow-up examination was performed after training of either precision movements (n = 8) or force (n = 8) of the left ADM. The size of the QuadS response (amplitude and area ratios) was greater in the dominant right hand than in the left hand (QuadS amplitude ratio: 47.1 +/- 18.1 versus 37.7 +/- 22.0%, Wilcoxon test: P < 0.05; QuadS area ratio: 49.7 +/- 16.2% versus 36.9 +/- 23.0%, Wilcoxon test: P < 0.05), pointing to a greater amount of repMNDs. Moreover, the QuadS amplitude and area increased significantly after finger precision training, but not after force training. This increase of repMNDs correlated significantly with the increase in performance in the finger tapping task. Our results demonstrate that repMNDs are related to handedness and therefore probably reflect supraspinal excitability differences. The increase of repMNDs after skills training but not after force training supports the hypothesis of a supraspinal origin of repMNDs.