A method to measure the distribution of latencies of motor evoked potentials in man.

Objective: To measure the intra-individual distribution of the latencies of motor evoked potentials (MepL) using transcranial magnetic stimulation.

Methods: We used the triple stimulation technique (TST) to quantify the proportion of excited spinal motor neurons supplying the abductor digiti minimi muscle in response to a maximal magnetic brain stimulus (Magistris et al., 1998). By systematically manipulating the TST delay, we could quantify the contribution of slow-conducting motor tract portions to the TST amplitude.

Results: Our method allowed the establishment of a MepL distribution for each of the 29 examined healthy subjects. MepLs of 50% of the motor tract contributing to the motor evoked potential laid between the intra-individually minimal MepL (MepL(min)) and MepL(min)+4.9 ms (range 1.6-9.2). The individual MepL distributions showed two peaks in most subjects. The first peak appeared at a MepL that was 3.0 ms longer on average (range 0.7-6.0) than MepL(min); the second peak appeared at MepL(min)+8.1 ms on average (range 3.7-13.0).

Conclusions: Slow-conducting parts of the motor pathway contribute notably to the motor evoked potential. Our data suggest a bimodal distribution of central conduction times, which might possibly relate to different fibre types within the pyramidal tract.

Significance: We present a non-invasive method to assess slow-conducting parts of the human central motor tract.