Investigation of excitability and fatigue phenomenon in the corticospinal tract using transcortical magnetic stimulation.

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(1) Experimental study in controlsBy using transcranial magnetic stimulation, fatigue phenomenon in the first dorsal interosseous muscle (FDI) was investigated in healthy volunteers during performance of two different manual tasks ; combination of flexion and abduction of index finger (FXA), and power grip (GP). Motor evoked responses (MEPs) were greatly suppressed in FXA task rather than in GP task. When conditionig digital nerve stimulation preceded the magnetic shocks, however, the decrement of MEP remained poor in ratio. In addition, peristimulus time histogram (PSTH) suggested that the suppression and maintenance of MEP in size were of cortical origin because of change of the number of motor unit counts (I-wave). The fatigue was also shown in other distal hand muscles, not in leg muscles.(2) Clinical studyIn order to verify whether corticospinal excitability was impaired in patients with Pakinson's disease, 10 patients and 10 age-matched controls were studied, after receiving info … More rmed consent. In the patient group, they were studied in two different days ; on the day they had ordinal L-DOPA therapy (ON-day) and on the day they were withdrawn from taking their medicine (OFF-day) at least over 12 hours. Motor evoked responses (MEPs) were recorded in the first dorsal interosseus muscle (FDI). Double magnetic stimulation technique (conditioning-test design) was applied. 10 trials for the test or conditioned shock were delivered, contralaterally to the target muscle at random. The intensity of the first conditionig shock was adjusted as 1.5 times of motor threshold of the target muscle at rest. The intensity of test shock was adjusted so that MEP at rest was around 1.0mV in the peak-to-peak amplitude. Conditioning-test intervals varied each 25 ms up to 125 ms. All applicants were studied in two situations ; at rest and and in activation. The conditioning shock induced silent period lasting up to around 150ms, following larger MEPs. At rest ; MEPs were greatly suppressed after 50 ms of CTI in both groups. In contrast, control subjects showed enhanced corticospinal excitability in activation between 25 and 125 ms of CTIs. Mean average of the excitability was 195%. In the patient group, it was 85% ON-DAY.Irrespective of L-DOPA therapy, the excitability in the patient group was greatly decreased, which suggests that the fatigue in patients with Parkinson's disease is probably in cortical origin. Less