Entladungscharakteristika der durch transkranielle Kortexreizung aktivierten motorischen Einheiten in den Handmuskeln des Menschen

Abstract

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 (see Fig. 1: 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 < 0.5 ms, see poststimulus time histogram in Fig. 2). With concurrent contraction of an ipsilateral neighbouring or of the contralateral homologuous 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 corticospinal impulses when there is no facilitation, whereas during facilitation the firing level is reached earlier. When using threshold electrical brain stimuli, the same motor unit with the same latency as with magnetic stimulation was activated (B in Fig. 6) and it also jumped to an earlier latency by the mentioned facilitatory measures (C in Fig. 6). A similar latency shift was sometimes observed when it was possible to increase the stimulus intensity without recruiting additional motor units within the pick up area of the needle electrodes, so that the original motor unit potential remained identifiable (B in Fig. 5; stimulus still more enhanced in C of Fig. 5). When increasing the stimulus intensity, the number of active cortico-spinal neurones impinging on a motoneurone is likely to be increased. This enhances the spatial summation of excitatory postsynaptic potentials in a motoneurone and may also result in an earlier firing of the motoneurone in response to the stimulus induced descending activity, since then less temporal summation is needed. Repetitive high frequency cortico-spinal trains in response to single brain stimuli must be assumed from animal experiments and epidural spinal cord recordings in humans. The interval between the descending impulses was found to be in the order of 1.4 (1.0-1.8) ms and therefore is similar to the latency shift we observed in motor unit potentials.