| Abstract |
The firing behavior of motor units (MUs) of the first dorsal interosseus muscle of the hand was examined during both steady and time-varying (sinusoidal or broadband random variations) isometric contractions in healthy adults. The emphasis was on the analysis of rhythmical MU synchrony with a new method which uses as tools the unit-to-aggregate coherence and cross-correlation functions. In this case, the unitary signal is MU activity and the aggregate signal is the muscle force. In steady contractions, the presence of regular and large tremor was accompanied by widespread and strong MU firing correlations at the tremor frequency, with the MUs in phase. In contrast, in cases of irregular and small tremor the MUs were uncorrelated; or they showed very weak correlations, the effects of which on tremor were negligible. Moreover, the tremor synchrony was suppressed during ischemia, while the tremor became minimal, and this strongly suggests an involvement of muscle spindle feedback in the generation of the tremor synchrony. These observations argue for the presence of two parallel tremor mechanisms in static contractions, one of which involves MU synchrony. In varying contractions, the firing of all MUs was modulated in the frequency (band) of the force variations. The MU modulations showed a phase advance over the force variations and were strongly correlated to the particular variations and among themselves. Thus, correlated modulations of MUs seem to provide the basic mechanism for the generation of time-varying force. Finally, when tremor was present in dynamic contractions, widespread MU correlations existed also at the tremor frequency. The ischemic block again suppressed the tremor and its synchrony, but left the synchrony of MU modulations unaffected. These observations argue for the presence of two parallel mechanisms of MU synchrony in varying contractions, the effects of which combine at the level of the membrane potential of amotoneurons.
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Μελέτη των δραστηριοτήτων κινητών μονάδων μυών του χεριού με έμφαση στην ανάλυση του συγχροισμού τους
