Abstract
A rhythmic oscillation of force at about 10 Hz called tremor is a well known feature of muscular contraction. The neuronal mechanism by which this is expressed has been extensively investigated and some hypotheses and models have been proposed recently. In this study, an attempt has been made to resolve the neuronal mechanism of tremor from the analysis of interspike intervals of the motor unit discharges.
Finger tremor of the extended second or third digit and bipolar wire electromyograms of the extensor digitorum were recorded. Tremor was measured with force transducer during steady voluntary contractions.
Many motor units displayed double discharges in which short interspike intervals alternated with long interspike intervals. That is, two adjacent spikes of a motor unit were phase-locked with each oscillatory wave of tremor.
When tension exerted by finger was large, motor units displayed intense double discharge firing pattern. But with decrease of tension, number of double discharges decreased and at last double discharges disappeared, that is, each wave of tremor corresponded with one spike. Mean firing frequency also decreased consequently.
Each motor unit showed different discharge characteristics even in the same level of tension. The motor units that displayed intense double discharges had low threshold for recruitment.
