The electrical activity of single motor units was recorded from the first dorsal interosseous muscles of nine patients with Parkinson's disease. Six of these patients had a combination of the following abnormal motor unit properies: (1) a variable delay period of 20 seconds to 3 minutes between the initiation of voluntary effort and the recruitment of the first group of motor units; (2) after recruitment, some of the motor units stopped firing for durations of 10s, 40s, 75s... 3 min.; (3) some of the motor units fired at abnormally low frequencies of 2-3 per second. All these six patients had slowed finger movement, and five of the six were studied while off levodopa for two to seven days. One of these patients, reinvestigated after levodopa therapy had been restarted, demonstrated improvement in motor unit control. The three remaining patients who were studied while on uninterrupted levodopa therapy could make rapid finger movements, could recruit motor units without delay, and could fire recruited motor units continuously at normal frequencies of 6-14 per second. These results suggest that levodopa therapy is effective in Parkinson's disease at least partly because of its ability to correct abnormalities in the recruitment of motor units. Levodopa also corrects the abnormal motor unit firing pattern. The abnormal motor unit properties found in these patients could account for some aspects of bradykinesia.
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