LONG PERIOD CONSTANCY OF THRESHOLD FORCE OF SINGLE MOTOR UNITS IN VOLUNTARY CONTRACTION AND ITS CHANGE INDUCED BY DESIRE FOR MICTURITION / 体力科学

ABSTRACT

In order to examine the characteristics of the activities of motor units over a long time, single spikes and consecutive discharges of single motor units were recorded by use of an inserted electrode. In addition, the tension at isometric contraction of knee extension or elbow flexion was simultaneously recorded. The results are summarized as follows.<BR>1) The relative threshold value of a single motor unit in <I>m. vastus medialis</I> stayed constant even for 13 hours. However, the threshold value increased with an increase in desire for micturition and returned to the original value upon disappearance of desire for micturition.<BR>2) It was observed in surface electromyograms of <I>m. rectus femoris</I> that both the amplitudes of potentials at the moment of single spike appearance and those corresponding to a constant sustained tension increased on raise of desire for micturition. Under the same circumstances, however, some motor units in <I>m. rectus femoris</I> showed an increase of, and the other motor units showed a decrease of, their relative threshold value.<BR>3) It was also found in <I>m. biceps brachii</I> that the relative threshold value of a single motor unit stayed constant for 6 hours. In this motor unit, no effect from the desire for micturition was observed.<BR>From these results, it can be presumed that each motor unit possesses a specific susceptibility for a long period. The relative threshold value, however, changed depending upon internal conditions such as desire for micturition. It can thus be concluded that the rotation of activity among the motor units is due to the variations of the relative threshold value.