抄録
Characteristics of the motor units recorded from the orbicularis oris muscle in 7 subjects were studied during voluntary closure of the lips. The activity of the single motor unit of the orbicularis oris inferior muscle were recorded with bipolar silver ball electrodes improved for this study and the force generated by the lip-closure was simultaneously recorded with a high sensitive sensor. The results obtained were as follows : (1) The contraction time of the motor unit, which was measured by the spike-triggered averaging method, varied from 9 to 59 ms. About 98% of the motor units studied had a contraction time of less than 55 ms, which would be classified as a fast twitch motor unit. (2) The motor unit which had a larger twitch tension tended to have a longer contraction time than that of the small-twitch tensioning units. Increasing the force gradually, a few motor units were recruited orderly and the firing rate of each separate unit increased progressively. These findings suggest that the control of the muscle force caused by the lip-closure might depend on the rate moduration of motor units rather than on the orderly recruitment fashion of the motor unit. (3) The motor units tended to be highly f atigable, as judged by both the spike amplitude and the duration during continuous lip-closing.
