Abstract
In the present study, we investigated the wave form of human single motor unit potentials recorded monopolarly using a surface electrode. Each motor unit potential consists essentially of three phases. However, we found a non-conductive component in the motor unit potentials, defined as“late positive deflection”. This non-conductive component appeared in and overlapped the third positive phase of the motor unit potential and showed the following properties 1) When surface electrodes were placed on the skin surface overlying the m. vastus medialis in line with the direction of muscle fibers belonging to the observed motor unit, the peaks of the non-conducting components were synchronized with each other and their amplitude increased exponentially with the distance from the motor end plate. 2) When the action potential was conducted to the myotendinous junction, the potential spread to the tendon electrotonically. The peak of the non-conducting component was also synchronized with the electrotonic potential. 3) The amplitude of the non-conducting component increased depending on the developed tension.
These results suggest that the appearance of the non-conducting component was due to synchronization of motor unit potentials that had just arrived at the myotendinous junction with the observed motor unit potential. When the motor unit potentials arrived at the myotendinous junction, the current flow to the forward local circuit of the action potential was cut off because of the high impedance of the tendon. Therefore the forward current flow was to be flowing the backward local circuit. The number of recruited motor units increased depending upon the developed tension. When many motor units fired randomly, their volume conducted-potentials canceled each other in the region of the muscle fiber. At the myotendinous junction, however, the directions of current flow due to the action potentials elicited by many motor units coincided and intensified each other.
Therefore, it is considered that this“late positive deflection”carries information on the number of motor units activated i. e., “recruitment”.
