Activities of Single Motor Units in Man during Sligiit Voluntary Efforts

Despite the vast literature concerning voluntary contractions of human muscles, there has not yet been presented an adequate description of the discharges from individual nerve cells of the spinal motor horn as they participate in the production of the various movements which occur. In this paper we are presenting material which deals with the responses of those fibers in a muscle which are in-nervated by one or by a very few motor nerve cells. The spinal motor neurone, which is the final common path (Sherrington, 1) for the activation of skeletal muscle has for its functional element the motor unit. This was defined by Liddell and Sherrington (2) as the ((motoneurone axone and its adjunct muscle fibres.-.. . " Adrian and bronk (3) and Denny-Brown (4) showed that it was possible to record the electrical discharges from single motor units which were responding during reflex activity and Adrian and Bronk recorded discharges from single units of muscles participating in voluntary contractions. Later Smith (5) and Lindsley (6) recorded the responses of single motor units in voluntary activity. Their work established a foundation on which we have attempted to place the beginning of a more complete structure. In selecting the experimental material which is to be considered below the attempt has been made to use records which would yield an outline description of the activities of single motor units under conditions ranging between threshold and slight to moderate voluntary effort. For the moment we shall avoid detailed quantitative considerations and shall describe the single unit response first as it occurs in quick movements of various intensities, either as isolated single efforts or as a rhythmic series of movements; second, as it participates in sustained movements begun or ended more or less suddenly; and third, as its activities are related to the activities of other nearby motor units whose responses may also be recorded.