Changes in excitability of motor units during preparation for movement

Summary

Single biceps motor units were recorded in two awake monkeys (Macaca fascicularis) during the preparation for and execution of a forearm flexion movement. The motor sequence was organized as follows: after a control period lasting 500 ms, the animal was informed that a preparatory period (PP) was beginning by a preparatory signal (PS) consisting of diffuse sound and light. One to 1.5 s later, the animal was instructed by a bright light response signal (RS) to perform a rapid forearm flexion movement. Two hundred motor sequences were run during each daily session. The single motor units (MU) were recorded and their discharges analysed. In both monkeys, two extreme groups were found to exist on the basis of their pattern of activity during the preparatory and movement execution phases. 56% of the MUs were silent during the PP and showed a brief burst of discharge after the RS, which was strictly correlated to the movement execution. Their high recruitment threshold and their firing frequency during resting and movement periods suggested that these MUs associated with the movement execution could be called presumed fast or phasic MUs. Among the remaining MUs (44%), 15% were active as early as the beginning of the PP (about 300 ms after the PS) and showed a progressively increasing discharge, which stopped just after the beginning of the movement execution. These MUs associated with the preparatory phase had low recruitment thresholds and firing frequencies, which is compatible with the possibility that they might be slow or tonic MUs. Two functional hypotheses can be proposed on the basis of these results. The first is that the presumed slow MUs associated with the preparatory period might modify the physiological state of the muscle, increasing its stiffness and thus enhancing the efficiency of the “fast phasic MUs” activated during the triggering of rapid movements. This would shorten the reaction time and make it possible to perform the fast movements required in these experiments. The second hypothesis is that the slow MUs may contribute to building up the nervous activity responsible for the forthcoming movement; and more specifically, to controlling the excitability of central neurons producing a phasic discharge which might activate the presumed fast MUs.