Comparison of voluntary and reflex activation of motor units. Functional organization of motor neurones.

The most important mechanism for gradation of muscle contraction is recruitment of new motor units with increasing effort. Whether voluntary and reflex contractions engage the different motor units in a similar order is not known. Since this question is not only of considerable physiological interest but also has clinical implications, we have compared the order in which the motor units are activated in voluntary contraction with that resulting from a polysynaptic and monosynaptic reflex. The voluntary contraction was increased very slowly or kept constant at a steady level. The polysynaptic reflex was the flexion reflex recorded from the anterior tibial muscle and evoked by painful electrical stimuli applied to the plantar surface of the foot. The monosynaptic reflex was the tendon jerk recorded mainly from the medial and lateral vastus of the quadriceps muscle. The voluntary activation was compared separately either with the flexion reflex or the tendon jerk. It was necessary to activate the motor units so that they could be identified separately and in order of appearance. This is difficult in the tendon jerk where the units discharge synchronously. No more than the first three potentials recruited within the picking up range of the electrode could be studied in any given needle position. For this reason the investigation was confined to weak contractions. It was found that in most instances identical motor units were recruited in the same order independently of the source of activation. The differences which did occur were almost always confined to motor units of nearby thresholds. The significance of this finding is discussed.