Single Responses of Motor Units in Consequence of Volitional Effort

Abstract

Since the demonstration by Adrian and Bronk 1 that action potentials could be recorded from single skeletal motor units∗ during voluntary muscular activity, several workers have published studies which have made use of this technic. The published material with which we are familiar has, however, dealt with more or less sustained contractions of the muscles voluntarily activated so that records of single motor units have, in all such cases shown repeated discharges. Stetson and Bouman 2 recorded muscle action potentials by means of electrodes placed against the skin. Their records, therefore, were led from many motor units. However, they found that when a quick, ballistic type of movement was made by the subject, the resulting record often revealed one or more groups of discharges, each of these having a duration of about 50 msec. There was, therefore, the possibility that, where a single unit burst appeared, it represented the activity of a considerable number of motor units discharging asynchronously over a period of about 50 msec, but with each motor unit discharging only once.

It seemed of interest to attempt the registration of a single motor unit discharge resulting from a single, quick, volitional effort. At the present time, 6 normal humans have been used. From each of these there have been obtained records of single discharges of single motor units from one or more of 11 muscles. A high degree of cooperation is required of the subject in order that the muscle unit near the recording electrodes may be made to respond with each volitional effort but without the complicating responses of other units.

A complete discussion of functional and technical aspects of the experiments will not be undertaken here but Fig. 1 indicates the type of record which has been obtained.