Effects of threshold, low frequency, long lasting stimulation on single motor unit electrical responses from the extensor digitorum brevis muscle in normal and myasthenic subjects.

The authors studied the recruitment of motor units of EDB muscle by long lasting threshold intensity stimulation at 1 c/sec of external popliteal nerve at the ankle in healthy subjects and in myasthenic patients. In the healthy volunteers, at steady intensity of threshold stimuli, maximum number of motor units never exceeded 5--6 electrical increments with a final incremental response of constant amplitude during 60 min of stimulation. In myasthenic patients, both an early exhaustion of threshold motor units and a marked facilitation phenomenon occurred with massive recruitment of incremental responses, while in a small number of cases no changes were observed. These data are discussed in the light of various morphofunctional hypotheses regarding the localization of the myasthenic impairment.