Insertion activity in electromyography with notes on denervated muscle response to constant current.

In clinical electromyography the muscle is examined not only in regard to voluntary activity and "spontaneous" activity, but also as regards the response on mechanical stimulation by the exploring needle electrode. According to Weddell and others (1944), the response of the normal muscle consists of a short outburst of action potentials which can be distinguished from normal motor unit action potentials of neural origin only by the mechanical manner of production. They term these action potentials "insertion motor unit action potentials." In denervated muscle Weddell and co-workers demonstrated that the outburst lasted appreciably longer, and that the action potentials evoked were identical with those of the spontaneously contracting individual muscle fibres of the denervated muscle, that is, the fibrillary action potentials. In view of the diagnostic importance of the mechanical irritability, we systematically examined it in the normal, and in the denervated muscle, as well as in different forms of muscular dystrophy. Our report includes also an observation on the response of denervated muscle to constant current.