THE RELATION OF ELECTRIC POTENTIAL CHANGES TO CONTRACTURE IN SKELETAL MUSCLE

Abstract

Expts. were performed on nerve-muscle fiber prepns. of the adductor longus muscle and on whole isolated sartorius muscles of frogs. Contractures were set up by constant current pulses and by application of drugs. Negative potential changes were recorded at the site of origin of contractures in isolated muscle fibers. Contractures, like propagated muscle responses, are initiated ensuing on a sufficient depolarization of the muscle membrane. Contractures may arise: (1) Following muscle impulses which may gradually fail to propagate fully from the region of their origin. In these prepns. a transition can be detected from normal to"abortive" impulses and to a maintained negative potential change which may give rise to contractures without appreciably exceeding the potential level at which the preceding propagated responses had been set up; (2) not preceded by propagated responses following on the depolarizing action of drugs or currents in fatigued narcotized or injured muscles. Contractures set up by chemical application are actively maintained by the depolarizing action of drugs. This action is analogous to the "cathodic shortening" effect which lasts for the duration of the current flow. "Chemical" or "electrical" contractures can be graded, depending on drug concn. or current strength. Relaxation of chemically produced contractures can be effected at the anode of constant currents. Novocaine does not raise the threshold at which contractures are set up by constant currents or by K application. The apparent threshold for brief shocks, however, is greatly increased. The electric time constant and the resting potential of the muscle membrane are not significantly affected. The connection between the muscle membrane and the contractile elements is discussed. It is suggested that the action currents which accompany depolarization or the propagated muscle impulse are not the essential link in the transmission of "excitation" from the membrane to the contractile elements.