Excitability of Muscle to Direct and Indirect Stimulation During Prolonged Direct Stimulation. ∗

Although electrical stimulation is used widely in diagnosis and in physical therapy and is the only practical procedure to test muscle function in animals, the effects of prolonged direct stimulation have not been studied extensively.

Alternating 60-cycle current was used in 3 independent but in phase circuits for stimulation through 3 pairs of electrodes, the first pair being inserted into the knee and tendon of the gastrocnemius muscle in situ of anesthetized rats (nembutal) and cats (dial), (D₁ electrodes); the second pair (D₂) inserted into the muscle itself at its upper and lower ends; the third pair (I) being employed for stimulation of the peripheral end of the cut sciatic nerve. The strength of the stimulus was adjusted with a rheostat and measured by a volt meter; in addition, the resistance in the muscle was measured by means of an oscilloscope in a shunt parallel to the muscle. Stimulation was continued either with D₁ or with D₂ until the muscle length reached the pre-stimulus resting value. During and after the continued stimulation, brief tetanic stimuli were applied at regular intervals through the 2 other pairs of electrodes. In all cases maximum stimuli were applied through a 10,000-ohm resistance placed in series with muscle and nerve. The muscle was loaded with 200 g and contraction was isotonic. Before continued stimulation the effect of summation was tested for all combinations of D₁, D₂, and I. There was no summation between D 1 and I, or D₂ and I, and a tendency to slight summation of D₁ + D₂.

Steel, platinum, silver, and nonpolarizable (Cu-CuSO_4l.0M - K₂S0_4sat. - NaCl_sat. NaCl_{0 9%}-Tissue) electrodes were employed in various experiments.