Abstract
1. The membrane potential and the tension development of a single muscle fiber of a toad or a frog were recorded simultaneously and their causal relation was investigated.
2. In normal saline the occurence of action potential was followed by the all-or-nothing contraction, the peak of which was attained in about 100 msec., the maximum value of the tension developed varied between 20 and 80 milligrams (temperature 12°C.).
3. In isotonic choline chloride solution the threshold for local contractions was about 40 mV in fibers of 90 mV resting potential. The contractions were graded with the stimulus intensity, maintained during the flow of the current. Tensions of several milligrams were obtained. The threshold membrane potential was approximately proportional to the resting potential.
4. In procain-Ringer solution the longitudinal current through the fibers could be raised up to 3 ×10-6 A without eliciting contraction. In the case of fibers in normal saline this critical stimulating current was about 2 × 10-6 A. These critical values were explicable by the leakage membrane current due to the finite internal resistance of the fiber.
5. Considering the estimation that the strength of the normal action current did not exceed the value of 0.8×10-6 A it was concluded that the longitudinal current inside the muscle fiber does not initiate contraction, and that the only indispensable factor is depolarization of the membrane.
