THRESHOLD EXCITATION AND OPTIMAL STIMULATION FOR FROG SKELETAL MUSCLE FIBERS STUDIED WITH INTRACELLULAR ELECTRODES

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The threshold depolarization in frog sartorius muscle fiber was relatively constant when stimulated with moderate pulse duration (1-10 msec). With shorter or longer pulse duration, the threshold depolarization increased. The pulse energy in the energy curve (i2t-t) obtained. from strength-duration relationships had a minimum value at approximately 1/4 of the combined time constant of the fiber membrane. The “apparent” input resistance was minimum with a stimulus whose duration was 1/4 of the combined membrane time constant. The maximum local response or subthreshold response reached the same amplitude regardless of the stimulus duration and reached its peak after the termination of the stimulus current. The time course of its development was slower, with a longer pulse duration. The safety factor calculated from maximal subthreshold response was approximately 8.<BR>The results obtained indicate that the frog sartorius muscle fiber has an optimal duration for square wave stimulation of approximately 1/4 of the combined membrane time constant of the fiber, and that sodium conductivity of the membrane during application of the square wave pulse seems to be largest at this moment.

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