Effect of short interstimulus intervals on the intra- and extracellular action potentials of isolated frog muscle fibres.

The changes in the time and space derivatives of the intracellular action potentials of isolated frog muscle fibers immersed in hypertonic sucrose-Ringer's solution were studied using paired stimulation with different interstimulus intervals. The peak-to-peak amplitude and the amplitudes of the separate phases of delta V/delta t and delta 2V/delta t2 decreased at intervals shorter than 10.25 ms as the duration of the phases lengthened. The velocity of spreading decreased at intervals below 7-10 ms and increased at longer intervals up to 40-100 ms. The peak-to-peak amplitude and the amplitudes of the separate phases of delta V/delta x and delta 2V/delta x2 decreased at intervals below 40-100 ms. The length of the separate phases of the space derivatives was almost constant. The extracellular action potentials near the membrane were similar to delta 2V/delta x2. When changing the interstimulus intervals the amplitude of the potentials recorded at different radial distances changed almost proportionally. In normal Ringer's solution the extracellular action potentials at different radial distances also changed almost proportionally depending on the intervals. They decreased at the shortest intervals and increased above the level measured at single stimuli for intervals longer than 7-10 ms, but shorter than 40-100 ms.