Changes in the muscle fibre extracellular action potentials in long-lasting (fatiguing) activity

The dependence of extracellular action potentials (ECAPs) of single frog muscle fibres on intracellular action potentials (ICAPs) was studied during long-lasting (fatiguing) activity. The conduction velocity, peak-to-peak amplitude and amplitudes of the separate phases of the first and second ICAP time derivatives decreased during long-lasting activity. The phases of the first and second ICAP space derivatives also decreased in amplitude and lengthened. ECAPs near the membrane were similar in shape and proportional in amplitude to \(\frac{{\partial ^2 V}}{{\partial x^2 }}\) when recording at a distance from both the end of the fibre and the point of stimulation. At long radial distances, the amplitudes of the separate ECAP phases depended on the amplitude and length of the corresponding phases of \(\frac{{\partial ^2 V}}{{\partial x^2 }}\) .Thus the decrease in ECAP amplitude during long-lasting activity at long radial distances was less than at points close to the muscle fibre membrane. The consequences of these findings for the changes in electromyograms recorded by needle or superficial electrodes during long-lasting (fatiguing) activity are discussed.