Spectral and time domain characteristics of single muscle fibre action potentials during continuous activity...

A model of the muscle fibre extracellular action potentials (ECAPs) calculation using experimentally recorded intracellular action potentials (ICAPs) has been applied to investigate the effect of repetitive stimulation on the electrical activity of isolated frog muscle fibres. The ECAPs were calculated both at small (0.01 mm) and at large (5 mm) radial distances to the fibre axis, and their relationship with the original ICAP parameters has been inferred. Fourier transformation of the calculated ECAPs in order to obtain the spectral characteristics and to trace out their behaviour during continuous fibre activity was performed. Stimulation frequency dependence on the ECAP time characteristics and on the shift of the maximum spectral density towards low frequencies at small and large radial distance were observed. The spectral density peak frequency is propagation velocity (PV)-dependent. The advantage of the presented method over the available experimental extracellular recording techniques from isolated muscle fibers is the possibility to show the effect of continuous muscle fibre activity on the parameters of the ECAPs and their spectral characteristics at large radial distance, which is not experimentally accessible. Our results are in agreement with those experimentally obtained. The results from the model prove the role of changes in PV of excitation along the muscle fibres (representing the last link in the complex organized motor system) in the development of fatigue.