STRETCH-INDUCED AND STIMULATION FREQUENCY-INDUCED CHANGES IN EXTRACELLULAR ACTION-POTENTIALS OF MUSCLE-FIBERS DURING CONTINUOUS ACTIVITY

The present investigation aimed to use the extracellular action potent ials (ECAPs) of muscle fibres for evaluation of the membrane functiona l state during long-lasting activity. Repetitive stimulation during 3 min trials at three different frequencies (5, 6.7 and 10 Hz) was appli ed to isolated frog muscle fibres at four different rates of stretch ( up to 35% of the initial length). The evoked changes in the ECAPs were estimated by alterations in: the peak-to-peak amplitude (A); the time intervals between the positive and the negative maxima (T-1) and betw een cross-points of the potential rise and fall with the base line (i. e. the negative phase duration) (T-0); as well as in the propagation v elocity (PV) of excitation. Due to the significantly shorter duration of continuous activity of the fast muscle fibres (FMFs), at the time-p oint of potential failure the decrease of PV and the increase in T-1 a nd T-0 were more pronounced in the slow muscle fibre (SMF) potentials than in FMF potentials. The amplitude decrease in most of the trials f or both fibre types was similar. Up until the end of the trials, the a ctivity of both fibre types was intermittent, and in the majority of t rials the percentage changes in the potential parameters held the valu es reached during the continuous activity. Only the time parameters gr adually increased throughout the trial at maximal stretch and 5 Hz sti mulation frequency, as for the FMF potentials they were more prolonged than those of the SMF potentials. T-1 and T-0 reflect the slowing of the depolarization phase and rapid repolarization of the intracellular potential. Hence the duration of the ECAPs was the parameter most aff ected by the maximal stretch. The changes in the ECAP parameters and P V induced by repetitive stimulation and fibre stretch reflect changes in ionic currents and muscle fibre membrane conductivity.