PATTERN OF CONTINUOUS MUSCLE-FIBER ACTIVITY DEPENDING ON FIBER STRETCH AND STIMULATION FREQUENCY

Isolated frog muscle fibres of four different lengths (L-0, initial; L -1, 15%; L-2, 25%; and L-3, 35% stretched vs. L-0) were stimulated con tinuously (3 min) by suprathreshold rectangular pulses at three differ ent frequencies (f(1) = 5 Hz, f(2) = 6.7 Hz and f(3) = 10 Hz). Using a pair of electrodes mounted at a fixed distance, the action potential at two different sites was recorded extracellularly and the propagatio n velocity (PV) of excitation was calculated. Throughout the trials tw o kinds of activity (continuous and intermittent) were observed. Two t ypes of muscle fibres, slow (SMFs) and fast (FMFs), were distinguished depending on the rate of decrease in the PV during the period of cont inuous fibre activity. The duration of this period decreased with the increase of L and f. The continuous activity was followed by intermitt ent activity. The variety of alternations of rest periods (i.e. failur e of potentials and periods of activity) characterized the pattern of SMF and FMF activity evoked by the protocol used. A coefficient of act ivity (k), i.e. the ratio between the number of action potentials and the number of stimulus pulses delivered for a determined time interval , was used to estimate the specificity of this pattern. A decrease in k was observed with an increase of L and f, and was differently expres sed for the two fibre types. The stretch of SMFs contributed more to t he shortening of the period of continuous activity and the decrease in k than did the increase of stimulation frequency. For FMFs the effect of the two factors (L and f) was identical. It is suggested that the disturbances of sarcolemmal and t-system membrane excitability during repetitive stimulation and stretch are fibre-type dependent and underl ie the pattern of muscle fibre activity.