The tetanic depression is phenomenon observed when higher frequency of stimulation of fast motor units immediately follows the lower one. However, it is not known does only one prolonged fi rst interpulse interval can evoke this effect. This study was conducted on 27 fast motor units of cat and 47 units of rat medial gastrocnemius muscles. In experiments performed on the two species the following stimulation protocol was applied in the main part of experiment: (1) the 6-pulses train of stimuli at 20 Hz (cat) or 40 Hz (rat), i.e. at 50 and 25 ms interpulse intervals, (2) the 6-pulses train of stimuli with the fi rst interpulse interval prolonged from 50 ms to 100 ms (cat) or from 25 to 50 ms (rat) followed by 5 stimuli at 20 Hz (cat) or 40 Hz (rat), (3) the 6-pulses train of stimuli at 20 Hz (cat) or 40 Hz (rat). Effects of two-time prolongation of the fi rst interpulse interval were observed as a decrease of the force following the sixth stimulus in a train. The tetanic depression in rat motor unit tetani for fast fatigable (FF) units amounted to 5.39 ± 4.94% and 9.24 ± 4.13% for fast resistant (FR) units whereas in cats the mean values were 15.00 ± 10.15% and 27.00 ± 13.95% for FF and FR motor units, respectively. This results prove that the studied phenomenon infl uence the force development when the motoneuron begins the activity even with only one, fi rst prolonged interpulse interval.
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