Wyniki wyszukiwania - Biblioteka Nauki

1

Variability of the twitch responses to indyvidual stimuli at random stimulation patterns of fast and slow motor units in rat medial gastrocnemius

100%

Celichowski J. , Raikova R. , Krutki P.

|

nr S

EN

The unfused tetani evoked by train of stimuli at variable interpulse intervals (IPIs) of fast fatigable (FF), fast resistant (FR) and slow (S) motor units (MUs) were recorded and then decomposed into trains of twitch-shape responses to successive stimuli. The mean stimulation frequency was matched for each MU to evoke tetani of a similar fusion degree (lower for slow MUs, higher for fast MUs), whereas the variability range of IPIs was 50 – 150% of the mean IPI value for a given MU. The decomposition was done using an experimentally verified mathematical algorithm, described previously. Ten MUs of each type were analyzed. For each MU the twitch force, the contraction time and the force-time area for the single twitch recording were calculated and compared to both, the strongest and the weakest of the responses to successive stimuli. For each of studied MUs considerable variability of twitch parameters for responses to successive stimuli was observed, although the largest range of variability characterized slow MUs. In general, the decomposed twitch responses were stronger and had longer duration than the single twitches, although especially for 9 FF and to a smaller degree for 6 FR MUs the smallest decomposed responses were weaker but not faster than the single twitches of these MUs. The mean value of a ratio of the strongest decomposed twitch force to the single twitch force amounted to 2.4, 1.8 and 5.5 for FF, FR and S MUs, respectively. The ratio of the strongest decomposed twitch force-time area to the single twitch force-time area amounted to 1.8, 2.2 and 9.5 for FF, FR and S MUs, respectively. Analogically calculated ratio for the contraction time amounted to 1.5, 1.5 and 2.2 for FF, FR and S MUs, respectively. In conclusion, the results evidence that during voluntary activity of muscles successive action potentials generated especially by motoneurons of slow MUs have considerable variability in relation to evoked contractile responses of muscle fibers.

2

Sex differences in the decomposition of motor unit tetanic contractions of rat soleus muscle

100%

Drzymala-Celichowska H. , Raikova R. , Krutki P. , Celichowski J.

Acta Neurobiologiae Experimentalis

|

2015

|

tom 75

|

nr Supl.

EN

BACKGROUND AND AIMS: The aim of this study was to reveal variability of twitch-shape decomposed components of motor unit tetanic contractions of rat soleus muscle, which is almost exclusively composed of slow motor units (MUs). Moreover, sex differences in ranges of the force amplitude or time parameters of these decomposed twitches were analyzed. METHODS: Experiments were performed on adult Wistar rats (three males and three females) under general anesthesia. Functional isolation of a MU was achieved by electrical stimulation of single axons from the ventral roots of L4–L5 spinal nerves. Unfused tetanic contractions were evoked by stimulation at variable interpulse intervals for 10 MUs of males and 10 MUs of females. RESULTS: Significantly higher variability between parameters of the decomposed responses was observed for male than female soleus MUs; the mean ratio of forces of the strongest decomposed twitch to the first (the weakest) decomposed twitch amounted to 3.8 for males and 2.8 for females. The ratios of the contraction times of the longest decomposed to the first twitch were less different, and amounted to 2.6 for male and 2.9 for female MUs. Consequently, the mean ratio of the force-time area for the strongest decomposed to the first twitch was considerably higher for male than for female MUs (7.35 vs. 5.07, respectively). The comparison to the data for slow or fast MUs in rat medial gastrocnemius indicates that high variability of responses to successive stimuli is a general property of slow MUs, but the mechanisms of summation of individual twitches into tetanic contractions of MUs are sex-related. CONCLUSIONS: A method of mathematical decomposition of tetanic contractions appears to be a useful and an effective tool to study differences in mechanisms of MU force development between different MU types, the same MU types in different muscles or the same muscles in different sexes.

3

Prolonged activity evokes potentiation and the “sag” phenomenon in slow motor units of rat soleus

100%

Drzymala-Celichowska H. , Raikova R. , Krutki P.

Acta Neurobiologiae Experimentalis

|

2016

|

tom 76

|

nr 2

EN

Slow motor units (MUs) have no sag in their unfused tetani. This study in anesthetized rats shows that the sag can be observed in slow soleus MUs after prolonged activity. Twitches and unfused tetanic contractions were recorded from male (n=35) and female (n=39) MUs before and after the four minutes of the fatigue test (trains of 13 pulses at 40 Hz repeated every second). After this activity twitch contractions potentiated and a shift in the steep part of the force-frequency curve towards lower frequencies was observed in both sexes. Initially no sag was visible in unfused tetani, but after the fatigue test the phenomenon was observed in 77% of male and in 13% of female MUs, with the sex difference possibly related to a higher content of IIa myosin and faster MU contraction in male soleus. Decomposition of tetani with sag into trains of separate twitches elicited by successive stimuli revealed higher forces for the initial than subsequent twitches. The newly revealed enhancement of the sag in force development following long-lasting activation is more pronounced in males than in females.

4

Force generated by fast motor units during stimulation with pulses at variable intervals

88%

Krutki P. , Pogrzebna M. , Drzymala H. , Raikova R. , Celichowski J.

|

nr 3

5

The successive contractions subtracted from unfused tetanus of fast and slow motor units

88%

Pogrzebna M. , Raikova R. , Drzymala H. , Celichowski J. , Aladjov H.

|

nr 3

6

Factors contributing to sag in unfused tetanic contractions of fast motor units in rat

88%

Celichowski J. , Krysciak K. , Raikova R. , Krutki P. , Drzymala-Celichowska H.

Acta Neurobiologiae Experimentalis

|

2017

|

tom 77

|

nr Suppl.1

7

Force generated by fast motor units of the rat medial gastrocnemius muscle during stimulation with pulses at variable intervals

88%

Krutki P. , Pogrzebna M. , Drzymala H. , Raikova R. , Celichowski J.

|

tom 59

|

nr 1

EN

During natural contractions of a muscle, motor units (MUs) are activated by irregular discharge patterns of motoneurones. The aim of this study was to analyze changes in contractile forces of MUs following patterns of stimulation at variable frequency. Experiments were performed on 33 functionally isolated MUs of the fast-type in the medial gastrocnemius (MG) muscle of adult Wistar rats, under pentobarbital anaesthesia. The MUs forces evoked at five different regular stimulation patterns of constant frequencies were compared to forces generated during five random patterns of irregular stimulation with the same mean values of interpulse intervals, between 10 and 75 ms, and variability of these intervals of ± 50% in each case. These values cover the natural range of the preferred firing rates of the MG motoneurones from unfused to nearly fused tetanic contractions. Analysis of changes in tetanic forces indicated a linear relationship between the interpulse interval as well as the initial level of the force and the amplitude of the force increase of the next contraction. It was demonstrated that variability of the instantaneous tetanic force during the irregular discharge pattern depends on the level of tetanic fusion. Moreover, it was demonstrated that for low and moderately-fused tetani, effectiveness of a MU contraction (expressed as the force-time area) is considerably higher for contractions evoked by irregular stimulation patterns. On the basis of the results of this study it was supposed that during voluntary contractions, the influence of changes in the motoneuronal firing rate on the motor unit force depends on the initial level of force.