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The area under the record of contractile tension: estimation of work performed by a contracting motor unit

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Celichowski J., Grottel K., Bichler E.
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issue 2
165-168
EN
The area under twitch tension records was measured for motor units in rat medial gastrocnemius. These measures were compared to measures of tension. The tension varied in significantly larger range than the area. The area of slow motor units was similar to the area of fast resistant units, whereas their tensions differed significantly. The area depended mainly on the amplitude of contraction and to a smaller degree on its time course. The measure of area under the tension record gives a more exact evaluation of the work performed by contracting motor units than the measure of tension alone. The obtained results show that motor units in mammalian muscle are less variable in their ability to perform contractile work and moreover, that slow motor units play a more significant role during contractions than was supposed based on tension measures.
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The influence of a doublet of stimuli at the beginning of the tetanus on its time course

100%
Celichowski J., Grottel K.
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vol. 58
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issue 1
47-53
EN
The influence of a pair of stimuli generated in a short time sequence (doublet) at the beginning of stimulation on the time course of the following tetanus was investigated. Experiments were performed on single motor units in rat's medial gastrocnemius. The doublet evoked an increase in tetanic tension, tetanic fusion and the area under tension record. These effects were measured in tetani fused to varying degrees. It was found that for all types of motor units the strongest influence of the doublet was observed in half-fused tetani. Moreover, the doublet influenced the first part of tetanus significantly more as compared to the second. Slow motor units showed greater sensitivity of the tension and the tetanus area to the doublet than fast units. The results show that slow units are characterized by better summation of their tension at the beginning of a tetanus. a tetan
3
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Tetanic potentiation in motor units of rat medial gastrocnemius

100%
Piotrkiewicz M., Celichowski J.
Acta Neurobiologiae Experimentalis
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2007
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vol. 67
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issue 1
35-42
EN
Tetanic potentiation is a phenomenon, which expresses the ability of a motor unit (MU) to increase its force output in tetanic contractions above that predicted with an assumption of algebraic summation of single twitch responses. To quantify tetanic potentiation, a coefficient TPC (tetanic potentiation coefficient) was defined as a ratio of the areas below tetanic force recording corresponding to the single stimulus contribution and that of the single twitch. Single MUs (27 Slow, 71 Fast, Fatigue Resistant, and 47 Fast, Fatigable) were isolated from the rat medial gastrocnemius muscle (MG) by ventral root splitting. TPC value was rate-dependent, with the maximum TPCmax at a certain optimal rate. The largest values of the TPCmax were obtained in the weakest and most fatigue resistant (S and weak FR) MUs. The different manifestation of staircase effect, post-tetanic and tetanic potentiation in individual MU types indicates that these phenomena may be independent of each other. We suggest that these phenomena as well as the fatigue resistance should be reexamined with protocols adjusted to the MU optimal frequency.
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The time course of the last contractions during incompletely fused tetani of motor units in rat skeletal muscle

100%
Celichowski J., Bichler E.
Acta Neurobiologiae Experimentalis
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2002
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vol. 62
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issue 1
7-17
EN
The time course of the contraction and the relaxation of individual contractions during incompletely fused tetani of motor units were analyzed. Investigations were performed on fast fatigable (FF), fast resistant (FR) and slow (S) motor units of the rat medial gastrocnemius muscle. Stimulation of a motor unit with a series of nine trains of stimuli at a frequency from 10 to 150 Hz was used and tetani fused to a variable degree were recorded. For fast motor units the procedure was repeated twice and observations were made on potentiated tetani in the second series of stimulation. For each tetanus, the amplitude of the tension increase, the peak amplitude of the contraction, the contraction time and the half-relaxation time were measured in the last contraction of the tension recording. It was observed in all three types of motor units that the last contraction was prolonged in parallel with the increase of fusion of a tetanus. In this contraction, the contraction time slightly decreased whereas the half-relaxation time strongly prolonged. The prolongation of the half-relaxation time was the strongest in tetani of slow units. Moreover, for the last contraction in a tetanus, the rate of changes in tension were studied. The rate of increase in tension during the contraction decreased in parallel with the increase of fusion of a tetanus, whereas the maximal rate of the tension decrease during the relaxation was found in tetani with fusion indices of 0.79, 0.98 and 0.95 for FF, FR and S motor units, respectively. Changes in the time course of contractions in tetani fused to a variable degree can shed light on processes of summation of contractions in unfused tetani at the level of individual motor units.
5
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Changes in fusion index during the fatigue test of fast motor units in the medial gastrocnemius muscle of the rat

100%
Celichowski J., Grottel K., Bichler E.
Acta Neurobiologiae Experimentalis
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1996
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vol. 56
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issue 4
881-887
EN
Changes in the fusion of tetani were investigated in fatigue tests of 50 fast motor units of the rat medial gastrocnemius muscle. Fusion of the tetani was measured using the fusion index, being the ratio of the tension to which motor unit relaxed before the last of tetanus to the peak tension of the last component. In both types of fast units (fast fatigable and fast resistant ) the changes in the fusion index were similar to those in tetanic tension: first they increased and then decreased. However, the increase of the fusion index was longer than that of tension and the subsequent decrease in the fusion index was smaller than that of tetanic tension. Furthermore, the initial increase in the fusion index of fast fatigable motor units was greater than in the tension. The dependence of the fusion index on twitch time enables the analysis of the influence of changes in the twitch time on changes in tension of unfused tetani observed during activity of fast motor units.
6
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Tetanic depression: a phenomenon influencing the production of tension in fast-twitch motor units in rat medial gastrocnemius

88%
Celichowski J.
Acta Neurobiologiae Experimentalis
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2001
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vol. 61
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issue 2
119-123
EN
The influence of an increase or a decrease in the stimulation frequency on tension development during a tetanus was studied in motor units of the rat medial gastrocnemius muscle. These effects were tested in one tetanus evoked at two frequencies of stimulation, a lower immediately followed by a higher one or the reverse. For all fast motor units it was observed that after the first part of a tetanus at a lower frequency of stimulation the tension of the following part, of the better fused contraction, was depressed. This effect was called a tetanic depression. When the lower stimulation frequency was followed by the higher one, the depression was visible in some motor units only whereas in the remaining units a potentiation of the second part of the tetanus was visible. The tetanic depression was larger in fast resistant than in fast fatigable motor units. In slow motor units tetanic depression was not observed. The tetanic depression is a phenomenon which can influence the production of contractile tension by fast motor units.
7
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Changes of motor unit contractile output during repeated activity

88%
Lochynski D., Celichowski J., Korman P., Raglewska P.
Acta Neurobiologiae Experimentalis
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2007
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vol. 67
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issue 1
23-33
EN
The aim of the study was to evaluate changes in the motor unit output and to determine changes in the optimal stimulation frequency (i.e., giving the maximal output per one pulse) during prolonged contractile activity when, successively, potentiation of force and fatigue developed. The influence of these phenomena was studied on three types of motor units: fast fatigable (FF), fast resistant (FR) and slow (S) in the rat medial gastrocnemius muscle. The motor units were isolated by a method of splitting of L5 ventral root into very thin bundles of axons which were electrically stimulated 10 times with repeated series of 10 trains of stimuli at duration of 500 ms and progressively increasing (1?150 Hz) frequency. The initial (the first series of stimulating trains), potentiated (the second series), as well as fatigued (the tenths series) force recordings were compared. The motor unit output was expressed as the area under the force-time record in response to one stimulus measured at a plateau phase of the tetanic force. The stimulation frequency when the force-time area per one pulse was maximal was accepted as the optimal frequency. In fast motor units, the maximal contractile output increased with potentiation and was reduced with fatigue, and the optimal frequency decreased and increased, respectively. Nevertheless, the fusion degrees of the optimal tetanic contractions were similar in initial state, potentiation and fatigue independently of the changes in force. The applied stimulation protocol had almost no influence on the mechanical activity of slow motor units. The study highlights the physiological importance of force potentiation induced by preceding contractile activity for the economy of motor performance. The observed changes of the optimal stimulation frequency are consistent with the known changes in the motor unit firing rates during voluntary activity when the two phenomena develop.
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