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Sag in motor unit unfused tetanus: extra force production at the beginning of activity

100%

Drzymala-Celichowska H.

Acta Neurobiologiae Experimentalis

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2019

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tom 79

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nr Suppl.1

Sag is an unexplained property of fast motor units (MUs) and skeletal muscles, and its presence (in fast) or absence (in slow MUs and muscles) is used in fast/ slow twitch recognition. Several series of experiments aimed to identify in the rat muscle factors that contribute to sag, i.e., an extra force production within first 100‑300 ms of activity in two types of fast MUs: fast fatigable (FF) and fast resistant to fatigue (FR). First, mathematical decomposition of sagging tetanic contractions of FF and FR MUs into twitch-shape responses to consecutive stimuli was performed. This method identified mechanisms of the sag, including a progressive increase in the amplitude of a few initial responses (a process shorter for FF and longer for FR MUs), followed by a decrease in the amplitude of later responses. In comparison to the first twitch, the relative increase in force amplitudes of the several subsequent decomposed responses was smaller, and their contraction and relaxation times were shorter for FF than for FR units, which corresponded to observed differences in sag profiles between FF and FR MUs. In other series of experiments, effects of occlusion of the blood circulation was studied and these experiments revealed that, under ischemic conditions, the sag disappeared, but it reappeared after restoration of the blood supply. Moreover, we have found a very high sensitivity of the sag amplitude to preceding activation, even for single twitches.

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The number of motor units in the medial gastrocnemius muscle of male and female rats

63%

Celichowski J., Drzymala-Celichowska H.

Journal of Physiology and Pharmacology

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2007

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tom 58

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nr 4

The number of motor units in the hindlimb locomotor muscle - the medial gastrocnemius - was estimated in male and female Wistar rats by comparison of the whole muscle tetanic force to the mean tetanic force of its motor units. Functional isolation of motor units was achieved by electrical stimulation of single axons from ventral roots of L4 - L5 spinal nerves, while the whole muscle force was measured during stimulation of the sciatic nerve. The medial gastrocnemius muscle is approximately 1.5 times bigger by mass, and is innervated by 10% more motoneurones in males compared to females. Estimation of the force that can be generated by the three types of motor unit in the studied muscle revealed that in both sexes fast fatigable units are responsible for over 60% of the total force output whereas the percentage of the force output contributed by slow units in females is approximately twice that in males. Concluding, the motor innervation of hindlimb skeletal muscles is sexually dimorphic and in the rat male medial gastrocnemius muscle contains about 10% more motor units than in females.

3

The transitory force decrease following high-frequency stimulation burst in unfused tetani of motor unit

51%

Grzesiak J., Drzymala-Celichowska H., Celichowski J.

Acta Neurobiologiae Experimentalis

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2015

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tom 75

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nr Supl.

BACKGROUND AND AIMS: The changes in force of motor units (MUs) following changes in activation pattern are still not well understood, especially in relation to the relaxation course at decreasing rate of stimuli. It is known that at linearly decreasing stimulation rate the force decrease is slower than expected when comparing to the constant stimulation frequency. The present study aimed to verify a hypothesis that at a sudden decrease of stimulation frequency the force decrease is also lower than expected. METHODS: The research was conducted on 4 adult female Wistar rats under pentobarbital anesthesia. 8 slow (S), 16 fast fatigable (FF) and 26 fast resistant (FR) MUs were isolated. Studied MUs were stimulated with the several trains of stimuli composed of three phases: first, 500 ms at low frequency, second, 300 ms at high frequency and third, 500 ms at the same low frequency for fast motor units and 1000 ms at low frequency, second, 300 ms at high frequency and third, 1000 ms at the same low frequency for slow motor units. The tested low frequencies for fast MUs were 10, 20, 30, 40 and 50 Hz, and high frequency amounted to 75 Hz, whereas for slow motor units low frequencies were 10, 12.5, 15, 20 and 25 Hz and high frequency amounted to 50 Hz. RESULTS: Surprisingly, for MU of the three types at the middlefused tetanic contractions (the fusion index 0.15–0.80) the sudden switch from high to low frequency evoked the transitory force decrease below the force level at initial low-frequency stimulation. On the average the decrease amounted to 15.99% and the highest noted decrease amounted to 40%. Among the three MU types the force decrease was most frequent and the strongest for FR MUs. CONCLUSIONS: The phenomenon most probably is related to low-force generating state of cross-bridges in muscle fibers at a new overlapping following the force decrease and/or to a slow adaptation of stretched collagen fibers to the lower force level in the contracting muscle.

4

Structural and electrophysiological characteristics of muscle compartments in rat medial gastrocnemius

51%

Taborowska M., Drzymala-Celichowska H., Bukowska D., Celichowski J.

Acta Neurobiologiae Experimentalis

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2013

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tom 73

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nr Suppl.1

Rat medial gastrocnemius is composed of the proximal and distal compartments. To diversify these subvolumes, glycogen depletion technique based on a stimulation protocol one of the two primary nerve branches to the muscle was applied. The area of compartments, number and diameter of muscle fibers in the two distinct subvolumes on five muscle levels (10, 25, 40, 75 and 90% of muscle length) were determined. It was shown that the two smallest, opposite serial sections: close to the knee (10% of muscle length) and close to the Achilles tendon (90% of muscle length) were occupied by only one compartment, i.e. proximal and distal, respectively. In the largest section (40% of muscle length), the proximal compartment constituted 27–38% of the muscle area. Maximal number of muscle fibers in the proximal compartment was 4536–6698, while in the distal one 4773–6241. The mean muscle fibers diameter in the proximal and the distal subvolumes ranged: 36.9–54.3 µm and 46.5–63.8 µm, respectively. Additionally, in electrophysiological experiments the forces evoked by common or separate stimulation of L4 and L5 ventral roots in whole muscle and in one of compartments were measured. The ratio of forces evoked at L5/L4 ventral roots stimulation amounted to 2.18 in the proximal compartment, whereas 64.67 in the distal compartment.

5

Differences in morphometric properties of muscle fibers and innervation ratio of male and female motor units in the rat medial gastrocnemius muscle

51%

Meirzejewska-Krzyzowska B., Drzymala-Celichowska H., Celichowski J., Bukowska D.

Acta Neurobiologiae Experimentalis

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2009

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tom 69

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nr 3

The study aimed at demonstration of sexual differences in morphology and innervation of the medial gastrocnemius muscle (MG) in male and female Wistar rats. The experimental project involved three stages: (1) the functional isolation and analysis of contractile properties of as many motor units (MUs) as possible, (2) recording the contraction force of the whole muscle evoked by stimulation of the sciatic nerve and, (3) the morphological examinations of MG. The experiments revealed that MG of males contained 46% of fast fatigable (FF) MUs, 40% of fast resistant (FR) MUs and 14% of slow (S) MUs, whereas in females 40% of FF, 37% of FR and 23% of S type MUs were found. The force of MUs in male muscle was higher, and the twitch time parameters were longer in males. Moreover, the muscle mass and force in males was signifi cantly higher. The relation of the muscle force to the mean value of MUs force revealed that male MG contained approximately 10% more MUs than in females. The male muscles which had approximately 1.5 times bigger mass than in females were composed of over 11 800 muscle fi bers, whereas in females 8 000 fi bers. These results strongly suggested that the average number of muscle fi bers in individual MUs of males was higher about 47% comparing to females. In addition, the mean diameter of muscle fi bers was 29% bigger in males. Finally, the cross-section area of muscle fi bers was also 15% larger in males.

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

51%

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

Acta Neurobiologiae Experimentalis

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2016

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tom 76

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nr 2

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.

7

Long-lasting effects of trans-spinal direct current stimulation on motoneuron firing properties in rats

51%

Krutki P., Drzymala-Celichowska H., Mrowczynski W., Baczyk M.

Acta Neurobiologiae Experimentalis

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2019

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tom 79

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nr Suppl.1

INTRODUCTION: Trans-spinal direct current stimulation (tsDCS) is a neuromodulative technique used to improve motor functions in neurological disorders and to facilitate sport performance. However, despite the broad application of spinal cord polarization, the physiological mechanisms behind the observed effects remain unclear. We have recently demonstrated that anodal or cathodal tsDCS can alter motoneuron membrane properties and firing characteristics during its application and beyond. AIM(S): The aim of this study was to determine whether these alterations persist over a longer period of time. METHOD(S): The study was performed on adult male Wistar rats under general anesthesia. Anodal or cathodal tsDCS (0.1 mA, 15 min) was applied through an electrode located on the lumbar vertebra above the recording site. The intracellular recordings from L4‑L5 spinal motoneurons were performed at various periods after the offset of polarization (up to 3 hours). The animals not subjected to tsDCS formed the control group. RESULTS: Anodal tsDCS evoked a significant decrease in the voltage threshold, the rheobase, the threshold for rhythmic steady-state firing, as well as, an increase in the steady‑state firing frequencies and the slope of the frequency‑current relationship. Some of these modulatory effects were observed up to 60 minutes after the offset of polarization. Cathodal tsDCS induced only modest changes in motoneuron threshold properties, which could be observed no longer than 30 minutes after the end of polarization. CONCLUSIONS: This study for the first time provides the direct evidence that tsDCS evokes long-term alterations in the threshold and rhythmic firing properties of spinal motoneurons. Modulatory effects of anodal polarization are stronger and last longer than those of cathodal tsDCS. We suppose that both autonomous cell mechanisms and synaptic effects contribute to the occurrence and long-term persistence of the indicated changes in motoneuron properties. FINANCIAL SUPPORT: Supported by the National Science Center grant No 2017/25/B/NZ7/00373.

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The influance of endurance, strength, and vibration training on sag in unfused tetanic contractions of fast motor units

51%

Krysciak K., Drzymala-Celichowska H., Smith I. C., Celichowski J.

Acta Neurobiologiae Experimentalis

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2019

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tom 79

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nr Suppl.1

INTRODUCTION: Unfused tetanic contractions of fast motor units exhibit a transitional decline in force following the initial extra-efficient force development, known as sag. Sag is sensitive to changing energy demands and the fuels metabolized to meet these demands. AIM(S): Since different training modes have varying effects on cellular energy systems in muscles, we aimed to determine how endurance, strength, and vibration training would affect sag in fast motor units of rat medial gastrocnemius. METHOD(S): Separate control groups were used for each training mode, with activity limited to normal cage movements. Endurance training (ET): 2, 4, or 8 weeks of treadmill training with weekly progressions in duration and speed. Strength training (ST): 5 weeks of voluntary progressive weight‑lifting. Vibration training (VT): 3 or 6 months of whole‑body vibration training at 50 Hz (4 × 30 s with 60 s rest periods). ET, ST, and VT groups were trained 5 days per week. Following training, functionally isolated fast motor units (divided into fast fatigable (FF) and fast fatigue-resistant (FR)) were investigated, and profiles of their unfused tetanic contractions at 40 Hz with sag were analyzed. RESULTS: The 40 Hz contractions of trained groups were less fused than those of untrained groups; this effect was attributable to shorter twitch time parameters in trained animals. Accordingly, numerous differences appeared in the sag profiles. However, when limiting the comparisons to motor units with comparable levels of fusion (fusion index 0.2 – 0.8), few differences were observed. With this constraint 1) ET had no effect on sag profiles, 2) the force decreased after the initial peak was delayed in FF of ST rats, and 3) VT resulted in a shorter duration of sag in FF and a smaller force decrease after the initial peak in FR. CONCLUSIONS: We conclude that while different training modes have differing effects on sag, these effects are primarily due to altered twitch time parameters.

9

Sex differences in motor unit contractile properties of the rat soleus muscle

51%

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

Acta Neurobiologiae Experimentalis

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2013

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tom 73

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nr Suppl.1

The soleus muscle has unique physiological characteristics, as a typical slow-twitch muscle, composed predominantly of slow motor units (MUs). In this study we examined electrophysiologically functionally isolated MUs of male and female adult Wistar rats. It was revealed that the mean mass of the soleus muscle in males was approximately by 80% bigger than in females, however, a relation of the muscle-to-body mass was higher for females. No differences were observed with respect to the MU twitch forces, but the maximum tetanus forces were substantially lower for female rats, what significantly influenced higher twitch-to tetanus ratios in females. The contraction and the half-relaxation times were significantly longer in female MUs, what might be due to differences in muscle architecture. The force-frequency curve of slow MUs in males was shifted rightwards with respect to females, indicating that the same relative level of a tetanic force could be achieved at a considerably higher stimulation frequency in males. The maximum force-time area per pulse was significantly higher for males, and the analysis of MU action potentials revealed about four times higher amplitudes in male rats. In conclusion, numerous sex differences in MUs of the rat soleus muscle are not directly influenced by differences in body and muscle size.

10

The content of myosin heavy chains in hindlimb muscles of female and male rats

51%

Drzymala-Celichowska H., Karolczak J., Redowicz M. J., Bukowska D.

Journal of Physiology and Pharmacology

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2012

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tom 63

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nr 2

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Changes in the contractile properties of motor units in functionally overloaded medial gastrocnemius muscle of the rat

51%

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

Acta Neurobiologiae Experimentalis

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2011

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tom 71

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nr S

The study aimed at examining the contractile properties of motor units (MUs) in medial gastrocnemius muscle subjected to 3 months compensatory overload, induced by bilateral tenotomy of its synergists (lateral gastrocnemius, plantaris and soleus). To assure that operated muscles were regularly voluntarily activated, surgical procedures were followed by keeping animals in wheelequipped cages and treadmill exercise. The final electrophysiological experiments were carried out on 106 MUs of the overloaded medial gastrocnemius muscle (OMG) and 88 MUs of the untreated, healthy gastrocnemius muscle (MG). Functionally isolated MUs were examined by electrical stimulation of thin filaments of the L4 – L5 ventral roots. MUs classification was based on 20 Hz tetanus index which divided MUs into fast and slow, whereas the fatigue index distinguished fast fatigable (FF) and fast resistant (FR) MUs. Results showed that both OMG mass and its relation to the body weight were higher in comparison to MG. MUs proportion was modified in response to altered functional demand and in OMG higher percentage of S and FF simultaneously with lower contribution of FR MUs were observed. Changes in MUs contractile properties of OMG in comparison to MG included: shorter half-relaxation time and lower twitch force in all types of MUs and in parallel higher (FR and S) or unchanged (FF) tetanus force. Changes in force parameters in OMG resulted in lower values of the twitch-to-tetanus ratio in all three types of MUs. Higher posttetanic potentiation was also noted for all MUs types in OMG. Changes in fatigue resistance were observed only in fast MUs: for FF type the mean value of the fatigue index was lower in OMG in comparison to MG, but for FR type this value was higher in OMG. In conclusion, the adaptation of the medial gastrocnemius muscle to overload included transformation of some MUs accompanied by changes in MUs contractile properties.

12

45%

Mierzejewska-Krzyzowska B., Bukowska D., Szydlowska J., Drzymala-Celichowska H., Celichowski J.

Acta Neurobiologiae Experimentalis

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2011

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tom 71

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nr S

Gender differences are a well-known phenomenon in animal kingdom and concern body mass and morphology. The aim of this study was to determine the gender differences in the number and size of alpha and gamma motoneurons in motor nucleus of the medial gastrocnemius (MG) in the rat. The retrogradely labeled cell bodies of motoneurons of the same age male and female Wistar rats were studied following a bath of the proximal stump of the transected MG nerve in the horseradish peroxidase solution. The number and the soma diameters of male and female MG motoneurons were determined from serial microscopic images of stained sections using the Sony CCD-Iris Camera and MultiScanBase System. It was shown that weight of the central nervous system was on average 19% higher in males than in females. The mean number of motoneurons was 11% higher in males than in females and amounted to 95 and 86 motoneurons, respectively. In each case, the average soma diameters of motoneurons in motor nucleus were distributed bimodally: motoneurons smaller than 32.5 micrometer were recognized as gamma and greater ones as alpha motoneurons. In the present material the percentage composition of gamma (22%) and alpha (78%) motoneurons were very similar in both sexes. The mean number of alpha motoneurons was 13% higher in males than in females, but differences between the mean soma diameter in the two kinds of motoneurons, gamma and alpha, in both sexes were not significant. It is concluded that a total number of motoneurons in the rat MG motor nucleus in males and females is different. Whereas in females the number of alpha and gamma motoneurons is smaller than in males, the size of motoneurons is similar. Therefore, the gender differences in weight of the central nervous system reflect rather a difference in a total number of neurons than in their size.

Ograniczanie wyników

Sag in motor unit unfused tetanus: extra force production at the beginning of activity

The number of motor units in the medial gastrocnemius muscle of male and female rats

The transitory force decrease following high-frequency stimulation burst in unfused tetani of motor unit

Structural and electrophysiological characteristics of muscle compartments in rat medial gastrocnemius

Differences in morphometric properties of muscle fibers and innervation ratio of male and female motor units in the rat medial gastrocnemius muscle

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

Long-lasting effects of trans-spinal direct current stimulation on motoneuron firing properties in rats

The influance of endurance, strength, and vibration training on sag in unfused tetanic contractions of fast motor units

Sex differences in motor unit contractile properties of the rat soleus muscle

The content of myosin heavy chains in hindlimb muscles of female and male rats

Changes in the contractile properties of motor units in functionally overloaded medial gastrocnemius muscle of the rat

Summation of motor unit forces in the rat medial gastrocnemius muscle

The motor unit contractile properties of rat medial gastrocnemius muscle subjected to three months compensatory overload

The transitory force decrease following high-frequency stimulation burst in unfused tetani of motor units

The mechanomyogram - a valuable tool to analyse mechanical activity of the muscle

Summation of NMG signals during co-activity of two isolated motor units in medical gastrocnemius muscle of the rat

Hypothermia induced changes in contractile properties of motor units in rat medial gastrocnemius

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

The influence of 12 weeks high fat diet and low dose streptozotocin-induced diabetes on contractile properties of motor units in medial gastrocnemius muscle of rat

Gender-related differences of motoreurons morphometric properties of the rat medial gastrocnemius muscle