Modeling of summation of individual twitches into unfused tetanus for various types of rat motor units.

Repeated stimulation of motor units (MUs) causes an increase of the force output that cannot be explained by linear summation of equal twitches evoked by the same stimulation pattern. To explain this phenomenon, an algorithm for reconstructing the individual twitches, that summate into an unfused tetanus is described in the paper. The algorithm is based on an analytical function for the twitch course modeling. The input parameters of this twitch model are lead time, contraction and half-relaxation times and maximal force. The measured individual twitches and unfused tetani at 10, 20, 30 and 40 Hz stimulation frequency of three rat motor units (slow, fast resistant to fatigue and fast fatigable) are processed. It is concluded that: (1) the analytical function describes precisely the course of individual twitches; (2) the summation of equal twitches does not follow the results from the experimentally measured unfused tetani, the differences depend on the type of the MU and are bigger for higher values of stimulation frequency and fusion index; (3) the reconstruction of individual twitches from experimental tetanic records can be successful if the tetanus is feebly fused (fusion index up to 0.7); (4) both the maximal forces and time parameters of individual twitches subtracted from unfused tetani change and influence the course of each tetanus. A discrepancy with respect to the relaxation phase was observed between experimental results and model prediction for tetani with fusion index exceeding 0.7. This phase was predicted longer than the experimental one for better fused tetani. Therefore, a separate series of physiological experiments and then, more complex model are necessary for explanation of this distinction.     

Fine structural changes in electrostimulated human skeletal muscle. Evidence for predominant effects on fast muscle fibres

Physical education students were subjected to electrical stimulation of relatively high frequency and current amplitude for 19 days. A quantitative study of several morphological parameters was performed on biopsy samples from gastrocnemius, using stereological methods at both light and electron microscopic levels. The main results were: muscle fibre size was increased; nuclear volume was also increased, suggesting that a proliferation of nuclei had occurred; this was paralleled by an increased content of nuclear DNA. The size of single myonuclei was increased, and their heterochromatin fraction was decreased, these changes being most pronounced in type II fibres. The increase in the mitochondrial fraction was also greatest in type II fibres. It is concluded that this type of electrical stimulation has predominant effects on type II fibres.     

Redox balance following magnetic stimulation training in the quadriceps of patients with severe COPD

In severe COPD patients, oxidative stress, which is involved in their peripheral muscle dysfunction, increases in response to exercise. In this study, muscle oxidative stress was explored after quadriceps magnetic stimulation training. A randomized controlled study was conducted on very severe COPD patients, who underwent quadriceps magnetic stimulation training for 8 weeks. A control group was also studied. In both groups, vastus lateralis specimens were obtained before and after the 8-week period. Muscle protein carbonylation and nitration and antioxidant enzymes were determined using immunoblotting and proportions and sizes of type I and II fibres using immunohistochemistry. Compared to controls, magnetic stimulation muscle training did not modify redox balance, whilst inducing a significant increase in type I fibre sizes. In severe COPD patients, it is concluded that quadriceps magnetic stimulation training was a well-tolerated therapeutic intervention, which did not enhance muscle oxidative stress, while increasing the size of slow-twitch fibres.     

Creatine kinase MB and citrate synthase in type I and type II muscle fibres in trained and untrained men

Total creatine kinase (CK), creatine kinase MB (CK-MB) and citrate synthase (CS) were determined in isolated and pooled type I and type II skeletal muscle fibres. Determinations were made on biopsies from 3 sedentary men, 3 junior cyclists and 2 elite cyclists. CS and CK-MB activities were higher in the trained groups in both fibre types. The total CK activity was not related to training status, although it was lower in type I fibres than in type II fibres (p less than 0.05). The reverse relation was observed for CS and CK-MB activities (p less than 0.01). The ratio of type I/type II for CS was not related to training status, while the corresponding ratio for CK-MB increased with a greater degree of endurance training. For a given increase in CS activity, the increase in CK-MB activity was greater in type I fibres than in type II fibres (p less than 0.01). Thus, with endurance training there seems to be a specific adaptation for CK-MB, particularly in type I fibres.     

Change in motor neurone activity modifies the differentiation of a slow muscle in chick embryo

Slow-tonic anterior latissimus dorsi (ALD) muscle properties were studied following chronic spinal cord stimulation in chick embryo. Stimulation at a fast rhythm was applied from day 7, 8 or 10 of development until the end of embryonic life. When stimulation was applied from day 7 up to day 18 of development, ALD muscle exhibited at day 18 a large decrease in half time to peak of tetanic contraction, a large proportion of fast type II fibres and an increase in fast myosin light chain content as compared to control muscle. When stimulation started at day 8 of development, changes in properties of ALD muscle were reduced when compared to the previous experimental series. Indeed, no fast type II fibres were observed within the muscle, even when stimulation was prolongated until the 20th day of embryonic development. In addition, chronic stimulation at a fast rhythm initiated at day 10 of development did not modify ALD muscle differentiation. The present results indicate that a fast pattern of motor neurone activity can induce some slow-to-fast transformations of ALD muscle fibres. However, after the first week of embryonic life, ALD myotubes appeared refractory to these transformations. The possible mechanisms responsible for the transformation of slow myotubes and for their further loss of plasticity are discussed.     

Are region-specific changes in fibre types attributable to nonuniform muscle hypertrophy by overloading?

Muscle fibre composition was compared among the proximal (25%), middle (50%) and distal (75%) regions of the muscle length to investigate whether compensatory overload by removal of synergists induces region-specific changes of fibre types in rat soleus and plantaris muscles. In addition, we evaluated fibre cross-sectional area in each region to examine whether fibre recruitment pattern against functional overload is nonuniform in different regions. Increases in muscle mass and fibre area confirmed a significant hypertrophic response in the overloaded soleus and plantaris muscles. Overloading increased the percentage of type I fibres in both muscles and that of type IIA fibres in the plantaris muscle, with the greater changes being found in the middle and distal regions. The percentage of type I fibres in the proximal region was higher than that of the other regions in the control soleus muscle. In the control plantaris muscle, the percentage of type I and IIA fibres in the middle region were higher than that of the proximal and distal regions. With regard to fibre size, type IIB fibre area of the middle and distal regions in the plantaris increased by 51% and 57%, respectively, with the greater changes than that of the proximal region (37%) after overloading. These findings suggest that compensatory overload promoted transformation of type II fibres into type I fibres in rat soleus and plantaris muscles, with the greater changes being found in the middle and distal regions of the plantaris muscle.     

The effect of strength training on estimates of mitochondrial density and distribution throughout muscle fibres

The purpose of this study was to investigate the effect of strength training (12 weeks, 3 days/week, four lower-body exercises) of young individuals (mean age 23.6 years) on estimates of mitochondrial distribution throughout muscle fibres. A control group (mean age 21. 7 years) was followed simultaneously. Skeletal muscle biopsy samples were obtained from the vastus lateralis, pre- and post-training. The regional distribution of subsarcolemmal and intermyofibrillar mitochondrial populations was determined using quantitative histochemical staining of succinate dehydrogenase (SDH) in type I and II muscle fibres. Strength training resulted in significant increases of 26% and 28% in the cross-sectional area of type I and II fibres, respectively (P < 0.05). Overall SDH activity decreased by 13% with strength training (P < 0.05). The decrease in SDH activity with strength training between fibre types and between subsarcolemmal and intermyofibrillar regions of muscle fibres was not different. Fibre area and SDH activity was unchanged in the control group. We conclude that the muscle hypertrophy associated with strength training results in reduced density of regionally distributed mitochondria, as indicated by the reduction in the activity of SDH.     

Modulation of the contractile activity of the fast and slow twitch rat muscle during continuous stimulation

The fast- and slow-twitch muscles were tested with single pulses in the course of unfused tetanus formation. The tetanus decreased differences in contractile parameters of the test-twitch contractions and, after continuous stimulation, eliminated them altogether.     

Twitch contractile adaptations are not dependent on the intensity of isometric exercise in the human triceps surae

Ultrastructural and twitch contractile characteristics of the human triceps surae were determined in six healthy but very sedentary subjects before and after 16 weeks of isometric training at 30% maximal voluntary contraction (MVC). Following training, twitch contraction time was approximately 16% shorter, although no differences were observed in one-half relaxation time or peak twitch torque. Percent fibre type was not changed by training. The mean area of type I and type II fibres in the soleus increased by approximately 30% but only type II fibres showed an increase in area in the lateral gastrocnemius (30%). Despite such changes in fibre area the volume density of the sarcoplasmic reticulum-transverse tubular network averaged 3.2 +/- 0.6% and 5.9 +/- 0.9% in type I and type II fibres respectively, before and after training in the two heads of the gastrocnemius. The results indicate that contractile adaptations to isometric training at 30% MVC were limited to twitch contraction time and were not directly related to changes in percent fibre distribution or the volume of sarcoplasmic reticulum and transverse tubules in either type I or type II fibres. The data further demonstrate that substantial fibre hypertrophy is achieved by training with low-intensity contractions.     

The number of satellite cells in slow and fast fibres from human vastus lateralis muscle

The aim of this investigation was to study the distribution of satellite cells in slow (type I fibres) and fast (type II fibres) fibres from human vastus lateralis muscle. This muscle is characterised by a mixed fibre type composition and is considered as the site of choice for biopsies in research work and for clinical diagnosis. Biopsy samples were obtained from five healthy young volunteers and a total of 1,747 type I fibres and 1,760 type II fibres were assessed. Satellite cells and fibre type composition were studied on serial muscle cross-sections stained with specific monoclonal antibodies. From a total of 218 satellite cells, 116 satellite cells were found in contact with type I fibres (53.6±8% of the satellite cells associated to type I fibres) and 102 satellite cells in contact with type II fibres (46.4±8% of the satellite cells associated to type II fibres). There was no significant difference (P=0.4) between the percentages of satellite cells in contact with type I and with type II fibres. Additionally, there was no relationship between the mean number of satellite cells per fibre and the mean cross-sectional area of muscle fibres. In conclusion, our results show that there is no fibre type-specific distribution of satellite cells in a human skeletal muscle with mixed fibre type composition.     

Prolonged force increase following a high-frequency burst is not due to a sustained elevation of [Ca2+]i

A brief high-frequency burst of action potentials results in a sustained force increase in skeletal muscle. The present study investigates whether this force potentiation is the result of a sustained increase of the free myoplasmic [Ca2+] ([Ca2+]i). Single fibers from mouse flexor brevis muscles were stimulated with three impulses at 150 Hz (triplet) at the start of a 350-ms tetanus or in the middle of a 700-ms tetanus; the stimulation frequency of the rest of the tetanus ranged from 20 to 60 Hz. After the triplet, force was significantly (P < 0.05) increased between 17 and 20% when the triplet was given at the start of the tetanus and between 5 and 18% when the triplet was given in the middle (n = 7). However, during this potentiation, [Ca2+]i was not consistently increased. Hence, the increased force following a high-frequency burst is likely due to changes in the myofibrillar properties.     

Model-generated decomposition of unfused tetani of motor units evoked by random stimulation

Unfused tetani of motor units (MUs) evoked by stimulation at variable interpulse intervals at mean frequencies of 20, 25, 33, 40 and 50 Hz were studied using ten functionally isolated fast-type MUs from the medial gastrocnemius muscle of adult Wistar rats. A previously proposed algorithm and computer program for mathematical decomposition of unfused tetani into a series of twitches, representing responses to individual pulses, were used. Analysis of the parameters of the decomposed twitches showed considerable variability in force of successive contractions. These twitches were extremely variable with up to 2-fold higher forces and longer contraction times than a single twitch evoked by one stimulus. However, when the stimulation frequency was decreased, the decomposed twitches became similar to the single twitch with respect to amplitude and contraction time. It was found that the basic contractile parameters of decomposed twitches could be predicted with high accuracy on the basis of the tetanus force level at which the next contraction begins. This analysis of the parameters of decomposed twitches demonstrated that the contractile responses of the muscle fibers to successive action potentials generated by motoneurons are highly variable and depend on the previous MU state.     

Differential changes in myoelectric characteristics of slow and fast fatigable frog muscle fibres during long-lasting activity.

The electrical activity of different muscle fibre types during fatigue at varying stimulation frequency and fibre stretch was studied. Extracellular action potentials (ECAPs) were recorded from isolated frog muscle fibres at initial length and stretched by 15%, 25% and 35% and stimulated for 180 s by suprathreshold pulses with frequencies of 5, 6.7 and 10Hz. The changes in ECAP negative phase duration (T(0)), propagation velocity of excitation (PV), potential power spectrum and its median frequency (MDF) were analysed for the period of uninterrupted activity (endurance time, ET). Slow (SMF) and fast (FMF) fatigable muscle fibre types were distinguished by the rate of PV decrease during ET. With the increase of stimulation frequency and fibre stretch, the rate of ECAP parameter changes increased and was larger in FMF, but this proportion was reversed with stretching over 25% and 10Hz stimulation. In both fibre types the power spectrum shift to lower frequencies during continuous activity was more pronounced with higher stimulation frequency. In FMFs the rates of MDF changes were positively and more strongly correlated with the rates of PV changes, whilst in SMFs the inverse correlation between the rates of changes of MDF and T(0) was stronger. The results indicate specific adaptation of slow and fast fatigable muscle fibres to stretch and activation frequency due to the differences in their membrane processes.     

Factors affecting the histochemical composition of reinnervated soleus muscle of rat.

The myofibrillar ATPase reaction was utilized to determine the relative proportion of type II fibres in reinnervated soleus muscle 6 months after transection and reunion of the nerve at various distances from the muscle. In self-reinnervated soleus muscle, a highly significant decrease in the percentage of type II fibres from 10.5 +/- 1.6% in normal muscle to 0.7 +/- 0.4% was noted. In randomly reinnervated soleus muscle the percentage of type II fibres gradually increased with the distance of the site of nerve transection from the muscle. After transection and reunion of the muscular branch of the tibial nerve, the type II fibres constituted 34.0 +/- 1.5% and that of the reinnervated soleus muscle after transection and reunion of the sciatic nerve stump was 73.7 +/- 1.7% of the total fibre population. Different factors which might be responsible for the observed differences in the degree of cytochemical transformation of muscle fibre types in the process of reinnervation are discussed.     

Post-tetanic potentiation of miniature end-plate potential frequency at neuromuscular junction of the rat soleus muscle

1. Changes in miniature end-plate potential (m.e.p.p.) frequency by repetitive nerve stimulation were examined in the rat soleus muscle. 2. The increase of m.e.p.p. frequency was induced by repetitive stimulation and persisted for several minutes after the tetanus. That is, post-tetanic potentiation (PTP) of neuromuscular transmission was first demonstrated here in the rat soleus muscle. 3. The time course of the decay of m.e.p.p. frequency after the tetanus showed a double exponential curve which consisted of a fast decaying component (augmentation) and a slow decaying component (potentiation). 4. The magnitude of PTP depended on the stimulation frequency and its duration. It increased with the increase of duration and was at its maximum at a frequency of 100 Hz. 5. No PTP was elicited by repetitive stimulation under conditions in which end-plate potential (e.p.p.) was completely suppressed, and, moreover, m.e.p.p. frequency tended to decrease after the tetanus.     

Tetanic depression in fast motor units of the cat gastrocnemius muscle.

Ability of muscle fibers to generate force is decreased when higher frequency of stimulation of motor units immediately follows lower frequency. This phenomenon called tetanic depression was found in rat medial gastrocnemius. However, it was not clear whether tetanic depression occurred only in rat muscle or it concerns all mammals. This study was conducted on motor units of cat medial gastrocnemius. Analyses were made at three successive trains of stimulation: 30 Hz, 20 and 30 Hz and again 30 Hz (the first pattern) or 40 Hz, 25 and 40 Hz and 40 Hz (the second pattern). In all fast units force generated within the middle tetanus was lower than force generated at the same, but constant frequency of stimulation applied earlier or later. The mean tetanic depression in 30 Hz tetani amounted to 10.9% for fast fatigable (FF) and 15.9% for fast resistant (FR) motor units, whereas in 40 Hz tetani mean values were 5.6% and 7.3% for FF and FR motor units, respectively. In slow motor units tetanic depression was not observed. These results proved the existence of tetanic depression in the feline muscle and indicated that its intensity depends on the fusion of tetanus. It has been concluded, that the tetanic depression is a general property of fast motor units in mammals.     

Histochemical and contractile responses of rat medial gastrocnemius to 2 weeks of complete disuse.

We studied the histochemical and in situ contractile changes in a rat ankle extensor, medial gastrocnemius, in which activation of muscle fibres by motoneurones was blocked for 14 days, using the sodium channel blocker tetrodotoxin applied to the sciatic nerve. Muscles were atrophied and showed slower twitch responses, greater fusion at subtetanic frequencies of stimulation, and higher twitch/tetanic ratios. Tetanic force/mm2 of fibre area and fatiguability were unchanged. Type II fibres were more atrophied and showed greater decreases in mitochondrial succinate dehydrogenase activity than type I fibres. The contractile changes resulting from complete disuse do not occur in models in which weight-bearing alone has been removed (space flight, hindlimb suspension), suggesting that the residual motoneurone activity reported in models of weightlessness is sufficient to prevent these responses. Similarly, the finding of a greater type II fibre susceptibility to complete disuse, which differs from the pattern seen in models of weightlessness, suggest that this residual motoneurone activity in the latter influences atrophic responses in a manner that is variable among motor unit types, to produce the reported preferential type I atrophy characteristic of removal of weight-bearing.     

Variations in fibre composition of the gastrocnemius muscle in rats subjected to speed training

Thirty-six adult Wistar rats were divided into three groups. One group was used as a control, and the other two underwent different training programmes in which greater relevance was attached to the intensity of exercise than to its duration. Samples of the red and mixed portions of m. gastrocnemius (caput lateralis) were stained with m-ATPase to determine the percentage of type I, IIA and IIB fibres, and with NADH-TR in order to quantify variations in the percentage of low staining intensity (FG) fibres. The most notable results obtained were: a) the ratio of type I type II fibres remained unchanged; b) the proportion of IIA fibres increased, while that of IIB fibres decreased correspondingly; c) FG fibres, which were virtually absent from the red portion, recorded a clear decrease which was more marked, and occurred more rapidly, than in IIB fibres. These differences were all statistically significant in the mixed portion of the muscle. Adaptive changes in fibre composition in the red portion were less marked.     

Analysis of melibiose mutants deficient in alpha-galactosidase and thiomethylgalactoside permease II in Escherichia coli K-12

Three types of mutants (mel(-)) unable to metabolize the alpha-d-galactoside, melibiose, were derived from Escherichia coli K-12. One type lacked alpha-galactosidase; another lacked a specific transport system, termed thiomethylgalactoside (TMG) permease II; and the third lacked both of these functions. The mutational sites were genetically mapped by recombination frequency with different markers and by determination of chromosomal transfer in interrupted-mating experiments. All three mel(-) mutant types mapped in a cluster near to the metA marker on the E. coli chromosome and were cotransducible. Induction studies revealed that the three alpha-d-galactosides, melibiose, melibiitol, and galactinol, induced alpha-galactosidase and TMG permease II coordinately; d-galactose also induced them but only in a galactokinaseless mutant. These data suggest that alpha-galactosidase and TMG permease II may be components of a common operon.     

Analysis of the unfused tetanus course in fast motor units of the rat medial gastrocnemius muscle

The course of unfused tetani with the sag effect in fast motor units of rat medial gastrocnemius was studied. The analysis of the course of successive contractions within these tetani showed that the high peak force at the beginning of tetanus before the sag resulted from temporary, very efficient sum mation of contractions at this phase, both in FF (fast fatigable) and FR (fast resistant to fatigue) units. The process of summation developed in spite of parallel shortening of the contraction and relaxation. The peak of tetanus force was visible on the average at the 2nd contraction in FF units and at the 5th contraction in FR units. After the tetanus peak the process of the efficient summation was completed and the force decreased what was visible as a sag. In the following part of the tetanus, mainly in FF units, the potentiation occurred and the force of successive contractions increased. The rise of force was visible in spite of shortening of the contraction time and was due to prolongation of the relaxation in this part of the tetanus. These observations indicated that the processes of the summation of successive contractions before sag and during the potentiation underwent different mechanisms discussed in this paper. Considerable release of Ca2+ ions from the sarcoplasmic reticulum was proposed as a possible mechanism responsible for a very efficient summation at the beginning of the tetanus whereas phosphorylation of regulatory light chain of myosin (RLC) in muscle fibers was considered as the reason of potentiation. Moreover, the present analysis revealed that previously found differences in tetani profiles between FF and FR units resulted from faster development of described changes in the course of contractions summating into the tetanus in FF motor units.