C-bouton components on rat extensor digitorum longus motoneurons are resistant to chronic functional overload

Mammalian motor systems adapt to the demands of their environment. For example, muscle fibre types change in response to increased load or endurance demands. However, for adaptations to be effective, motoneurons must adapt such that their properties match those of the innervated muscle fibres. We used a rat model of chronic functional overload to assess adaptations to both motoneuron size and a key modulatory synapse responsible for amplification of motor output, C-boutons. Overload of Extensor Digitorum Longus (EDL) muscles was induced by removal of their synergists, Tibialis Anterior (TA) muscles. Following 21 days survival, EDL muscles showed an increase in fatigue resistance and a decrease in force output, indicating a shift to a slower phenotype. These changes were reflected by a decrease in motoneuron size. However, C-bouton complexes remained largely unaffected by overload. The C-boutons themselves, quantified by expression of vesicular acetylcholine transporter, were similar in size and density in the control and overload conditions. Expression of the post-synaptic voltage-gated potassium channel (KV2.1) was also unchanged. Small conductance calcium activated potassium channels (SK3) were expressed in most EDL motoneurons, despite this being an almost exclusively fast motor pool. Overload induced a decrease in the proportion of SK3+ cells, however there was no change in density or size of clusters. We propose that reductions in motoneuron size may promote early recruitment of EDL motoneurons, but that C-bouton plasticity is not necessary to increase the force output required in response to muscle overload.

Microscopic changes in the spinal extensor musculature in patients experiencing chronic spinal pain: protocol for a systematic review

IntroductionChronic spinal pain (CSP) is the most common musculoskeletal disorder and is a leading cause of disability as per the Global Burden of Diseases. Previous reviews of microscopic changes in the spinal extensor muscles of people with CSP have focused on the lumbar region only and the results have been inconclusive. Therefore, in this protocol, we aim to assess microscopic changes in the extensor muscles of all spinal regions, investigating regionally specific changes in muscle fibre types of the spinal extensor muscles in patients with non-specific CSP.Methods/analysisThis protocol was designed using Preferred Reporting Items for Systematic Review and Meta-Analysis Protocols (PRISMA-P) guidelines. Searches will use the following databases: MEDLINE, Embase, PubMed, CINAHL Plus and Web of Science along with relevant grey literature searches. Two reviewers will conduct the searches, perform data extraction, apply inclusion criteria and conduct risk of bias assessment using Newcastle-Ottawa Scale. Data will be synthesised and analysed independently. If there is sufficient homogeneity, then meta-analysis will be conducted by the reviewers jointly. If not, meta-synthesis or narrative reporting will be performed. The quality of the evidence will be assessed using Grading of Recommendations, Assessment, Development and Evaluation (GRADE) guidelines.Ethics and disseminationThe results of this study will be submitted for publication to a peer-reviewed journal and will be presented at conferences. Ethical approval for this systematic review was not required due to no patient data being collated.PROSPERO registration numberCRD42020198087.

Effect of aerobic exercise on muscle structure and expression of proteins promoting hypertrophy and metabolism in aged rats

Aging and physical inactivity lead to histochemical changes in muscles. The expression of many muscle proteins, including brain-derived neurotrophic factor (BDNF), silent information regulator of transcription 1 (SIRT1), and peroxisome proliferator-activated receptor γγ coactivator-1α (PGC-1a), declines with age. However, the effect of aerobic exercise on muscle structure and the expression profile of these proteins in elderly rats is unknown. Here, we investigated whether short-term aerobic exercise improves muscle structure and increases BDNF, SIRT1, and PGC-1a levels in aged rats. Ten male Wistar rats (95-week-old) were assigned to sedentary (SED) or exercise (Ex) groups. The Ex group performed running on a treadmill for 1 h, 6 times per week, for 2 weeks. The extensor digitorum longus muscles were removed to examine the muscle fibre type composition, cross-sectional area, and capillary-to-fibre (C/F) ratio. BDNF, SIRT1, and PGC-1a levels were evaluated by western blotting. Relative to the SED group, the Ex group showed increased proportion of Type I fibres (P<0.05), cross-sectional area of all muscle fibre types (P<0.05), succinate dehydrogenase activity (P<0.001), C/F ratio (P<0.05), and expression of BDNF, SIRT1, and PGC-1a (P<0.05).Thus, 2 weeks of aerobic exercise is sufficient to improve muscle histology and hypertrophic marker protein expression, indicating that it could prevent skeletal muscle atrophy in elderly rats.

Muscle Fibre Types and Their Relation to Meat Quality Traits in Pigs

The authors have been studying various characteristics of muscle fibres and their relationship to the meat quality parameters for many years. However, the conclusions drawn by researchers often differ. A higher proportion of glycolytic IIB fibres in pig muscles is usually related to paler meat with lower water holding capacity. On the other hand the relationship between muscle fibres and meat texture parameters is not clear. Studies using immunohistochemistry methods that allow a more detailed classification of individual muscle fibre types could bring new findings in this area. It would thus be possible to influence muscle fibre type composition in the muscle to achieve the desired meat quality using various extrinsic and intrinsic factors. The main aim of this review is to summarise current knowledge on the description of muscle fibres typology and the effect of their morphological traits on pork meat quality.

Preferred metabolic pathway of bovine muscle fibre revealed by synchrotron–deep ultraviolet fluorescence imaging

The different bovine muscle fibre types I, IIA and IIX are characterised by their preferred metabolic pathway, either oxidative (I, IIA) or glycolytic (IIX), and their contraction speed, either slow-twitch (I) or fast-twitch (IIA, IIX). These physiological specificities are associated with variations in intracellular composition and their fluorescence spectra signatures. We hypothesised that these slight differences in autofluorescence responses could be used to discriminate the muscle fibre types by fluorescence imaging. Serial histological cross-sections of beef longissimus dorsi were performed: the start set was used to identify the metabolic and contractile type of muscle fibres by both immunohistoenzymology and immunohistofluorescence, and the following set was used to acquire synchrotron–deep ultraviolet (UV) autofluorescence images after excitation in the UV range (275 nm and 315 nm). This strategy made it possible to explore the label-free autofluorescence of muscle cells previously subtyped by histochemistry. Glycolytic cells (IIX) showed more intense fluorescence than oxidative cells (I and IIA) with near-90 % accuracy. This discrimination is more specifically assigned to the fluorescence of nicotinamide adenine dinucleotide. UV autofluorescence was unable to discriminate contractile type.

Metabolic cost underlies task-dependent variations in motor unit recruitment

Mammalian skeletal muscles are comprised of many motor units, each containing a group of muscle fibres that have common contractile properties: these can be broadly categorized as slow and fast twitch muscle fibres. Motor units are typically recruited in an orderly fashion following the ‘size principle’, in which slower motor units would be recruited for low intensity contraction; a metabolically cheap and fatigue-resistant strategy. However, this recruitment strategy poses a mechanical paradox for fast, low intensity contractions, in which the recruitment of slower fibres, as predicted by the size principle, would be metabolically more costly than the recruitment of faster fibres that are more efficient at higher contraction speeds. Hence, it would be mechanically and metabolically more effective for recruitment strategies to vary in response to contraction speed so that the intrinsic efficiencies and contraction speeds of the recruited muscle fibres are matched to the mechanical demands of the task. In this study, we evaluated the effectiveness of a novel, mixed cost function within a musculoskeletal simulation, which includes the metabolic cost of contraction, to predict the recruitment of different muscle fibre types across a range of loads and speeds. Our results show that a metabolically informed cost function predicts favoured recruitment of slower muscle fibres for slower and isometric tasks versus recruitment that favours faster muscles fibres for higher velocity contractions. This cost function predicts a change in recruitment patterns consistent with experimental observations, and also predicts a less expensive metabolic cost for these muscle contractions regardless of speed of the movement. Hence, our findings support the premise that varying motor recruitment strategies to match the mechanical demands of a movement task results in a mechanically and metabolically sensible way to deploy the different types of motor unit.

Effect of high-fat mixed lipid diet and swimming on fibre types in skeletal muscles of rats with colon tumours

Skeletal muscle fibre types, whose characteristics are determined by myosin heavy chain (MyHC) isoforms, can adapt to changed physiological demands with changed MyHC isoform expression resulting in the fibre type transitions. The endurance training is known to induce fast-to-slow transitions and has beneficial effect in carcinogenesis, whereas the effect of an excessive fat intake and its interaction with the effect of swimming are less conclusive. Therefore, we studied the effect of high-fat mixed lipid (HFML) diet and long-term (21-week) swimming on fibre type transitions and their average diameters by immunohistochemical demonstration of MyHC isoforms in slow soleus (SOL), fast extensor digitorum longus (EDL), and mixed gastrocnemius medialis and lateralis (GM, GL) muscles, divided to deep and superficial portions (GMd, GMs, GLd, GLs), of sedentary and swimming Wistar rats with experimentally (dimethylhydrazine) induced colon tumours and fed either with HFML or low-fat corn oil (LFCO) diet. HFML diet induced only a trend for fast-to-slow transitions in SOL and in the opposite direction in GMd. Swimming triggered significant transitions in unexpected slow-to-fast direction in SOL, whereas in GMs the transitions had tendency to proceed in the expected fast-to-slow direction. The average diameters of fibre types were mostly unaffected. Hence, it can be concluded that if present, the effects of HFML diet and swimming on fibre type transitions were counteractive and muscle-specific implying that each muscle possesses its own adaptive range of response to changed physiological conditions.