The influence of jaw-muscle fibre-type phenotypes on estimating maximum muscle and bite forces in primates

Numerous anthropological studies have been aimed at estimating jaw-adductor muscle forces, which, in turn, are used to estimate bite force. While primate jaw adductors show considerable intra- and intermuscular heterogeneity in fibre types, studies generally model jaw-muscle forces by treating the jaw adductors as either homogeneously slow or homogeneously fast muscles. Here, we provide a novel extension of such studies by integrating fibre architecture, fibre types and fibre-specific tensions to estimate maximum muscle forces in the masseter and temporalis of five anthropoid primates: Sapajus apella ( N = 3), Cercocebus atys ( N = 4), Macaca fascicularis ( N = 3), Gorilla gorilla ( N = 1) and Pan troglodytes ( N = 2). We calculated maximum muscle forces by proportionally adjusting muscle physiological cross-sectional areas by their fibre types and associated specific tensions. Our results show that the jaw adductors of our sample ubiquitously express MHC α-cardiac, which has low specific tension, and hybrid fibres. We find that treating the jaw adductors as either homogeneously slow or fast muscles potentially overestimates average maximum muscle forces by as much as approximately 44%. Including fibre types and their specific tensions is thus likely to improve jaw-muscle and bite force estimates in primates.

Nebulin and Lmod2 are critical for specifying thin-filament length in skeletal muscle

Regulating the thin-filament length in muscle is crucial for controlling the number of myosin motors that generate power. The giant protein nebulin forms a long slender filament that associates along the length of the thin filament in skeletal muscle with functions that remain largely obscure. Here nebulin’s role in thin-filament length regulation was investigated by targeting entire super-repeats in the Neb gene; nebulin was either shortened or lengthened by 115 nm. Its effect on thin-filament length was studied using high-resolution structural and functional techniques. Results revealed that thin-filament length is strictly regulated by the length of nebulin in fast muscles. Nebulin’s control is less tight in slow muscle types where a distal nebulin-free thin-filament segment exists, the length of which was found to be regulated by leiomodin-2 (Lmod2). We propose that strict length control by nebulin promotes high-speed shortening and that dual-regulation by nebulin/Lmod2 enhances contraction efficiency.

Adaptive Mechanisms of a Mouse Locomotor Muscles "M. Soleus" and "M. Edl" the Conditions of the Allergic Modification of the Organism

The relevance of the problem discussed in the article is connected to the fact that mandatory athletes’ vaccination before competitions leads to the change in the function of the muscular system, the mechanisms of which have not yet been fully clarified. The purpose of the article is to determine the mechanism of a mouse skeletal muscles adaptation (SM) ("fast" (in case of m.edl) and "slow" (in case of m.soleus) in case of allergic alteration. The following research methods were used in the presented work: registration of the constrictive function of the abovementioned muscles in vitro to the humoral constriction initiators (carbacholinum and KCI) and determination of malonyldialdehyde (MDA) level in them, just as the indicators of the oxidant and antioxidant equilibrium. It has been demonstrated that the change in the “slow” muscle strength correlates with the MDA level dynamics, evidently, reflects the adaptation processes during the allergic modification. "Fast" muscles turn out to be more sustainable to oxidative stress which is most probably achieved by the work of compensatory mechanisms and is expressed in quite minor changes in the MDA dynamics. The article can be used in the search of the new possibilities for the correction of the locomotor muscles function in the conditions of the allergy, аnd also while the therapeutic impact strategy is determined, taking into account their fiber composition.

Muscle cell type diversification facilitated by extensive gene duplications

The evolutionary mechanisms underlying the emergence of new cell types are still unclear. Here, we address the origin and diversification of muscle cells in the diploblastic sea anemone Nematostella vectensis. We discern two fast and two slow-contracting muscle cell populations in Nematostella differing by extensive sets of paralogous genes. The regulatory gene set of the slow cnidarian muscles and the bilaterian cardiac muscle are remarkably similar. By contrast, the two fast muscles differ substantially from each other, while driving the same set of paralogous structural protein genes. Our data suggest that extensive gene duplications and co-option of individual effector modules may have played an important role in cell type diversification during metazoan evolution.One Sentence SummaryThe study of the simple sea anemone suggests a molecular mechanism for cell type evolution and morphological complexity.

Synaptic silencing of fast muscle is compensated by rewired innervation of slow muscle

For decades, numerous studies have proposed that fast muscles contribute to quick movement, while slow muscles underlie locomotion requiring endurance. By generating mutant zebrafish whose fast muscles are synaptically silenced, we examined the contribution of fast muscles in both larval and adult zebrafish. In the larval stage, mutants lacked the characteristic startle response to tactile stimuli: bending of the trunk (C-bend) followed by robust forward propulsion. Unexpectedly, adult mutants with silenced fast muscles showed robust C-bends and forward propulsion upon stimulation. Retrograde labeling revealed that motor neurons genetically programmed to form synapses on fast muscles are instead rerouted and innervate slow muscles, which led to partial conversion of slow and intermediate muscles to fast muscles. Thus, extended silencing of fast muscle synapses changed motor neuron innervation and caused muscle cell type conversion, revealing an unexpected mechanism of locomotory adaptation.