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The diaphragm as a striated muscle is characterized by the repetition of a single element arranged in series: the sarcomere containing two kinds of myofilaments: a thick one constituted by the myosin, and a thin one primarily composed of actin. The myosin molecule consists of two heads where two myosin heavy chains (MHC) are fixed, a flexible hinge with two light (MLC) chains, and long rod-shaped tails. The diaphragm contains 4 MHC isoforms (MHC-slow, MHC-2A, MHC-2B, MHC-2X) and 6 MLC isoforms (MLC-1f, MLC-3f, MLC-1sa, MLC-1sb, MLC-2f, MLC-2s/v). In humans, the diaphragm contains mainly fibers expressing the isoforms MHC-slow, MHC-2A, and MLC-2f, MLC-2s et MLC-1f. For the mechanical properties of the different isoforms, there is a gradient from the MHC-slow to the MHC-2A, MHC-2B and MHC-2X/2B. According to the circumstances, the diaphragm will adapt towards a slow profile (COPD, cardiac failure and in animals: Duchenne muscular dystrophy, denervation-1 week, age-female, corticosteroids, chronic stimulation), or a fast profile (in animals: chronic hypoxia, denervation-2 weeks, age-males) or a more oxidative profile (in animals: cachexia, obesity). The reasons why the diaphragm adapts towards a slower or a faster muscle are not known. In fact, for a given pathological situation, several factors are able to influence the fiber composition of the diaphragm. Therefore, the net result of the influence of these different factors in terms of MHC and MLC diaphragm adaptation is difficult to predict.

Cytoskeleton based molecular motors support most of the cellular movements and by consequence they are associated with a variety of human disorders. The wide functional diversity of these molecular motors is now explained by the presence of three different families: the myosin, kinesin and dynein families. Although they are functionally distinct, these motors present unexpected structural homologies at the ATP and actin or microtubule binding sites. However, these homologies do not seem sufficient to design a common molecular mechanism which allows these proteins to move along the cytoskeleton.

The crossed nerve anastomosis between the peripheral end of the vagus nerve, cut above the nodose ganglion, and the peripheral end of the accessory nerve has demonstrated the capacity of some vagal afferents to reinnervate, via the accessory nerve stump, certain sternocephalicus muscle fibers in the rabbit. These results add to our understanding of the capacity of these afferents to counter the past-denervational atrophying process that occurs in the reinnervated muscles and to evaluate the changes induced in these muscles during reinnervation. Our work shows that within 3 months, the vagal sensory reinnervation of previously denervated sternocephalicus muscles induces their total weight recovery. This recovery is concomitant on the one hand with the hypertrophy of the four muscle fiber types (I, IIBD, IIC and IIA) identified histochemically in the normal muscles and, on the other, with the appearance of small newly formed myofibers, which are often underlined by characteristic central nuclei. The vagal sensory neurones induce important changes in the percentages and the muscle cross-sectional distribution of the fibers in reinnervated muscles. In these muscles we see also the disappearance of the fast myosin heavy chains MHCIIB and MHCIID, the upholding of the fast MHCIIA percentage and an increase in the slow MHCI isoform.

Skeletal muscle sarcolemmal vesicles from control (C) and hindlimb suspended (S) rats were used to investigate the effect of unweighting on the lactate transporter activity. Sarcolemmal preparations were not different between the two groups. The efficiency of 4 weeks of hindlimb suspension was confirmed by a 40% decrease of citrate synthetase activity and a shift towards faster myosin isoforms in soleus muscle. The time course of 1 mM lactate uptake showed that the equilibrium was reached faster in group C (20 s) than in group S (40 s). The initial rate of 1 mM of lactate uptake decreased significantly (p < 0.05) after 4 weeks of hindlimb suspension. The initial rate of 50 mM lactate uptake did not differ significantly between the two groups. We conclude that 4 weeks of unweighting decreases significantly the skeletal muscle sarcolemmal lactate transport activity in rats. This result suggests that the level of physical activity probably plays a role on lactate transport regulation in muscle.

During the metabolic differentiation of developing human skeletal muscle fibers, two different generations of fibers can be characterised by their profiles of protein expression (in particular of the myosin heavy chains). This differentiation is regulated by both genetic and epigenetic factors (motor innervation, levels of thyroid hormone). The anomalies in different muscle pathologies can be better understood by comparison with the modifications occurring during normal skeletal muscle development.

Systolic function of normal hearts is characterised by the sequence isometric-isotonic contraction and dominated by the force-time relationship which, more than the pressure graph, determines myocardial oxygen consumption. It is the main factor of heat production whether or not related to mechanical activity. Apart from the loading conditions, there are two important biological factors during contraction: the structure of the myosin and the quantity of free intracytoplasmic calcium on which the membrane proteins depend. In cardiac hypertrophy, the decrease of Vmax allows the fibre to develop a normal active tension more economically at the expense of its velocity. Depending on the species and tissues, this decrease may be due to a sarcoplasmic and/or a membrane protein modification. The therapeutic inotropic effect is obviously deleterious at fibre level but it may, on the contrary, be economically beneficial from the thermodynamic point of view to the heart as a whole.