How Does a Skeletal Muscle Twitch Differ from a Tetanic Contraction
The rate at which a motor neuron triggers action potentials affects the tension generated in skeletal muscle. If the fibers are stimulated while a previous contraction is still occurring, the second contraction will be stronger. This response is called wave summation because the excitation-contraction coupling effects of successive motor neuron signaling are added or added together (Figure 4a). At the molecular level, summation occurs because the second stimulus triggers the release of more Ca++ ions, which become available to activate additional sarcomeres while the muscle contracts again from the first stimulus. Summation leads to a stronger contraction of the motor unit. Although muscles are unidirectional active force generators, shortening and extension contractions constantly occur during natural movements. They are sometimes called concentric or eccentric contractions. Indeed, all joints are crossed by at least one pair of muscles acting in opposite directions. As a result, with all joint movements, one muscle expands, while the other shortens. In general, both muscles show non-zero non-activation values during movements. This means that one of them performs a concentric contraction, while the other contracts eccentrically.
If a muscle is supposed to lift a constant load (isotonic conditions) after the start of stimulation, the force increases, just like with isometric contraction, and when the force is equal to the load, the muscle begins to shorten and lift the load. When the activity of the muscle and the strength it contains begins to decrease, the load stretches the muscle to its original length. The tension in the muscle is equal to the load during the shortening and lengthening of the muscle, except during the short acceleration phases, when the muscle begins to move. It is only once the muscle has regained its original length that the tension begins to subside. The size of the load also determines the speed of shortening; this relationship between load and speed also applies to the heart and running muscles. Here, we used time-resolved X-ray diffraction of intact and electrically stimulated mouse extensor digitorum longus (EDL) muscles to fill these fundamental gaps in understanding the mechanisms that determine the amplitude and complete temporal course of contractions in mammalian muscles, focusing on X-ray reflections that signal the structure and regulatory state of thick filaments and conformations of myosin engines. The results complement those of a groundbreaking time-resolved X-ray study of contractions and tetanus in amphibian muscle that focused on radiographic reflections based on thin filaments (Kress et al., 1986). All available measurements on the mouse`s EDL muscle were made at 28°C, a temperature at which the muscles contract reproducibly, as is necessary for the average of the signals in X-ray experiments. In addition, the temporal evolution of the transient [Ca2+]i and its binding to troponin in mouse EDLs at 28°C have been characterized (Baylor and Hollingworth, 2003), and the maximum strength is very close to that at mammalian body temperature (Caremani et al., 2019). More importantly, the external structure of the thick filament in mammalian muscle is maintained at this temperature (Caremani et al., 2021), unlike almost all published studies on the regulation of thin filaments, for example, those that use pCa titrations with Ca buffer at steady state in flayed muscle fibers made at a relatively low temperature to counter the irreversible effects of persistent high to high activation. temperature.
minimize. The role of regulating thick filaments was inadvertently excluded in such experiments, as thick filaments were already lit at low temperature used, even in the absence of calcium. Half-time (t1/2) for the ascending phase of tetanus (activation) and for relaxation after tetanus and contractions. t1/2 values for X-ray parameters were determined by adjusting the sigmoidal curves to the 5 ms time interval data. The t1/2 values for tetanus activation and contraction relaxation are reported with respect to the first stimulus at time 0 and those for tetanus relaxation with respect to the last stimulus at 100 ms. The lines are sorted by t1/2 for tetanus activation, with the fastest signals at the top. No significant changes in AAL1 were observed during contractions. LM3, MM3 and HM3 are the fractions of the low, medium and high angular peaks of the M3 reflection in Figure 5E.
n = 5 muscles for tetanus and n = 4 muscles for contractions. The data displayed are average values ± SEM. *A significant difference in t1/2 for the X-ray parameter in terms of force in this phase. *S<0.05; **S<0.001; S<0.0001. IML1 did not recover during isometric relaxation after tetanus stimulation (Figure 3D, circles filled), which is consistent with the above conclusion that myosin motors do not reform the folded helical conformation at this stage despite the decrease in force and detachment of myosin motors from actin. . . .