The amount of force generated by a muscle fiber in tetanus contractions and tetanus contractions depends on the length of that fiber and its rate of change (speed). The dependence of muscle strength on length and speed depends in part on the properties of transverse bridges and in part on the properties of passive connective tissue, including the properties of tendons. Under static conditions, longer muscle fibers generate greater forces in response to a standard stimulus up to a certain length, after which their strength shows a decrease (Figure 3.4). Note, however, that this decrease is usually observed at fiber length values that are outside the typical anatomical range. Sometimes muscle behavior is described in the analogy of a non-linear spring with a damping element. This is a questionable analogy, as some of the characteristics of muscle contractions (see, for example, the description of Fenn and Hill`s studies in Chapter 2) make them very different from those expected from moistened feathers. The simplest models of muscle contraction include a force-generating element (transverse bridges), a parallel elastic element, a dampening element parallel to the contractile element, and another elastic element in series with the first three that reflects the elastic properties of tendons and other elements connected in series (Figure 3.5). The muscular effect also depends on the external load. Regulated current stimulation provides more reproducible results than regulated voltage stimulation.
Regardless of the impedance of the electrode, a reproducible electric field can be generated in the stimulated tissue [41]. Therefore, in the present study, we used stimulation with a controlled current. Roszek et al. [29] stimulated the ischiadica nerve with silver bipolar electrodes and recorded the strength of the median gastrocnemius muscle in a rat. They used 200 ms trains with pulses of 0.1 ms of 3 mA current at different frequencies. The gradation of the stimulation frequency from 15 to 100 Hz led to a gradation of muscle strength. Frieswijk et al. [42] looked for the threshold current for a single pulse of 0.1 ms (monopolar stimulation of the peroneal nerve with a NiCr wire), which can produce a minimal muscle contraction response in the extender digitorum longus in rats.
They found that the threshold current can be as low as 0.0026 mA. When a muscle is stimulated by a single action potential, it contracts and then relaxes. The time between the stimulus and the onset of contraction is called the latency period, which is followed by the contraction period. At maximum contraction, the muscle relaxes and returns to its resting position. Taken together, these three periods are called contractions. Tetanus is made from an exotoxin produced by Clostridium tetani, a gram-positive anaerobic cancer that can infect soft tissue wounds. The toxin leads to neuronal hyperexcitability; This results in convulsions, autonomic instability and persistent “tetanus” muscle contractions that affect the jaw (“lockjaw”), neck, back and respiratory muscles. Treatment aims to (1) support ventilation, which may require intubation and administration of neuromuscular block agents; (2) neutralization of the toxin with intramuscular or intrathecal tetanus immunoglobulin 250 U (single dose); and (3) eradication of soft tissue infection with penicillin procaine 1.2 million U every 6 hours for 10 days. The results presented above make it possible to track structural changes in thick filaments and myosin head or motor domains with a temporal resolution of 5 ms during activation and relaxation of the intact EDL muscle of the mouse.
The functional significance of these structural changes is clearer for tetanus than for Twitch, where activation and relaxation are clearly separated by a period of stationary sarcomeric-isometric contraction in which thin filaments are activated to the maximum by calcium. Changes in the thick structure of the filament during tetanus can be correlated with changes in strength in five successive phases, which we call activation, tetanus plateau, isometric relaxation, chaotic relaxation and mechanically relaxed, the latter being different from the resting state reached a few minutes after a previous contraction. When analog protocols were used, the changes in thick filament-based X-ray reflections in these five phases are qualitatively similar to those previously described in rapidly contracting amphibian muscles, as described below. The structural mechanisms of regulation of thick filaments in skeletal muscles have been well preserved throughout the evolution of this species, making it possible to integrate the current results into the vast corpus of physiological, structural and mechanical studies on individual fibers isolated from the amphibian muscle, taking advantage of the greater homogeneity and inferior final conformity of this preparation. However, most published X-ray studies on amphibious muscle fibers were conducted before the importance of thick filament regulation was recognized; Thus, the synthesis leads to new perspectives on the underlying mechanisms. .