Titin molecules connect the Z line to the M line and form a scaffolding for myosin myofilaments. Their elasticity forms the basis of muscle contraction. Titin molecules are thought to play a key role as a molecular rule that maintains parallel alignment in the sarcomere. Another protein, haze, is thought to play a similar role in actin myofilaments. To allow muscle contraction, tropomyosin must modify the conformation and expose the myosin binding site to an actin molecule, allowing the formation of a transverse bridge. Troponin, which regulates tropomyosin, is activated by calcium, which is maintained at extremely low concentrations in the sarcoplasm. When present, calcium ions bind to troponin, resulting in conformational changes in troponin that allow tropomyosin to move away from myosin binding sites on actin. Once tropomyosin is eliminated, a transverse bridge can form between actin and myosin, triggering contraction. Cycling on deck will continue until Ca2+ ions and ATP are no longer available. Tropomyosin again covers actin binding sites. The binding of the myosin head to muscle actin is a highly regulated process. When a muscle is at rest, actin and myosin are separated. To prevent actin from binding to the active center of myosin, regulatory proteins block molecular binding sites.
Tropomyosin blocks myosin binding sites on actin molecules and prevents the formation of transverse bridges, which prevents contraction in a muscle without nerve input. The troponin protein complex binds to tropomyosin and helps position it on the actin molecule. Muscle contraction: Calcium remains in the sarcoplasmic reticulum until it is released by a stimulus. The calcium then binds to the troponin, which causes a change in the shape of the troponin and removes the tropomyosin from the binding sites. The adhesion of the transverse bridge continues until calcium ions and ATP are no longer available. Muscles can contract for different durations and at different speeds and forces. Such regulation does not take place at the level of each individual myofiber, because each fiber is contracted or relaxed. Instead, this regulation takes place at the level of motor neurons connected to myofibers. Motor neurons are connected to a myofiber by the neuromuscular connection, also known as the neuromuscular synapse. This synapse functions as a neuron-neuron synapse, in which the action potential of an axon is transferred to a downstream neurite.
At the neuromuscular junction, the myofiber is located downstream of the axon, and the action potential of the axon leads to the depolarization of the sarcolemma, which moves rapidly along the sarcolemma and tubules T, resulting in a contraction of the myofiber. Each motor neuron is connected to one or more myofibers and forms a motor unit (English and Wolf, 1982). However, each myofiber has only one neuromuscular connection, which is connected to a single motor neuron. Therefore, muscle length and speed are regulated by the number of activated motor units. The more the motor units are activated, the higher and faster the contraction. On the other hand, the rate of stimulation by the motor neuron controls the force generated by each myofiber in a motor unit. Dihydropyridine dervivatives, e.B. Nifedipine, amlodipine: directly relaxes smooth vascular muscles and therefore have pronounced peripheral vasodilatory properties. Drugs of this class can produce transient reflex tachycardia; The other two classes do not.
The amount of force and movement generated by a single sarcomere is small. However, multiplied by the number of sarcomeres in a myofibril, myofibrils in a myocyte and myocytes in a muscle, the amount of force and movement generated is significant. Lorand, L. “Adenosine triphosphate creatine transphosphorylase” as a relaxing muscle factor. Nature 172, 1181–1183 (1953) doi:10.1038/1721181a0. Is muscle contraction fully understood? Scientists are always curious about several proteins that clearly affect muscle contraction, and these proteins are interesting because they are well preserved in animal species. For example, molecules like titin, an unusually long, “elastic” protein that covers sarcomeres in vertebrates, appear to bind to actin, but this is not well understood. In addition, scientists have made many observations of muscle cells that behave in a way that does not match our current understanding of them.
For example, certain muscles in molluscs and arthropods produce strength over long periods of time, a little-understood phenomenon sometimes referred to as “capture tension” or force hysteresis (Hoyle 1969). Studying these and other examples of muscle changes (plasticity) is an exciting path for biologists. Ultimately, this research can help us better understand and treat neuromuscular systems and better understand the diversity of this mechanism in our natural world. Muscle contractions consume energy provided by carbohydrates, fats and rarely proteins. High-intensity exercise requires large amounts of blood sugar and muscle glycogen to produce ATP, while low-intensity exercise consumes free fatty acids. Levels of fatty acids increase in the blood during rest after high-intensity exercise, which requires carbohydrates. Thus, the consumption of fatty acids during recovery is increased. This example implies that the intensity and duration of exercise and recovery intervals can affect metabolism in complex ways and choosing the right diet can be difficult. Nutrition is of particular importance in the preparation of a competition or regeneration and influences athletic performance. This chapter focuses on designing diets to maximize performance.
A peculiarity of the sarcolemma is that it penetrates the sarcoplasm of the muscle cell and forms membranous tubules radially and longitudinally in the fiber, called T tubules or transverse tubules. On both sides of the transverse tubules are terminal cisternal enlargements of the sarcoplasmic reticulum (called endoplasmic reticulum in non-muscle cells). A transverse tubule surrounded by two SR tanks is called a triad, and contact between these structures occurs at the intersection of bands A and I. At the level of the sliding filament model, expansion and contraction occur only in the I and H bands. The myofilaments themselves do not contract or expand and the A band therefore remains constant. In 1954, scientists published two groundbreaking papers describing the molecular basis of muscle contraction. This work described the position of myosin and actin filaments at different stages of contraction in muscle fibers and suggested how this interaction produced contractile strength. Using high-resolution microscopy, A. F. Huxley and R. Niedergerke (1954) and H.
E. Huxley and J. Hanson (1954) observed changes in sarcomas as muscle tissue shortened. They observed that an area of repeated arrangement of the sarcoma, the “A-band”, remained relatively constant during contraction (Figure 2A). The A-band contains thick myosin filaments, suggesting that the myosin filaments remained central and constant in length, while other regions of the sarcomaer were shortened. The researchers found that the “I-band,” rich in thinner actin filaments, changed length with the sarcomere. These observations led her to propose the sliding wire theory, which states that the sliding of actin beyond myosin creates muscle tension. Because actin is bound to structures at the lateral ends of each sarcoma called Z discs, or “Z bands,” any shortening of the length of the actin filament would result in a shortening of the sarcoma, and therefore of the muscle. This theory has remained incredibly intact (Figure 2B). Muscle contractions caused by transcutaneous electrical stimulation can prevent joint fixation and prepare muscles before voluntary recruitment. However, the high electrical resistance of the skin limited the application of stimulation, as the high charge levels of the electrical impulses needed to induce muscle contractions are painful and not tolerated. There is no systematic evidence of improvement in regeneration or functional recovery.
In animals, repetitive and effective daily electrical stimulation of denervated muscles reduces and reverses denervation atrophy in the hind legs (Al Amood, Lewis and Schmalbruch, 1991; Schmalbruch, Al-Amood and Lewis, 1991). The same reversal of denervation atrophy was achieved with daily electrical stimulation of the denervated quadriceps muscles in patients with equine tail lesions. In these patients, the loss of skin sensation allowed effective muscle stimulation via large electrodes placed on the motor point (Kern et al., 2010). Figure 12. Sequence of events leading to muscle contraction. Skeletal muscle connects to the skeletal system primarily through tendons to maintain posture and control movement. .
