Which of the following statements about muscle contraction is true? 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 to actin molecules, prevents the formation of transverse bridges, and prevents contraction in a muscle without nerve input. Troponin binds to tropomyosin and helps position it on the actin molecule; It also binds calcium ions. The muscle contraction cycle is triggered by calcium ions that bind to the troponin protein complex and expose the active binding sites on actin. Once the actin binding sites are exposed, the high-energy myosin head closes the gap and forms a transverse bridge. Once the myosin binds to the actin, the pi is released and the myosin undergoes a conformational change at a lower energy state. When myosin consumes energy, it moves through the “force snap” and pulls the actin filament towards the M line. When the actin is pulled about 10 nm in the direction of the M line, the sarcomere shortens and the muscle contracts. At the end of the strength race, the myosin is in a low-energy position.
The second work by Hugh Huxley and Jean Hanson is entitled “Changes in the crossed streaks of muscle during contraction and stretching and their structural interpretation”. It is more complex and based on their study of rabbit muscle with phase contrast and electron microscopes. According to them: Exercise often requires the contraction of skeletal muscle, as can be seen when the biceps muscle of the arm contracts and pulls the forearm towards the trunk. The sliding filament model describes the process that muscles use to contract. It is a cycle of repetitive events that causes actin and myosin myofilaments to slide over each other, contracting the sarcoma and creating tension in the muscle. The concentration of calcium in muscle cells is controlled by the sarcoplasmic reticulum, a unique form of the endoplasmic reticulum in the sarcoplasm. Muscle contraction ends when calcium ions are pumped into the sarcoplasmic reticulum, allowing the muscle cell to relax. When stimulating the muscle cell, the motor neuron releases the neurotransmitter acetylcholine, which then binds to a postynaptic nicotinic acetylcholine receptor. When the myosin head is “tense”, it contains energy and is in a high-energy configuration. This energy is consumed when the myosin head moves through the power stroke; At the end of the coup, the myosin head is in a low-energy position.
After the power stroke, ADP is released. However, the formed transverse bridge is still present, and actin and myosin are connected to each other. ATP can then bind to myosin, allowing the transverse bridge cycle to start again and additional muscle contraction (Figure 1). The movement of the myosin head to its original position is called a recovery stroke. Resting muscles store ATP energy in myosin heads while waiting for another contraction. To enable muscle contraction, tropomyosin must alter conformation, expose the myosin binding site to an actin molecule, and allow the formation of transverse bridges. This can only occur in the presence of calcium, which is maintained in the sarcoplasm at extremely low concentrations. 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. Transverse cycling will continue until Ca2+ ions and ATP are no longer available and tropomyosin again covers actin binding sites. Excitation-contraction coupling is the link between the electrical action potential and mechanical muscle contraction. • Calcium ions and the proteins tropomyosin and troponin control muscle contractions stimulus-contraction coupling is the physiological process of converting an electrical stimulus into a mechanical response.
This is the link (transduction) between the action potential generated in the sarcolemma and the appearance of muscle contraction. The sliding wire theory explains the mechanism of muscle contraction based on muscle proteins that slide on top of each other to create movement.  According to the sliding filament theory, the myosin filaments (thick) of the muscle fibers slide beyond the (thin) actin filaments during muscle contraction, while both groups of filaments remain relatively long. Myocytes can be incredibly large, with diameters of up to 100 microns and lengths of up to 30 centimeters. Sarcoplasm is rich in glycogen and myoglobin, which store the glucose and oxygen needed to produce energy, and is almost entirely filled with myofibrils, the long fibers made up of myofilaments that facilitate muscle contraction. ACh is broken down into acetyl and choline by the enzyme acetylcholinesterase (AChE). AChE is located in the synaptic cleft and breaks down ACh so that it does not remain bound to ACh receptors, which would lead to prolonged unwanted muscle contraction. The process of muscle contraction occurs through a number of key stages, including: as with skeletal muscle, the heart muscle is scratched; However, it is not consciously controlled and is therefore classified as involuntary. Heart muscle can be distinguished from skeletal muscle by the presence of intercalated discs that control the synchronized contraction of heart tissue. Cardiac myocytes are shorter than skeletal equivalents and contain only one or two nuclei located in the center.
Myofibrils are made up of smaller structures called myofilaments. There are two main types of myofilaments: thick filaments and thin filaments. Thick filaments are mainly made up of myosin proteins, the tails of which are connected to each other, so the heads are exposed to the thin intertwined filaments. Thin filaments consist of actin, tropomyosin and troponin. The molecular model of contraction, which describes the interaction between actin and myosin myofilaments, is called the cross-bridge cycle. With substantial evidence, Hugh Huxley formally proposed the filament sliding mechanism and is variously referred to as the vibrating transverse bridge model, the cross-bridge theory or the cross-bridge model.   (He himself preferred the name “vibrant Crossbridge model” because, as he recalled, “it was [the discovery] after all the 1960s.” 2]) He published his theory in the June 20, 1969 issue of Science under the title “The Mechanism of Muscle Contraction.”  According to his theory, the filament is slipped by cyclic fixation and detachment of myosin to actin filaments. Contraction occurs when myosin pulls the actin filament towards the center of the A-band, detaches from the actin, and creates a force (stroke) to bind to the next actin molecule.  This idea was later proven in detail and is better called the transverse bridge cycle.  Figure 1. It shows the muscular contraction cycle of the transverse bridge triggered by the binding of Ca2+ to the active center of actin.
With each cycle of contraction, actin moves relative to myosin. ATP is crucial for muscle contractions because it breaks the myosin-actin transverse bridge and releases myosin for the next contraction. The first muscle protein discovered was myosin by german scientist Willy Kühne, who extracted it and named it in 1864.  In 1939, a Russian couple Vladimir Alexandrovich Engelhardt and Militsa Nikolaevna Lyubimova discovered that myosin has an enzymatic property (called ATPase) that ATP can break down to release energy.  Albert Szent-Györgyi, a Hungarian physiologist, turned to muscle physiology after receiving the Nobel Prize in Physiology or Medicine in 1937 for his work on vitamin C and fumaric acid. He showed in 1942 that ATP was the energy source for muscle contraction. He actually observed that muscle fibers containing myosin B were shortened in the presence of ATP, but not with myosin A, the experience he later described as “perhaps the most exciting time of my life.”  In Brunó Ferenc Straub, he quickly discovered that myosin B was associated with another protein they called actin, while myosin A was not. Straub cleaned actin in 1942 and Szent-Györgyi cleaned myosin A in 1943. It turned out that myosin B was a combination of myosin A and actin, so myosin A retained the original name while renaming myosin B to actomyosin. In the late 1940s, Szent-Györgyi`s team applied with evidence that actomyosin contraction corresponds to muscle contraction as a whole.
 But the idea was generally rejected, even by Nobel laureates such as Otto Fritz Meyerhof and Archibald Hill, who adhered to the prevailing dogma that myosin was a structural protein and not a functional enzyme.  However, in one of his recent contributions to muscle research, Szent-Györgyi showed that ATP-powered actomyosin is the basic principle of muscle contraction.  Skeletal muscle connects to the skeletal system primarily through tendons to maintain posture and control movement….