What Makes Skeletal Muscles Contract
The mechanism of muscle contraction has eluded scientists for years and requires continuous research and updating.  The sliding wire theory was developed independently by Andrew F. Huxley and Rolf Niedergerke, as well as Hugh Huxley and Jean Hanson. Their results were published as two consecutive papers published in the May 22, 1954 issue of Nature under the common theme “Structural Changes in Muscles During Contraction.”   Cytoplasmic calcium binds to troponin C and displaces the tropomyosin complex from the actin binding site so that the myosin head can bind to the actin filament. From this point on, the contractile mechanism is essentially the same as in skeletal muscle (above). In short, using ATP hydrolysis, the myosin head pulls the actin filament towards the center of the sarcomere. Although smooth muscle contractions are myogenic, the speed and strength of their contractions can be modulated by the autonomic nervous system. Postnodal nerve fibers in the parasympathetic nervous system release the neurotransmitter acetylcholine, which binds to muscarinic acetylcholine receptors (mAChR) on smooth muscle cells. These receptors are metabotropic or G protein-coupled receptors that initiate a second cascade of messengers. Conversely, the postnodal nerve fibers of the sympathetic nervous system release the neurotransmitters epinephrine and norepinephrine, which bind to adrenergic receptors that are also metabotropic. The exact effects on smooth muscle depend on the specific characteristics of the activated receptor – parasympathetic input and sympathetic entry can be excitatory (contractile) or inhibitory (relaxing).
When a muscle contracts, actin is pulled along the myosin to the center of the sarcoma until the actin and myosin filaments completely overlap. In other words, for a muscle cell to contract, the sarcoma must shorten. However, thick and thin filaments – the components of sarcomeres – do not shorten. Instead, they slide in front of each other, shortening the sarcomor while keeping the filaments of the same length. The sliding thread theory of muscle contraction was developed to adjust the differences observed in the ligaments mentioned on the sarcoma with different degrees of muscle contraction and relaxation. The contraction mechanism is the binding of myosin to actin and forms transverse bridges that produce filament movements (Figure 5). Botulinum toxin is a remedy that alters neuromuscular function. This toxin, which is produced by C. botulinum prevents the release of ACh from the presynaptic membrane of the motor neuron. Therefore, skeletal muscles cannot contract, which leads to flaccid paralysis.
 Heart muscle tissue is only found in the heart, and heart contractions pump blood through the body and maintain blood pressure. Like skeletal muscle, heart muscle is scratched, but unlike skeletal muscle, heart muscle cannot be consciously controlled and is called an involuntary muscle. It has one nucleus per cell, is branched and is characterized by the presence of intercalated discs. When actin binding sites are exposed, a transverse bridge is formed; That is, the myosin head extends over the distance between the actin and myosin molecules. Pi is then released so that the myosin can consume the stored energy as a conformational change. The myosin head moves in the direction of the M line and pulls the actin with it. When the actin is fired, the filaments move about 10 nm in the direction of the M line. This movement is called a force stroke because it is the step in which the force is generated. When the actin is pulled in the direction of the M line, the sarcomere shortens and the muscle contracts. Malignant hyperthermia is a life-threatening disease that occurs mainly in people with a genetic predisposition with a mutation in the ryanodine receptor of the sarcoplasmic reticulum.
When these people are exposed to volatile anesthetics or the muscle relaxant succinylcholine, there is a massive release of intracellular Ca2+ from ryanodine receptors and insufficient sequestration of Ca2+ by the SERCA pump. This mechanism leads to muscle contraction, rhabdomyolysis, severe hyperthermia and eventually death. The only treatment for malignant hyperthermia is dantrolene, which binds to the ryanodine receptor to prevent the release of Ca2+.  DMD is an inherited disease caused by an abnormal X chromosome. It mainly affects men and is usually diagnosed in early childhood. DMD usually occurs first as a difficulty with balance and movement, and then develops into an inability to walk. It progresses higher in the body from the lower limbs to the upper body, where it affects the muscles responsible for breathing and circulation. It eventually causes death due to respiratory failure, and sufferers usually do not live beyond the age of 20. • The contraction of skeletal muscles is achieved by sliding actin and myosin filaments As already mentioned, the thick and thin filaments of myofibrils are arranged in units called sarcomeres. The sarcoma is the fundamental contractile unit of myofibril.
Z lines separate each sarcomere. The A stripes, which are located in the middle of each sarcomere, contain the thick filaments that can overlap with thin filaments. The A band divides further into the H zone, which does not contain thin filaments. The distinctive M line divides the H zone and is used to connect the central parts of the thick filaments. On both sides of the A strip are the I bands, which contain both the thin filaments and the Z line running in the middle of each I band. The essence of the sliding filament model of muscle contraction is the action of actin and myosin sliding on top of each other. When this happens, the sarcoma shortens and the muscle contracts. The process begins when a command or pulse is sent through a neuron connected to a muscle called a motor neuron. Skeletal muscle tissue forms skeletal muscles that attach to bone or skin and control locomotion and any movement that can be consciously controlled. Since it can be controlled by thoughts, skeletal muscle is also known as arbitrary muscle.
Skeletal muscles are long and cylindrical in appearance; Seen under a microscope, skeletal muscle tissue looks scratched or scratched. Strips are caused by the regular arrangement of contractile proteins (actin and myosin). Actin is a globular contractile protein that interacts with myosin for muscle contraction. Skeletal muscle also has several nuclei present in a single cell. In skeletal muscle, this sequence begins with signals from the somatic motor department of the nervous system. In other words, the “no excitation” in skeletal muscle is always triggered by signals from the nervous system. Once the muscle fiber is stimulated by the motor neuron, actin, and myosin protein filaments in the skeletal muscle fiber, they slide over each other and create a contraction. The sliding wire theory is the most widely used explanation for how this happens.
According to this theory, muscle contraction is a cycle of molecular events in which thick myosin filaments repeatedly attach to thin actin filaments and pull on them so that they slide on top of each other. The actin filaments are attached to Z discs, each of which marks the end of a sarcomere. The sliding of the filaments brings the Z discs closer to a sarcomere, thus shortening the sarcoma. When this happens, the muscle contracts. Figure 7. This diagram shows the excitation-contraction coupling in a skeletal muscle contraction. The sarcoplasmic reticulum is a specialized endoplasmic reticulum found in muscle cells. In general, epimysium, perimisium and endomysium extend beyond the fleshy part of the muscle, abdomen or gastric a, forming a thick rope-shaped tendon or a wide, flat leaf-shaped aponeurosis. The tendon and fascia form indirect attachments of the muscles to the periosteum of the bones or to the connective tissue of other muscles. As a rule, a muscle extends over a joint and is attached to the bone at both ends with tendons. One of the bones remains relatively firm or stable, while the other end moves as a result of muscle contraction. The area where the thick and thin filaments overlap has a dense appearance because there is little space between the filaments.
This area, where thin and thick filaments overlap, is very important for muscle contraction because it is where the movement of the filament begins. Thin filaments anchored at their ends through the Z disks do not extend completely into the central zone, which contains only thick filaments anchored to their bases in a place called the M line. A myofibril consists of many sarcomeres that run along its length; Thus, myofibrils and muscle cells contract when sarcomeres contract. When a sarcomere shortens, some regions shorten while others remain the same length. A sarcomere is defined as the distance between two successive Z disks or Z lines; When a muscle contracts, the distance between the intervertebral Z discs is reduced. Zone H – the central region of zone A – contains only thick filaments (myosin) and is shortened during contraction. The H-zone is getting smaller and smaller due to the increasing overlap of actin and myosin filaments and muscle shortening. Thus, when the muscle is completely contracted, the H-zone is no longer visible. The I strip contains only thin filaments and is also shortened. The A band does not shorten – it remains the same length – but the A bands of different sarcomeres get closer during contraction and eventually disappear. .