Acetylcholine (ACh) is a neurotransmitter released by motor neurons that binds to receptors in the motor end plate. The release of neurotransmitters occurs when an action potential moves through the motor neuron axon, resulting in impaired permeability of the synaptic terminal membrane and an influx of calcium. Ca2+ ions allow synaptic vesicles to move and bind to the presynaptic membrane (on the neuron) and release neurotransmitters from the vesicles into the synaptic cleft. Once released from the synaptic terminal, ACh diffuses through the synaptic cleft to the engine end plate, where it binds to the ACh receptors. When a neurotransmitter binds, these ion channels open and Na+ ions cross the membrane into the muscle cell. This reduces the voltage difference between the inside and outside of the cell, which is called depolarization. Since the ACh binds to the engine end plate, this depolarization is called end plate potential. Depolarization then propagates along the sarcolemma, creating an action potential when the sodium channels detect and open the voltage change next to the initial depolarization site. The action potential moves over the entire cell and creates a wave of depolarization.
The cross-bridge cycle is a sequence of molecular events that underlies the slippery filament theory. A transverse bridge is a projection of myosin consisting of two myosin heads that extends from thick filaments.  Each myosin head has two binding sites: one for ATP and one for actin. Binding ATP to a myosin head dissolves actin myosin, allowing myosin to bind to another actin molecule. Once attached, ATP is hydrolyzed by myosin, which uses the released energy to move into the “tense position,” weakly binding to part of the actin binding site. The rest of the actin binding site is blocked by tropomyosin.  In hydrolyzed ATP, the tense myosin head now contains ADP + Pi. Two Ca2+ ions bind to troponin C in the actin filaments.
The troponin-Ca2+ complex slides tropomyosin over the rest of the actin binding site and releases it. By unlocking the remaining actin binding sites, the two myosin heads can close and strongly bind myosin to actin.  The myosin head then releases the inorganic phosphate and triggers a force stroke that produces a force of 2 pN. The force stroke moves the actin filament inward, shortening the sarcomere. The myosin then releases the ADP, but remains firmly attached to the actin. At the end of the power stroke, ADP is released from the myosin head, so the myosin remains attached to the actin in a strict state until another ATP binds to the myosin. A lack of ATP would lead to the state of rigor characteristic of Rigor Mortis. As soon as another ATP binds to the myosin, the myosin head detaches again from the actin and another transverse bridge cycle occurs. The end of the transverse bridge cycle (and leaving the muscle in the latch state) occurs when myosin`s light-chain phosphatase removes phosphate groups from myosin heads. Phosphorylation of 20 kDa myosin light chains is well correlated with the speed of shortening of smooth muscles.
Meanwhile, there is a rapid increase in energy consumption, as measured by oxygen consumption. A few minutes after the start of induction, calcium levels decrease significantly, phosphorylation of light chains 20 kDa-myosin decreases, and energy use decreases; However, the strength of the tonic smooth muscles is preserved. During muscle contraction, rapidly circular transverse bridges form between activated actin and phosphorylated myosin, which generate strength. Power maintenance is thought to result from dephosphorylated “locking bridges” that slowly become cyclical and maintain power. A number of kinases such as rho kinase, DAPK3 and protein kinase C are thought to participate in the prolonged phase of contraction, and the flow of Ca2+ may be significant. The most widely used theory that explains how muscle fibers contract is called the sliding filament theory. According to this theory, myosin filaments use the energy of ATP to “walk” along actin filaments with their transverse bridges. As a result, the actin filaments are brought closer together. The movement of the actin filaments also brings the Z lines closer together, shortening the sarcomere.
In the type of thyrotoxic hypertrophy, calcium is eliminated more quickly, while there is a change in myosin. At the molecular level, there are more sarcoplasmic reticular calcium pumps, while the transverse bridgehead of myosin changes faster and remains bound to the energy-generating state for a shorter period of time. The result is a heart that contracts much faster, but economically less than normal, and can meet the peripheral need for large amounts of blood at normal pressure. When actin binding sites are exposed, a transverse bridge is formed; That is, the myosin head covers 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 pulled, the filaments move about 10 nm in the direction of the M line. This movement is called a force stroke because it is the stage where the force is generated. When the actin is pulled towards the M line, the sarcoma shortens and the muscle contracts. There are two types of heart muscle cells: autorythmic and contractile. Autorythmic cells do not contract, but determine the rate of contraction of other heart muscle cells that can be modulated by the autonomic nervous system. In contrast, contractile muscle cells (cardiomyocytes) make up the majority of heart muscle and can contract.
With eccentric contraction, isometric tension is not sufficient to overcome the external load on the muscle, and muscle fibers lengthen as they contract.  Instead of working to pull a joint towards muscle contraction, the muscle acts to slow down the joint at the end of a movement or otherwise control the repositioning of a load. This may be unintentional (for example. B, when you try to move a weight that is too heavy for the muscle to lift) or voluntarily (for example. B when the muscle “smoothes” a movement or resists gravity, by. B example when you walk downhill). In the short term, strength training, which involves both eccentric and concentric contractions, seems to increase muscle strength more than training with concentric contractions alone.  However, exercise-induced muscle damage is also greater during prolonged contractions.  The contractile activity of smooth muscle cells can be tonic (persistent) or phasic (transient) and is influenced by multiple inputs such as spontaneous electrical activity, neuronal and hormonal inputs, local changes in chemical composition, and stretching.  This contrasts with the contractile activity of skeletal muscle cells, which is based on a single neuronal input. Some types of smooth muscle cells are able to spontaneously generate their own action potentials, which usually occur after pacemaker potential or slow wave potential. These action potentials are generated by the influx of extracellular Ca2+ and not Na+.
Like skeletal muscle, cytosolic Ca2+ ions are needed for the Crossbridge cycle in smooth muscle cells. In concentric contraction, muscle tension is sufficient to overcome the load, and the muscle shortens as it contracts.  This happens when the force generated by the muscle exceeds the load that counteracts its contraction. In 1952, the term excitation-contraction coupling was coined to describe the physiological process of converting an electrical stimulus into a mechanical reaction.  This process is fundamental to muscle physiology, with the electrical stimulus usually being an action potential and the mechanical response being a contraction. Excitation-contraction coupling can be deregulated in many diseases. Although excitation-contraction coupling has been known for more than half a century, it is still an active area of biomedical research. The general pattern is that an action potential arrives to depolarize the cell membrane.
Thanks to mechanisms specific to the muscle type, this depolarization leads to an increase in cytosolic calcium, called transient calcium. This increase in calcium activates calcium-sensitive contractile proteins, which then use ATP to cause cell shortening. The movement of muscle shortening occurs when the myosin heads bind to the actin and pull the actin inward. This action requires energy provided by ATP. Myosin binds to actin at a binding site on the globose actin protein. Myosin has another ATP binding site where enzyme activity hydrolyzes ATP to ADP, releasing an inorganic phosphate molecule and energy. Each muscle fiber contains hundreds of organelles called myofibrils. Each myofibril consists of two types of protein filaments: actin filaments, which are thinner, and myosin filaments, which are thicker.
Actin filaments are anchored in structures called Z-lines (Figure 13.13.2). The area between two Z lines is called sarcomeres. In a sarcoma, the myosin filaments overlap the actin filaments. Myosin filaments have tiny structures called transverse bridges that can attach to actin filaments. Remember that the filaments of actin and myosin themselves do not change in length, but slide in front of each other. With the exception of reflexes, all contractions of skeletal muscle occur as a result of conscious exertion that comes from the brain. .