The myocytes are spindle-shaped muscle cells. Myosin is a protein that plays an important role in its anatomy and function. Antoni van Leeuwenhoek first described muscle cells in the 17th century. The entire musculature of the skeleton is made up of these basic cellular units. The muscle cells are also called muscle fibers. The smooth muscles of the organs do not consist of myocytes. The muscle cells consist of fused myoblasts and are thus multinucleated, which makes the concept of the muscle cell misleading.
A muscle cell actually contains several cells and cell nuclei. However, the individual cells of the cell networks are no longer differentiable as such in the muscle fiber, but form a widely branched syncitium. Different types of fibers are distinguished in the skeletal mucus and taken under the generic term of the myocytes. The most important fibers are the S-fibers and the F-fibers. S-fibers contract more slowly than F-fibers. Unlike the F-fibers, they only tire slowly and are designed for permanent contractions.
Foothills of the cell membrane invagine the muscle fiber in tubular folds and form a system of transverse tubules. Thus, action potentials at the cell membrane also reach the deeper cell layers of the muscle fiber. In the depths of the muscle fibers is a second cavity system of protuberances of the endoplasmic reticulum. In this system of longitudinal tubules, the calcium ions are stored. Laterally, the Ca2 + chambers encounter a folding of the tubule system, so that the individual membranes abut the folded cell membrane.
The receptors of these membranes can thus communicate directly with each other. Each muscle fiber joins with the associated nerve tissue to form a motor unit whose motor neuron lies on the motor end plate. In the cytoplasm of the fibers are mitochondria, which contain partially oxygen-storing pigments, glycogen and specialized enzymes for the energy metabolism of the muscles. There are also several hundred myofibrils in a muscle fiber. These myofibrils are a fan system that corresponds to the contractile units of the muscle. A connective tissue layer connects the muscle fibers to a tendon and can combine several muscles to form a box.
The myocytes play a role in energy metabolism as well as in general motor function. The motor function is ensured by the contractility of the myocytes. This ability to contraction holds the muscle fibers by the communication ability of its two proteins actin and myosin. By means of these two proteins, a skeletal muscle fiber can reduce its length in the sense of a concentric contraction. It can also maintain the length against resistance, which is known as isometric contraction. Lastly, she may respond to an extension with resistance. This principle is also known as eccentric contraction.
Contractility results from the ability of myosin to bind to the actin. The protein tropomyosin prevents binding in the resting state of the muscles. However, when an action potential arrives, calcium ions are released which keep the tropomyosin from blocking the binding sites. On the basis of filament gliding, this causes contraction. A single action potential only makes a skeletal muscle twitch. In order to achieve a strong or longer lasting shortening of the muscle fiber, action potentials arrive in rapid succession. The individual convulsions overlap so gradually and add up to contraction.
The muscle power is regulated in the fibers, inter alia, by different pulse frequencies of motor neurons. The energy metabolism of the muscles is relevant to the execution of the described muscle work. The energy supplier ATP is stored in all cells of the body. The energy supply runs either under consumption of oxygen or oxygen-free. When oxygen is consumed, the ATP breaks down and in the muscle, new ATP is produced with the help of creatine phosphate.
A faster form of energy supply is the oxygen-free form that takes place under the consumption of glucose. Since glucose is not completely decomposed within this framework, the energy yield of this process is only small. A glucose molecule produces two ATP molecules. When the same process takes place with the help of oxygen, whole 38 ATP molecules are formed from one sugar molecule. Fats can also be used as part of this.
Various diseases have an effect on the myocytes. Disorders of the energy metabolism can, for example, limit the motor function of the muscle fibers. In mitochondriopathies, for example, there is an ATP deficiency, which can trigger a multi-organ disease. Mitochondrial disorders can have different causes. For example, inflammation can damage the mitochondria. Mental and physical stress, malnutrition or toxic trauma can also jeopardize the provision of ATP. The result is a disturbed energy metabolism.
In addition to such disturbances of the energy metabolism, diseases of the nervous system can make the work of the myocytes more difficult. If damage to the central or peripheral nerve tissue, for example, disturbs the signal transmission, it can lead to paralysis. Certain muscles can only be moved atactically or not at all, because in the motor units signals are no longer arriving directly behind one another at a reduced line speed and thus can no longer be overlapped and summed up. Even muscle tremors can occur in the context of this phenomenon.
Muscle fibers can also be affected by illnesses themselves. Hereditary Naxos disease, for example, causes extensive loss of myocytes. A more well-known phenomenon is the torn muscle. This phenomenon manifests itself in a sudden and severe pain in the muscles. The affected muscles are only limited mobility and a swelling occurs. Also common are muscle fiber infections caused by infections or immune disorders. It should be distinguished from muscle hardening, which usually after long-term stress due to a change in muscle metabolism adjust, but in rare cases may also be related to muscle inflammation.Tags: