Myocytes are multinucleated muscle cells. They make up the skeletal muscles. In addition to contraction, energy metabolism also falls within their range of functions.
What are myocytes?
Myocytes are spindle-shaped muscle cells. Myosin is a protein that plays an important role in their 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 are not composed of myocytes. Muscle cells are composed of fused myoblasts and are thus multinucleated, which makes the term muscle cell misleading. Thus, a muscle cell actually contains multiple cells and nuclei. However, the individual cells of the cell composites 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 mucosa and are grouped under the generic term myocytes. The most important fibers are the S-fibers and the F-fibers. S-fibers contract more slowly than F-fibers. Unlike F-fibers, they fatigue slowly and are designed for continuous contractions.
Anatomy and structure
Extensions of the cell membrane invert into tubular folds at the muscle fiber, forming a system of transverse tubules. Thus, action potentials at the cell membrane reach the deeper cell layers of the muscle fiber. In the depths of the muscle fibers lies a second cavity system of endoplasmic reticulum protrusions. Calcium ions are stored in this system of longitudinal tubules. 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 its associated neural tissue to form a motor unit, whose motoneuron is located at the motor end plate. Mitochondria are located in the cytoplasm of the fibers, some of which contain oxygen-storing pigments, glycogen, and specialized enzymes for muscle energy metabolism. In addition, several hundred myofibrils are located 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 may combine several muscles into a lodge.
Function and tasks
Myocytes play a role in energy metabolism as well as general motor function. Motor function is provided by the myocytes’ ability to contract. Muscle fibers hold this ability to contract through the communication ability of their two proteins, actin and myosin. Through these two proteins, a skeletal muscle fiber can reduce its length in terms of concentric contraction. However, it can also maintain length against resistance, which is known as isometric contraction. Last, it can respond to lengthening with resistance. This principle is also known as eccentric contraction. The contractility results from the binding ability of myosin to actin. The protein tropomyosin prevents binding when muscles are at rest. However, when an action potential arrives, calcium ions are released to prevent tropomyosin from blocking the binding sites. Contraction is thus initiated on the basis of filament sliding. A single action potential only causes a skeletal muscle to twitch. To cause a strong or prolonged shortening of the muscle fiber, action potentials arrive in rapid succession. The individual twitches thus gradually overlap and add up to contraction. Muscle force is regulated in the fibers, among other things, by different pulse frequencies of the motoneurons. The energy metabolism of the muscles is relevant for the execution of the described muscle work. The energy supplier ATP is stored in all cells of the body. Energy supply proceeds either with consumption of oxygen or without oxygen. When oxygen is consumed, the ATP decays and new ATP is produced in the muscle with the help of creatine phosphates. A faster form of energy supply is the oxygenless form, which takes place under the consumption of glucose. However, since glucose is not completely decomposed in this process, the energy yield of this process is low.Two ATP molecules are formed from one glucose molecule. If the same process takes place with the aid of oxygen, a full 38 ATP molecules are created from one sugar molecule. Fats can also be utilized as part of this process.
Diseases
Several diseases have an effect on myocytes. Diseases of energy metabolism, for example, can limit the motor function of muscle fibers. In mitochondriopathies, for example, ATP deficiency is present, which can cause multiorgan disease. Mitochondriopathies can have various causes. For example, inflammation can cause mitochondria to become damaged. However, mental and physical stress, malnutrition or toxic trauma can also compromise the provision of ATP. The result is a disturbed energy metabolism. In addition to such disturbances in energy metabolism, diseases of the nervous system can also make it difficult for myocytes to work. If, for example, signal transmission is disturbed by damage to the central or peripheral nervous tissue, this can result in paralysis. Certain muscles can only be moved ataxically or not at all, because signals no longer arrive in the motor units in immediate succession only at reduced conduction velocity and thus can no longer overlap and add up. Muscle tremor can also occur as part of this phenomenon. Muscle fibers can also be affected by disease themselves. In hereditary Naxos disease, for example, there is extensive loss of myocytes. A more familiar phenomenon is muscle fiber rupture. This phenomenon manifests itself in a sudden and severe pain in the muscles. The affected muscles have limited mobility and swelling occurs. Muscle fiber inflammations caused by infections or immune disorders are just as common. To be distinguished from this are muscle stiffnesses, which usually develop after continuous strain due to an altered muscle metabolism, but in rare cases can also be related to muscle inflammation.
