Muscle fibrils are muscle fiber elements consisting predominantly of the proteins actin and myosin. These two proteins are the contractile elements of the muscles that work together to create muscle movement. In nemaline myopathy, the muscle fibrils change into a spindle-shaped shape and cause muscle weakness.

What is a muscle fiber?

Muscle fibers or muscle fiber cells are myocytes and correspond to the spindle-shaped cell basic units of striated skeletal muscle. Smooth muscle does not consist of muscle fibers.

In the muscle fibers of the skeletal muscle, the muscle fibrils form a functional unit at the level of the cell organelles. The elements are also referred to as myofibrils and allow by filament gliding the contraction of skeletal muscle. Each skeletal muscle of the striated muscle contains many individual fibrils that form a composite with each other. The protein myosin is a major component of muscle fibrils. Ontologically, the functional unit muscle fibrils is subordinated to the organelle.

Subordinate components include sarcomere and costamer. Individual muscle fibrils are always bound in the musculature by the intermediate filament desmin to fibril bundles, and are spun by a variant of the endoplasmic reticulum, which is referred to as a sarcoplasmic reticulum.

Anatomy & Construction

Muscle fibrils contain cascaded assemblies of similar internal structure, also known as sarcomeres. Under the light microscope, fibrils show typical transverse striations, which give the striated muscles their name.

The apparent transverse stripes are caused by the regular distribution of individual muscle filaments. The typical band pattern arises. The individual sarcomeres of the muscle fibrils consist of protein fibers, which are arranged parallel to each other. These thick protein fibers correspond to the so-called myosin, which is better known as muscle protein. Between the individual filaments of myosin are each thinner filaments of actin.

The complex of actin and myosin is kept stable by the largest protein in the human organism: the so-called titin. At regular intervals are solid discs that hang on thin threads of titanium. The individual discs are separated from myosin molecules and overlap proportionally with the Aktinfäden.

Function & Tasks

Myosin and actin are collectively referred to as contractile elements of the muscle. The muscle fibrils consist of these contractile elements. Thus, the main task of the fibrils is the contraction of the skeletal muscles. Muscle contractions always require an interaction of nerve tissue and muscle tissue.

Only the neuromuscular unit of motor nerve and associated skeletal muscle causes the muscle to respond to commands from the central nervous system. Although the contractile muscle elements actin and myosin contribute to muscle motor function and therefore to the ability to move the human body, motor skills are far from being the only ones to do it. Each skeletal muscle contraction is preceded by a release of calcium ions. These ions come from the sarcoplasmic reticulum and are released whenever contraction commands from the central nervous system reach the muscle via efferent nerves. The commands arrive in the form of bioelectric excitation to the muscle and are transmitted to the motor end plate.

Once this happens, the said calcium ions are released and then diffuse between myosin and actin filaments. After diffusion, the calcium ions between the filaments trigger interactions. It comes to the so-called filament gliding. Due to the interactions between the myosin and actin filaments, the actin filaments slide between individual myosin filaments. This phenomenon is also referred to as a sliding filament mechanism and underlies any muscle contraction of striated muscle.

Due to the filament sliding, the sarcomere is shortened. This shortens all muscle fibers of the muscle fiber, causing a twitch of the fiber. The eventual contraction differs from this muscle twitch in that it requires the shortening of multiple muscle fibers.

Diseases

Myopathies are muscular disorders that are not based on a neuronal cause. Skeletal muscle can suffer from different forms of myopathy. One of these is nemaline myopathy.

Nemaline myopathy is an extremely rare disease that corresponds to congenital myopathy. Muscular fibrils show rod-shaped changes called nemaline in disease. The term nemaline myopathy can refer to genetically different disorders. Each variant of the disease is preceded by a genetic mutation, which is passed on partly in autosomal dominant and partly in autosomal recessive inheritance. In patho-histological analysis of muscle biopsy, all forms of the disease are manifested by rod or filamentous structures in the myocytes. The clinical picture of the disease is extremely variable.

The course of the disease varies between moderate symptoms and severe limitations. In most cases, the myopathy is associated with a more or less pronounced weakness of the muscles. Severe courses of nemaline myopathy are associated with the loss of spontaneous movements or respiratory movements, which can begin at birth and can lead to death within a few months at such an early onset. In moderately severe gradients, the muscle weakness progresses only slowly, stagnates or even decreases over time.

Characteristically, diminished muscle tension or muscle weakness in lighter gradients affects the trunk muscles as well as the bulbar and facial muscles. When breathing muscles are involved, hypoventilation is the most common symptom. Partly frequent airway infections occur. Weaknesses of the Bulbärmuskulatur express themselves mostly by speech disturbances and problems with swallowing. Muscle fibrils can be affected by myopathies as well as myopathies. For example, atrophy of the myofibrils can also manifest as muscle weakness. Depending on the cause of atrophy, however, local weaknesses do not have to be significant.

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