Muscle Fibrils: Structure, Function & Diseases

Muscle fibrils are muscle fiber elements composed primarily of the proteins actin and myosin. These two proteins are the contractile elements of muscle that work together to implement muscle movement. In nemaline myopathy, the muscle fibrils change to a spindle shape, causing muscle weakness.

What is a muscle fibril?

Muscle fibers or muscle fiber cells are myocytes and correspond to the basic spindle-shaped cell units of striated skeletal muscle. Smooth muscle is not composed of muscle fibers. In skeletal muscle fibers, muscle fibrils form a functional unit at the cell organelle level. The elements are also called myofibrils and enable the contraction of skeletal muscles by filament sliding. Each skeletal muscle of striated muscle contains many individual fibrils that form a composite with each other. The protein myosin forms a major component of muscle fibrils. Ontologically, the organelle is superordinate to the functional unit muscle fibril. Subordinate components include sarcomeres and costamer. Individual muscle fibrils are always bound into bundles of fibrils in muscle by the intermediate filament desmin, and are surrounded by a variant of the endoplasmic reticulum called the sarcoplasmic reticulum.

Anatomy and structure

Muscle fibrils contain back-to-back building units with similar internal structure, also known as sarcomeres. Under the light microscope, fibrils show typical transverse striations that give striated muscle its name. The apparent transverse stripes result from the regular distribution of individual muscle filaments. The typical banding pattern is formed. The individual sarcomeres of the muscle fibrils consist of protein fibers that are arranged parallel to each other. These thick protein fibers correspond to the so-called myosin, which is better known as muscle protein. Between each filament of myosin are thinner filaments of actin. The complex of actin and myosin is held stable by the largest protein in the human organism: so-called titin. Regularly spaced solid discs are attached to thin filaments of titin. The individual discs are separated by myosin molecules and overlap proportionately with the actin filaments.

Function and tasks

Myosin and actin are collectively referred to as the contractile elements of muscle. Muscle fibrils are composed of these contractile elements. Thus, the main function of fibrils is to contract skeletal muscle. Muscle contractions always require an interaction of nerve tissue and muscle tissue. Only the neuromuscular unit of motor nerve and associated skeletal muscle allows the muscle to respond to commands from the central nervous system. Thus, although the contractile muscle elements actin and myosin contribute a part to muscle motor function and thus to human locomotion, they by no means implement motor function alone. Every skeletal muscle contraction is preceded by a release of calcium ions. These ions originate from the sarcoplasmic reticulum and are released whenever contraction commands from the central nervous system reach the muscle via efferent nerves. The commands reach the muscle in the form of bioelectrical excitation and are transmitted to its motor end plate. Once this happens, said calcium ions are released and subsequently diffuse between myosin and actin filaments. After diffusion, the calcium ions trigger interactions between the filaments. This results in what is known as filament sliding. The interactions between the myosin and actin filaments cause the actin filaments to slide between individual myosin filaments. This phenomenon is also known as the sliding filament mechanism and underlies every muscle contraction of striated muscle. Filament sliding causes shortening of the sarcomere. This eventually shortens all the muscle fibrils of the muscle fiber, causing the fiber to twitch. The ultimate contraction differs from this muscle twitch in that it requires the shortening of multiple muscle fibers.

Diseases

So-called myopathies are inherent diseases of the musculature that have no underlying neuronal cause. Skeletal muscle can suffer from different forms of myopathy. One of these is nemaline myopathy.Nemalin myopathy is an extremely rare disease that corresponds to a congenital myopathy. In the disease, rod-shaped changes called nemaline bodies appear on the muscle fibrils. The term nemaline myopathy may refer to genetically distinct disorders. Each variant of the disease is preceded by a genetic mutation that is passed on partly in autosomal dominant and partly in autosomal recessive inheritance. On patho-histological analysis of muscle biopsy, all forms of the disease are manifested by rod-like or filamentous structures in myocytes. The clinical presentation of the disease is extremely variable. The course of the disease varies from moderate symptoms to severe limitations. In most cases, myopathy is associated with more or less severe weakness of the muscles. Severe courses of nemaline myopathy are associated with failure of spontaneous movements or respiratory movements, which may begin at birth and, with such early onset, may lead to death in a few months. In moderately severe courses, muscle weakness progresses slowly, stagnates, or even decreases over time. Characteristically, in milder courses, the decreased muscle tone or weakness affects the trunk muscles and the bulbar and facial muscles. When the respiratory muscles are involved, hypoventilation turns out to be the most common symptom. In some cases, frequent respiratory infections occur. Weaknesses of the bulbar musculature are usually manifested by speech disorders and problems with swallowing. Muscle fibrils can be affected by numerous diseases in addition to myopathies. Atrophies of myofibrils, for example, can also manifest as muscle weakness. Depending on the cause of atrophy, however, local weakness need not be a major concern.