Muscle fibers form the cellular base and work unit of all human skeletal muscles. They can assume a length of less than 1 mm to 50 cm with a thickness of about 0.01 to 0.2 mm. Several muscle fibers become muscle fiber bundles, which - combined in several ways - form the muscle in its entirety. The multinucleated muscle fibers may respond to electrical nerve stimuli with contraction or relaxation. There are several types of muscle fibers that differ in response, fatigue and energy metabolism.

What are muscle fibers?

Muscle fibers are the striated muscle cells that make up skeletal muscle. Other common names are muscle fiber cells or myocytes. The multinucleated muscle fibers reach a length of a few mm to 50 cm and have a diameter of 0.01 to 0.2 mm. Several parallel-aligned muscle fibers are grouped into muscle fiber bundles and enveloped by a membrane.

The actual contractile function is fulfilled by myofibrils, of which several hundred are present in each muscle fiber. A contraction of the muscle is achieved by the sliding parallel juxtaposed actin and Myosinfilamenten without shortening themselves. To fulfill different functions of the muscles such as speed with the shortest reaction time or endurance, different types of muscle fibers can be distinguished, which differ in the reaction time and in their energy metabolism.

Responsive muscle fibers that fatigue quickly operate in the anaerobic region, while the muscle fibers, which are characterized by longer reaction times, operate predominantly in aerobic continuous operation. The distribution of different types of muscle fibers within a muscle is largely genetic and can most likely not be changed by strength and endurance training.

Anatomy & Construction

Muscle fibers are the cellular building blocks of striated skeletal muscle. It is a syncytium, a fusion of many single cells, whose cytoplasm and nuclei are preserved and become part of the new large cell. In a muscle fiber up to 40 cell nuclei per mm can be present. Each individual muscle fiber contains several hundred myofibrils, which are composed of only about 2 μm long sarcomeres.

The sarcomeres resemble tiny "compartments" in which are parallel aligned actin myofilaments and, laterally offset, myosin motor proteins. They are arranged so regularly one behind the other that the typical transverse striation can be recognized in polarized light. In a muscle fiber with 10 cm length cica 40, 000 sarcomeres are lined up in a row. Upon receipt of an appropriate action potential, the actin and myosin filaments glide into each other and lead to a shortening of the muscle cell.

Each myofibril, along with its associated organelles, is enveloped by a membrane, the sarcolemma. To increase mechanical strength, myofibrils also contain connective tissue fibers grown on the basement membrane. A very important function is carried out by so-called muscle spindles or proprioceptors, which are interposed between the muscle fibers and inform the central nervous system (CNS) of the momentary contraction state of the muscle via afferent nerve fibers.

Function & Tasks

Skeletal muscles can achieve their main functions such as stabilization of the body, movement of individual limbs and release of heat to the body only through the interaction of their individual muscle fibers. In order to ensure that all muscle fibers shorten at the same time during a muscle contraction, all muscle fibers must receive the action potential for contraction (almost) at the same time, otherwise punctiform muscle tensions and relaxations would occur.

The sarcolemma is responsible for transmitting the contraction command to all the muscle cells of a specific muscle, which offers the anatomical prerequisites because of its many invaginations into the myofibrils. The extremely fast-reacting FT fibers (almost twitch), which appear pale because of their low levels of myoglobin and mitochondria, are also referred to as white muscle fibers. They develop a high power potential, but tire quickly. The body needs this kind of muscle fibers for escape or attack reactions as well as for high jump or impact force.

In contrast, there are the so-called slower twitch fibers (slow twitch), which are also called red muscle fibers because of their high content of myoglobin and mitochondria. They develop less strength, but work in the aerobic area and tire much slower. In case of hypothermia of the body muscle cells of the skeletal muscles from the vegetative nervous system can be caused to deliberately uncontrollable chills (muscle tremors), which ultimately glucose is converted into heat and the body temperature can increase again.

Diseases & complaints

Muscle fiber diseases and disorders can be caused either by direct disease and inflammation of the muscle fibers, or by lesions on the innervating nerves or their superior nerve nodes. The former case is a myriad of possible myofibrillar myopathies and, in the latter case, a neuromuscular disease.

A direct mechanical damage of muscle fibers can be caused by a torn muscle, if the muscle is punctured too punctiform. In most cases, several muscle fibers or even entire muscle fiber bundles are affected. Myofibrillar myopathies manifest in progressive muscle weakness and muscle wasting, which can be attributed to one or more genetic defects. While muscle tremors in the form of a chills can be considered normal, muscle tremors (tremors) can also be triggered by a variety of neurological diseases. A distinction is made between a rest, action, movement or intention tremor.

The various types of tremor provide initial clues to the type of possible lesions present in the brain. Serious impairment of muscle fibers may be caused by motor neurone disease. Either the first (primary) motor neurons, whose axons originate in the motor cortex, are then affected, or the second motor neurons, which arise from the spinal cord. Amyotrophic lateral sclerosis (ALS) belongs to the group of motor neuron diseases. It is characterized by muscle weakness or muscle stiffness and takes a different progressive course.

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