As a muscle fiber , including muscle cell or myocyte , is referred to the spindle-shaped cellular basic unit of (striated) muscle of the skeleton . The muscle cells of the smooth muscles are not muscle fibers. The heart muscle cells are sometimes referred to as heart muscle fibers, but they differ in structure from the skeletal muscle fibers.
As carriers of their function, muscle fibers contain up to several hundred muscle fibrils (myofibrils) each about 1 μm in diameter, which run through the cell along their entire length, aligned parallel to one another. Depending on their number, muscle fibers measure around 0.01 mm to 0.1 mm in cross-section and, depending on the type and length of the individual muscle , can be a few millimeters to a few centimeters long. In a skeletal muscle , several muscle fibers are combined into fiber bundles (so-called "meat fibers") with a diameter of 0.1 mm to 1 mm, the ends of which are usually attached to the bones via tendons . When the muscle fiber contracts , these skeletal elements can be brought closer together.
A muscle fiber is an elongated, multinucleated cell, with the cell nuclei usually lying just below the cell membrane of the muscle cell, the sarcolemma . Extensions of the sarcolemma turn into tube-like folds in many places and thus form a system of transverse tubules ( T-tubules , transverse tubules; Latin: tubulus 'tube'), perpendicular to the surface and transversely to the longitudinal axis of the muscle cell , over which also deep Areas located in the muscle cell can be quickly reached by an excitation if an action potential is passed over the sarcolemma.
The invaginations of the T-system pull down into the immediate vicinity of the cavities of another tube system, the extensions of the (smooth) endoplasmic reticulum ( sarcoplasmic reticulum SR). These chambers are now oriented parallel to the longitudinal axis of the muscle cell - i.e. located longitudinally between the myofibrils or surrounding them - and thus form a closed system of longitudinal tubules ( L-tubules , longitudinal tubules) that serves as a reservoir for calcium ions. On both sides, the Ca 2+ -storing chambers of the L-system meet the traversing folds of the T-system, so that the folded membrane of the sarcolemma lies on both sides with SR membranes ( triad ) and receptors on the opposite membrane regions are in direct contact with each other can kick.
If a muscle fiber is excited by a nerve cell via the myoneural synapse - at the motor end plate by the associated (lower) motor neuron of its motor unit (mE) - and the action potential (AP) of the muscle cell is now conducted via the T-system, its tension-sensitive ones are then conducted (DHPR) receptors addressed. Their conformational change results in Skelettmuskelfasen directly to activation of the associated opposite (RYR1) receptors in the SR membrane, after which open ion channels and here out of the L-system Ca 2+ ions are released, which then in the myofibrils as electromechanical coupler a Trigger contraction: the muscle fiber shortens.
In the cytoplasm of muscle fibers, there are also different numbers of mitochondria ( sarcosomes ). The sarcoplasm can also contain myoglobin as an oxygen- storing pigment , energy-rich substances such as glycogen and enzymes for a metabolism of aerobic energy production in various concentrations , according to which different types of muscle fibers can also be differentiated biochemically.
→ Main article: Sarcomere
Mainly, however, muscle fibers contain a few hundred myofibrils , densely packed and arranged in parallel , which extend over the entire length of the muscle fiber. A myofibril is composed of several sharply delimited, uniform compartments that follow one another in a longitudinal direction, the approximately 2 µm long sarcomeres , which form the actual contractile units and (in polarized light) have typical transverse stripes. The reason is the high degree of order to the next to each other aligned in the sarcomere contractile proteins , the pro sarcomere thin each 2,000 actin - myofilaments and 1000 thicker myosin - motor proteins , which form during contraction move against each other. Since not only the myofilaments of a myofibril, but also the myofibrils in a muscle fiber - in contrast to smooth muscle cells - all pull in one direction and strictly next to each other, the transverse striation is also evident in the entire cell. Because in a muscle belly between two attachment points, origin and insertion , of a skeletal muscle, the muscle fibers of a fiber bundle now work together in the same direction.
Between the muscle fibers there is connective tissue as Endomysium they with the radiating tendon connects several muscle fibers are a Perimysium internum taken by connective tissue and so designated as the primary bundle. Some primary bundles together form a secondary bundle if they are enclosed by a common external perimysium . The epimysium , which surrounds the muscle as a whole, extends around these secondary bundles, merges into the muscle fascia , which can also combine several muscles in a muscle box , and separates when shifted from the environment. The task of the connective tissue structures is therefore to tie in the muscle in a force-fitting, tear-proof and movable manner and to secure its supply. The endomysium consists largely of reticular fibers , the perimysium of parallel collagen fibers. This is where the supplying blood vessels, draining lymph vessels and impulse-giving nerves run.
A distinction is made between two main types of muscle fibers according to their equipment with mitochondria and the enzymes of aerobic metabolism as well as their myoglobin content , or according to the course of the muscle fiber twitching. Muscle with a relatively higher number of said functional elements and thus provide an improved equipment for the longer paths a - available energy compounds thorough bailing - oxidative (oxygen consuming) metabolism are S low-twitch fibers called (or type 1) and twitch slower . The fast-twitch muscle fibers use rather the shorter paths anaerobic energy supply (preferably from glycogen) - therefore also have a lower content, for example to sauerstoffspeicherndem red dye muscle myoglobin - hot and F ast-twitch fibers (or Type 2). The difference between these types is an adaptation to the prevailing stress pattern; it is not a fixation in so far as the muscle fibers of a motor unit of one type can be converted into another type over time if the activity pattern (determined by the AP frequency of the responsible motor neuron) changes. Recent research at top athletes in cross-country skiing , however, have shown that with appropriate training and F ast-twitch fibers a mitochondria volume and a capillary density as S may have low-twitch fibers.
S-fibers (type 1)
Type S (slow) fibers are muscle fibers that twitch more slowly and therefore tend to contract slowly. However, they are still shortened so quickly that cyclical movements with high repetition frequencies such as cycling with cadence frequencies above 100 / min are possible - and can be maintained over a longer period of time. They are also called dark or red fibers because they have a dark red color due to the high myoglobin concentration . They are designed for continuous performance with limited effort and tire very slowly. The S-fiber is supplied by fine capillaries and gains its energy aerobically, whereby the oxygen required for this is taken from the blood . They are also called type 1 fibers or oxidative fibers .
F-fibers (type 2)
Type F (fast) fibers are fast-twitch muscle fibers. They consume more energy, tire more quickly and are also called light-colored or white fibers because of their low myoglobin content, or as type 2 fibers or glycolytic fibers . F-fibers are usually further divided into two (sub-) types: the FR (fast-resistant) fibers or type 2A , which are more similar in their properties to the S-fibers, and the very strong, fast, but quickly tiring FF (fast- fatigue) fibers or type 2X or type 2B ; in addition, further intermediate types (-2C, -2AC, -2AB) can be differentiated histochemically (according to myosin-ATPase activity). These differences correspond to different expression patterns of the skeletal muscle-specific isoforms of heavy chains in myosin (MyHC-IIa, -IIx, -IIb). While type 2B fibers are common in smaller mammals, isoform IIb is not found in adult humans in the fast muscle fiber type, but IIx, which is why the more recent literature speaks of type 2X.
Other fiber types of the skeletal muscles
Other fiber types, some with specific myosin types, can be found, for example, in the jaw muscles that develop enormous chewing pressure (e.g. in the masseter muscle ) or in the muscles of the eyeball that perform extremely fine eye movements (e.g. in the lateral rectus muscle ) . Furthermore, the fetus has a partially special set of myosins, which enable some muscle fibers to have their own specificity beyond the rough classification into S and F fibers.
Development and growth
The skeletal muscles of vertebrates emerge on both sides along their primary axis ( notochord ) from the (paraxial) mesoderm , which is divided transversely into somites and, through inductive signals, acquires a spatially oriented pattern of its positional relationships (in three dimensions: dorso-ventral, anterio- posterior, medio-lateral axis). Myotomes for muscles with muscle precursor cells are formed from dorsal parts, some of which remain local (later autochthonous muscles ), others migrate to the extremities ( cell migration ).
From these progenitor cells, divisible immature and ultimately mature myoblasts develop . These mononuclear cells fuse together ( fusion ) to a syncytium with multiple nuclei and form thin long tube-like structures, the myotubes. In these the first striated myofibrils are formed along the central nuclear chains. As they mature into muscle fibers, the cell nuclei migrate towards the edges and the basement membrane is formed around the muscle fiber as a separate envelope, in which some resting myoblasts are also enclosed, called satellite cells .
Because the cell nuclei within a mature muscle fiber are no longer able to divide, these mononuclear satellite cells become important for the later growth of muscle fibers, as they can be used to integrate additional cell nuclei if the length or cross-section increases. For this purpose, satellite cells can be made to divide by signals from various growth factors ; one daughter cell then fuses with the muscle fiber, while the other may later divide again. Additional nuclei are not only needed when the muscle fiber enlarges, be it with normal growth or training-related hypertrophy , but also for possible healing processes within the basement membrane envelope.
If a muscle is no longer used, inactivity atrophy occurs , in which the thickness of its fibers and the number of satellite cells decreases.
The mobilization, the transport and the breakdown of energy-rich substrates while producing ATP in the muscle cells are used to perform muscle work. See the
→ Main article: muscle contraction
Through the interaction of the two proteins in a sarcomere , actin and myosin, a muscle cell can reduce its length (concentric contraction), maintain resistance (isometric contraction), or resist its elongation (eccentric contraction). In the resting state, the areas on the actin to which the myosin is supposed to bind are covered by another protein, tropomyosin . When an action potential occurs in the SR, the release of calcium ions, which was first demonstrated by Setsuro Ebashi, is stimulated, which dissolves the blockage caused by the tropomyosin and thus triggers a contraction of the sarcomere through so-called filament sliding.
A single action potential only triggers a single short muscle fiber twitch in the skeletal muscles, during which the muscle fiber is only slightly shortened. In order to achieve a greater shortening or to cause a sustained contraction, action potentials must arrive in rapid succession so that individual twitches are gradually superimposed and added up (superposition). This must be distinguished from the tetanic contraction , which only occurs at an even higher action potential frequency , in which there is a complete fusion of individual twitches such as the maximum possible contraction of the muscle fiber (smooth or complete tetanus). Slowly twitching S-fibers can be tetanized by action potential series with frequencies from about 20 Hz, F-fibers need significantly higher fusion frequencies (above about 60 Hz).
In the muscle, the muscle strength is not only graded by different impulse frequencies of the motor neurons, but primarily by the type and number of alternately (and asynchronously) recruited motor units. Even with low muscle tension such as the reflex-induced muscle tone ("reflex tone "), which, for example, can involuntarily hold a body upright despite gravity, usually no individual twitches are visible. Nor does tetanus of the muscle occur under normal stress in vivo.
The muscle fibers were first described by Antoni van Leeuwenhoek in 1677 .
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