Muscle fiber

definition

Under a muscle fiber (also: muscle fiber cell, Myocyte) one understands the smallest unit of a skeletal muscle; The muscle cells of the smooth muscles and the heart muscle show certain similarities to the muscle fibers, but are not given that name.

Structure of a muscle fiber

A muscle fiber is a so-called Syncytium. This means that this is not simply a single cell. Several myoblasts have divided and grown to form a muscle fiber, which is why one Variety of cell nuclei usually located on the outside of the cell along the sarcolemma, up to 40 cell nuclei per millimeter are not uncommon. A muscle fiber cell is usually spindle-shaped, 1 mm to 15 cm long, and 10 to 200 µm in diameter.

Illustration of a muscle fiber

Figure skeletal muscle (A) and construction diagram of a cross-section I-I (B)
  1. Muscle fiber
    of a skeletal muscle
    Muscle fibra
  2. Muscle fiber bundles -
    Muscular Fasciculus
  3. Epimysium (light blue) -
    Connective tissue sheaths around groups
    of muscle fiber bundles
  4. Perimysium (yellow) -
    Connective tissue sheaths
    around muscle fiber bundles
  5. Endomysium (green) -
    Connective tissue between muscle fibers
  6. Myofibrils (= muscle fibrils)
  7. Sarcomere (myofibril segment)
  8. Myosin threads
  9. Actin threads
  10. artery
  11. vein
  12. Muscle fascia
    (= Muscle skin) - Fascia
  13. Transition of muscle fibers
    in tendon fibers -
    Junctio myotendinea
  14. Skeletal muscle
  15. Tendon fibers -
    Fibrae tendineae

You can find an overview of all Dr-Gumpert images at: medical illustrations

Of the most important component of a muscle fiber are those Myofibrilsthat allow a muscle to contract (contract).
A myofibril is made up of several connected lengthways Sarcomeres. These represent the smallest contractile unit They mainly consist of the Proteins actin and myosinthat are arranged very regularly, which is why there is one in polarized light visible horizontal stripes comes - hence the name striated muscles, which is also often used for the skeletal muscles.
They also come in a muscle fiber cell Cell organelles as in other body cells.
The Sarcolemmathat one Plasma membrane corresponds to, surrounds the muscle fibers from the outside.
It has several indentations, which are known as T system (transverse system, T-tubules).
It runs perpendicular to it L system (longitudinal system, L-tubules, sarcoplasmic reticulum), which is similar to the endoplasmic reticulum. It serves as a Storage for calcium ions and thus fulfills a very important task in the context of muscle contraction.
The also contained, lying between the myofibrils Mitochondria are responsible for the energy supply of the muscle fibers.

Between the individual myofibrils there is also a connective tissue structure that Endomysium is called.
Several myofibrils accumulate and form one of Perimysium internum enclosed primary bundle. The amalgamation of several primary bundles is called a secondary bundle Perimysium externum is surrounded. After all, around the secondary bundles is that Epimysiumthat in the Muscle fascia transforms.
This network of connective tissue is there to make the muscle fibers tear-resistant and thus to protect them against external forces.

composition

A muscle fiber is made up of around three quarters in total water, to 20% off Proteins (Half of which is made up of the contractile proteins actin and myosin) and 5% off Ions, Fats, Glycogen (an energy storage device) and nitrogenous substances.

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Types of muscle fibers

Be based on their function two different types of muscle fibers distinguished.
On the one hand there is phasic, fast-twitch muscle fibers (FT fibers) and on the other hand the tonic, slowly twitching muscle fibers (ST fibers).

The slow twitch muscle fibers will too red or type 1 fibers called. Their red coloring comes from the fact that they are a high concentration of myoglobin own, which represents an oxygen storage. Due to the high oxygen content, these fibers can limited power for a long time they tire very slowly, but they are flat not capable of high performance. Such muscle fiber cells can be found in the diaphragm or the Eye muscles, muscles that are relatively permanently active, but usually do not have to perform very well.

The rapidly twitching muscle fibers (Type 2 or white fibers) on the other hand contain less myoglobin, but have a more pronounced sarcoplasmic reticulum. This makes it possible to release a lot of calcium ions very quickly and also to take them up again, which quickly results in a High performance can be provided. However, these fibers also consume more energy and tire faster than the slowly twitching fibers. This type of muscle fiber is found mainly in muscles that are on quick, short-lived activities sprinters, for example, generally have a high proportion of white muscle fiber cells.

Injuries

If a muscle fiber is suddenly subject to very strong stretching and the connective tissue is no longer strong enough to absorb it, it can lead to a Torn hamstring come. This injury is a very painful event and is often described by those affected as feeling like being stabbed with a knife. As a rule, the crack is as Indentation palpable and goes with one bruise hand in hand.
Torn muscle fibers are particularly common in the Calf muscles, this one particularly high loads, especially the dangerous braking movements.
The care of a torn muscle fiber is subject to this PECH principle:
P like Break, E like ice, C like Compression (Compression) and H like Elevate. If this therapy is given early, a muscle fiber tear will normally heal on its own within a few days without further complications or consequences.