Definition
A muscle fiber (also: muscle fiber cell, myocyte) is the smallest unit of a skeletal muscle; the muscle cells of the smooth muscle and the heart muscle show certain similarities to the muscle fibers, but are not called so.
Structure of a muscle fiber
A muscle fiber is a so-called syncytium. This means that it is not simply a single cell. Several myoblasts have divided and grown to form a muscle fiber, which therefore has a large number of nuclei that are usually located on the outside of the cell along the sarcolemma, up to 40 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. The most important component of a muscle fiber is the myofibrils, which are responsible for the contraction of a muscle. A myofibril is made up of several sarcomeres arranged longitudinally in series.
These represent the smallest contractile unit. They consist mainly of the proteins actin and myosin, which are arranged in a very regular pattern, resulting in cross-stripping visible in polarized light – hence the name cross-striped muscles, which is also frequently used for skeletal muscles. In addition, a muscle fiber cell also contains cell organelles as in other body cells.
The sarcolemma, which corresponds to a plasma membrane, surrounds the muscle fibers from the outside. It has several invaginations, which are called the T-system (transverse system, T-tubules). The L-system (longitudinal system, L-tubules, sarcoplasmic reticulum), which is similar to the endoplasmic reticulum, runs vertically.
It serves as a store for calcium ions and thus fulfils a very important task in the context of muscle contraction. The mitochondria, which are also contained between the myofibrils, are responsible for the energy supply of the muscle fibers. Between the individual myofibrils there is also a connective tissue structure called endomysium.
Several myofibrils are arranged together and form a primary bundle surrounded by perimysium. The combination of several primary bundles is called a secondary bundle, which is surrounded by the perimysium externum. Finally, the epimysium is located around the secondary bundles and merges into the muscle fascia. This network of connective tissue is there to make the muscle fibers tear-resistant and thus protect them against external forces.
