Definition
The heart muscle (myocardium) is a special type of muscle that is only found in the heart and forms most of the wall of the heart. Through its regular contractions, it is responsible for the blood being squeezed out of the heart (the heart’s task) and pumped through our body, making it vital.
Structure of the heart muscle
The heart muscle has both smooth and striated muscle characteristics and therefore represents a special form. In terms of structure, it corresponds more to the striated, i.e. skeletal, musculature. The individual muscle fibers are structured in such a way that the proteins responsible for contraction, actin and myosin, are arranged so regularly that this special structure ensures that the cells exhibit a kind of striated structure under polarizing light.
The tubule system (membrane-bounded spaces within the cytoplasm that serve as calcium stores and thus play an extremely important role in muscle contraction) is also similar to that of striated muscles, which is why the heart, like the skeletal muscles, is capable of rapid and, above all, powerful contraction. A characteristic that the heart muscle cell (cardiomyocyte) has in common with the cells of the smooth muscle, however, is that each cell has its own nucleus, which is usually located in the middle of the cytoplasm. Only rarely are there two nuclei per cell, whereas in a skeletal muscle cell there can be hundreds.
Therefore, in contrast to the striated muscle cells, we speak here only of a “functional” syncytium, since the cells are closely coupled but not fused together. In addition, there are properties that only the heart muscles possess: For example, an important feature is that the individual heart muscle cells are connected to each other by so-called shiny stripes (disci intercalares). These shiny stripes contain desmosomes and adherence contacts. These are both structures that help to stabilize the cell structure and enable the transfer of force between the individual cells. On the other hand, the glossy strips also contain gap junctions, i.e. practically small “gaps” between neighboring cells, through which an ion flow and thus electrical coupling is possible.
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