The heart muscle or myocardium ( Latin myocardium ) makes up most of the wall of the heart . The heart muscles are surrounded by the epicardium on the outside and the inner lining of the heart ( endocardium ). The heart muscle is a hollow muscle which has a macroscopic (loop-shaped, networked) structure specific for its contraction with a reduction in the volume of the cavity. The muscles of the ventricles pull superficially (subepicardially) to the apex of the heart and beat inwards at the heart vertebrae ( vortex cordis ) and pull back as a deep (subendocardial) muscle layer to the heart skeleton .
Fine structure of the heart muscles
Although the structure of the heart muscles is very similar to the skeletal muscles , it also has properties that are known from smooth muscles : For example, it has medium-sized cell nuclei . Heart muscle cells (cardiomyocytes) usually contain one nucleus per cell, in rare cases two, that is, they only represent a syncytium from a functional point of view . The heart muscles have in common with the skeletal muscles their regular structure of special striated muscle fibers and the system of fast calcium -Ion influx through diads (in skeletal muscles triads ) from terminal SR cisterns and T-tubules of the cell membrane . This structure is essential for the synchronization of the fast and powerful contraction and distinguishes the heart and skeletal muscles significantly from the smooth muscles.
Special features are the branch and the connection of the individual cardiac muscle cells (cardiomyocytes) by intercalated discs (1914 Victor von Ebner described intercalated discs or Disci intercalates ; English intercalated disks ) merge, whereas skeletal muscle precursor cells during Säugerembryonalentwicklung to real multinuclear syncytia (and thus long Develop muscle fibers). The glossy strips contain gap junctions for impulse transmission and desmosomes ( maculae adhaerentes ) and adherence contacts ( fasciae adhaerentes ) to stabilize the cell structure and force transmission . New molecular studies of the glossy stripes show that typical desmosomal and fascia adherent proteins (in contrast to their respective localizations in epithelia ) do not occur separately between the mammalian myocardial cells and thus heart-specific adherence connections of a complex mixed type ( area composita ) predominate. The intermediate filaments of the cytoskeleton that attach to these cell-cell connections consist mainly of desmin . Histologically, there is also a poorly developed sarcoplasmic reticulum and smaller cisterns than in skeletal muscle cells. In the heart muscle, there is always a setting of the plasma membrane (T-tubule) and an enlargement of the sarcoplasmic reticulum (L-tubule or SR cistern). In this context, one speaks of a “diad”, in contrast to the “triads” of a T-tubule and two L-tubules found in the striated skeletal muscles.
Specialized cardiac muscle cells, which can spontaneously trigger action potentials themselves, take over the basic control of the heart's action . They are called the arousal formation system. The impulse is also passed on to the actual work muscles by means of gap junctions via specialized heart muscle fibers and not via nerve fibers. The adaptation of the cardiac muscle activity to short-term fluctuations in blood pressure takes place via the Frank Starling mechanism in the heart muscles itself; the adaptation to changing physical activities is controlled via the circulatory center in the brain stem and the autonomic nervous system and is therefore also not subject to the will of the individual.
Human heart muscle
The heart muscle cells of the heart chambers of the human heart are 10-25 µm thick and 50-100 µm long. The number of cardiac muscle cells in the left ventricle, which provide the main pumping capacity, is initially estimated at 6 billion cells. In the course of life, the number decreases spontaneously and continuously and is given as 2–3 billion cells in older people.
Heart muscle disease
Causes of diseases of the heart muscle can be:
- Inflammation ( myocarditis )
- Blood supply disorders ( coronary heart disease , heart attack , "old age heart ")
- Degeneration due to metabolic disorders and disorders of internal secretion (hormonal diseases, thyroid diseases, stress cardiomyopathy ; poisoning)
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