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Cardiac muscle
Cardiac muscle
Cardiac muscle
Cardiac muscle tissue is only found in the heart.
An isolated cardiac muscle cell, beating
Cardiac muscle (heart muscle) is an involuntary, striated muscle that is found in the
walls and histological foundation of the heart, specifically the myocardium. Cardiac
muscle is one of three major types of muscle, the others being skeletal and smooth
muscle. These three types of muscle all form in the process of myogenesis. The cells
that constitute cardiac muscle, called cardiomyocytesor myocardiocytes, predominantly
contain only one nucleus, although populations with two to four nuclei do
exist.[1][2][page needed] Themyocardium is the muscle tissue of the heart, and forms a thick
middle layer between the outer epicardium layer and the innerendocardium layer.
Coordinated contractions of cardiac muscle cells in the heart pump blood out of
the atria and ventricles to the blood vessels of the left/body/systemic and
right/lungs/pulmonary circulatory systems. This complex mechanism illustrates systole of
the heart.
Cardiac muscle cells, unlike most other tissues in the body, rely on an available blood
and electrical supply to deliver oxygen and nutrients and remove waste products such
as carbon dioxide. The coronary arteries help fulfill this function.
Structure
Cardiac muscle
Intercalated discs are part of the cardiac muscle sarcolemma and they contain gap junctions and
desmosomes.
3D rendering showing thick myocardium within heart wall.
Striation[
Cardiac muscle has cross striations formed by rotating segments of thick and thin protein
filaments. Like skeletal muscle, the primary structural proteins of cardiac muscle are
myosin and actin. The actin filaments are thin, causing the lighter appearance of the I
bandsin striated muscle, whereas the myosin filament is thicker, lending a darker
appearance to the alternating A bands as observed withelectron microscopy. However, in
contrast to skeletal muscle, cardiac muscle cells are typically branch-like instead of
linear.
T-tubules[
Another histological difference between cardiac muscle and skeletal muscle is that the Ttubules in the cardiac muscle are bigger and wider and track laterally to the Z-discs.
There are fewer T-tubules in comparison with skeletal muscle. The diad is a structure in
the cardiac myocyte located at the sarcomere Z-line. It is composed of a single Ttubule paired with a terminal cisterna of the sarcoplasmic reticulum. The diad plays an
important role in excitation-contraction coupling by juxtaposing an inlet for the action
potential near a source of Ca2+ ions. This way, the wave of depolarization can be coupled
to calcium-mediated cardiac muscle contraction via the sliding filament mechanism.
Cardiac muscle forms these instead of the triads formed between the sarcoplasmic
reticulum in skeletal muscle and T-tubules. T-tubules play critical role in excitationcontraction coupling (ECC). Recently, the action potentials of T-tubules were recorded
optically by Guixue Bu et al.[3]
Intercalated discs]
Main article: Intercalated disc
The cardiac syncytium is a network of cardiomyocytes connected to each other
by intercalated discs that enable the rapid transmission of electrical impulses through the
network, enabling the syncytium to act in a coordinated contraction of the myocardium.
There is an atrial syncytium and a ventricular syncytium that are connected by
cardiac connection fibres.[4] Electrical resistance through intercalated discs is very low,
thus allowing free diffusion of ions. The ease of ion movement along cardiac muscle
fibers axes is such that action potentials are able to travel from one cardiac muscle cell to
the next, facing only slight resistance. Each syncyntium obeys the all or none law.[5]
Intercalated discs are complex adhering structures that connect the single
cardiomyocytes to an electrochemical syncytium (in contrast to the skeletal muscle,
which becomes a multicellular syncytium during mammalian embryonic development).
The discs are responsible mainly for force transmission during muscle contraction.
Intercalated discs are described to consist of three different types of cell-cell junctions:
the actin filament anchoring adherens junctions, the intermediate filament
anchoring desmosomes , and gap junctions. They allow action potentials to spread
between cardiac cells by permitting the passage of ions between cells, producing
depolarization of the heart muscle. However, novel molecular biological and
comprehensive studies unequivocally showed that intercalated discs consist for the most
part of mixed-type adhering junctions named area composita (pl. areae compositae)
representing an amalgamation of typical desmosomal and fascia adhaerens proteins (in
contrast to various epithelia).[6][7][8] The authors discuss the high importance of these
findings for the understanding of inherited cardiomyopathies (such as arrhythmogenic
right ventricular cardiomyopathy).
Under light microscopy, intercalated discs appear as thin, typically dark-staining lines
dividing adjacent cardiac muscle cells. The intercalated discs run perpendicular to the
direction of muscle fibers. Under electron microscopy, an intercalated disc's path appears
more complex. At low magnification, this may appear as a convoluted electron dense
structure overlying the location of the obscured Z-line. At high magnification, the
intercalated disc's path appears even more convoluted, with both longitudinal and
transverse areas appearing in longitudinal section.[9]
Physiology[
In contrast to skeletal muscle, cardiac muscle requires extracellular calcium ions for
contraction to occur. Like skeletal muscle, the initiation and upshoot of the action
potential in ventricular cardiomyocytes is derived from the entry of sodium ions across
the sarcolemma in a regenerative process. However, an inward flux of extracellular
calcium ions through L-type calcium channels sustains the depolarization of cardiac
muscle cells for a longer duration. The reason for the calcium dependence is due to the
mechanism of calcium-induced calcium release (CICR) from thesarcoplasmic
reticulum that must occur during normal excitation-contraction (EC) coupling to cause
contraction. Once the intracellular concentration of calcium increases, calcium ions bind
to the protein troponin, which allows myosin to bind to actin and contraction to occur.
Regeneration of heart muscle cells
Dog cardiac muscle (400X)
Until recently, it was commonly believed that cardiac muscle cells could not be
regenerated. However, a study reported in the April 3, 2009 issue of Science contradicts
that belief.[10] Olaf Bergmann and his colleagues at theKarolinska
Institute in Stockholm tested samples of heart muscle from people born before 1955 who
had very little cardiac muscle around their heart, many showing with disabilities from this
abnormality. By using DNA samples from many hearts, the researchers estimated that a
4-year-old renews about 20% of heart muscle cells per year, and about 69 percent of the
heart muscle cells of a 50-year-old were generated after he or she was born.
One way that cardiomyocyte regeneration occurs is through the division of pre-existing
cardiomyocytes during the normal aging process.[11] The division process of pre-existing
cardiomyocytes has also been shown to increase in areas adjacent to sites of myocardial
injury. In addition, certain growth factors promote the self-renewal of endogenous
cardiomyocytes and cardiac stem cells. For example, insulin-like growth factor
1, hepatocyte growth factor, and high-mobility group protein B1 increase cardiac stem
cell migration to the affected area, as well as the proliferation and survival of these
cells.[12] Some members of the fibroblast growth factorfamily also induce cell-cycle reentry of small cardiomyocytes. Vascular endothelial growth factor also plays an important
role in the recruitment of native cardiac cells to an infarct site in addition to
its angiogenic effect.
Based on the natural role of stem cells in cardiomyocyte regeneration, researchers and
clinicians are increasingly interested in using these cells to induce regeneration of
damaged tissue. Various stem cell lineages have been shown to be able to differentiate
into cardiomyocytes, including bone marrow stem cells. For example, in one study,
researchers transplanted bone marrow cells, which included a population of stem cells,
adjacent to an infarct site in a mouse model. Nine days after surgery, the researchers
found a new band of regenerating myocardium.[13] However, this regeneration was not
observed when the injected population of cells was devoid of stem cells, which strongly
suggests that it was the stem cell population that contributed to the myocardium
regeneration. Other clinical trials have shown that autologous bone marrow cell
transplants delivered via the infarct-related artery decreases the infarct area compared to
patients not given the cell therapy.[14]