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Cardiac muscle (also called heart muscle or myocardium) is one of three types of
vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an indiv ...
muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly cat ...

muscle
s, with the other two being
skeletal The skeleton refers to the frames of support of animal bodies. There are several different skeletal types: the exoskeleton An exoskeleton (from Greek έξω, ''éxō'' "outer" and σκελετός, ''skeletós'' "skeleton") is the external sk ...

skeletal
and
smooth Smooth may refer to: Mathematics * Smooth function is a smooth function with compact support. In mathematical analysis, the smoothness of a function (mathematics), function is a property measured by the number of Continuous function, continuo ...

smooth
muscles. It is involuntary,
striated
striated
muscle that constitutes the main tissue of the walls of the
heart The heart is a muscular MUSCULAR (DS-200B), located in the United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,Usage is mixed. The Guardian' and Telegraph' use ...

heart
. The myocardium forms a thick middle layer between the outer layer of the heart wall (the
epicardium The pericardium, also called pericardial sac, is a double-walled sac containing the heart The heart is a cardiac muscle, muscular Organ (biology), organ in most animals, which pumps blood through the blood vessels of the circulatory sys ...
) and the inner layer (the
endocardium The endocardium is the innermost layer of tissue that lines the chambers of the heart The heart is a muscle, muscular Organ (anatomy), organ in most animals, which pumps blood through the blood vessels of the circulatory system. The pumped ...
), with blood supplied via the
coronary circulation Coronary circulation is the circulation of blood in the blood vessels that supply the cardiac muscle, heart muscle (myocardium). Coronary arteries supply oxygen saturation (medicine), oxygenated blood to the heart muscle. #Cardiac veins, Cardiac ve ...
. It is composed of individual heart muscle cells (
cardiomyocytes Cardiac muscle cells or cardiomyocytes (also known as myocardiocytes or cardiac myocytes) are the muscle cells (myocytes) that make up the cardiac muscle (heart muscle). Each myocardial cell contains myofibrils, which are specialized organelles c ...
) joined together by
intercalated disc Intercalated discs(also called intercalated discs or glossy stripes) are microscopic identifying features of cardiac muscle Cardiac muscle (also called heart muscle or myocardium) is one of three types of vertebrate Vertebrates () comprise all ...

intercalated disc
s, encased by
collagen Collagen () is the main structural protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowder ...

collagen
fibers and other substances that form the
extracellular matrix In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...
. Cardiac muscle
contracts A contract is a legally binding document between at least two parties that defines and governs the rights and duties of the parties to an agreement. A contract is legally enforceable because it meets the requirements and approval of the law. A ...

contracts
in a similar manner to
skeletal muscle Skeletal muscles (commonly referred to as muscles) are Organ (biology), organs of the vertebrate muscular system that are mostly attached by tendons to bones of the skeleton. The muscle cells of skeletal muscles are much longer than in the other ...

skeletal muscle
, although with some important differences. Electrical stimulation in the form of an
action potential In physiology Physiology (; ) is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence ...

action potential
triggers the release of calcium from the cell's internal calcium store, the
sarcoplasmic reticulum The sarcoplasmic reticulum (SR) is a membrane-bound structure found within muscle cells that is similar to the smooth endoplasmic reticulum in other Cell (biology), cells. The main function of the SR is to store calcium ions (Ca2+). Calcium ion le ...
. The rise in calcium causes the cell's
myofilament Myofilaments are the two protein filament In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular inte ...

myofilament
s to slide past each other in a process called
excitation contraction coupling Muscle contraction is the activation of tension Tension may refer to: Science * Psychological stress * Tension (physics), a force related to the stretching of an object (the opposite of compression) * Tension (geology), a stress which stretches ...
. Diseases of the heart muscle are of major importance. These include conditions caused by a restricted blood supply to the muscle including
angina Angina, also known as angina pectoris, is chest pain Chest pain is pain or discomfort in the chest, typically the front of the chest. It may be described as sharp, dull, pressure, heaviness or squeezing. Associated symptoms may include pain in ...

angina
pectoris and
myocardial infarction A myocardial infarction (MI), commonly known as a heart attack, occurs when blood flow Hemodynamics American and British English spelling differences#ae and oe, or haemodynamics are the Fluid dynamics, dynamics of blood flow. The circulatory sy ...

myocardial infarction
, and other heart muscle diseases known as cardiomyopathies.


Structure


Gross anatomy

Cardiac muscle tissue or myocardium forms the bulk of the heart. The heart wall is a three-layered structure with a thick layer of myocardium sandwiched between the inner
endocardium The endocardium is the innermost layer of tissue that lines the chambers of the heart The heart is a muscle, muscular Organ (anatomy), organ in most animals, which pumps blood through the blood vessels of the circulatory system. The pumped ...
and the outer
epicardium The pericardium, also called pericardial sac, is a double-walled sac containing the heart The heart is a cardiac muscle, muscular Organ (biology), organ in most animals, which pumps blood through the blood vessels of the circulatory sys ...

epicardium
(also known as the visceral pericardium). The inner endocardium lines the cardiac chambers, covers the cardiac valves, and joins with the
endothelium Endothelium is a single layer of squamous Epithelium () is one of the four basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. Wit ...
that lines the blood vessels that connect to the heart. On the outer aspect of the myocardium is the
epicardium The pericardium, also called pericardial sac, is a double-walled sac containing the heart The heart is a cardiac muscle, muscular Organ (biology), organ in most animals, which pumps blood through the blood vessels of the circulatory sys ...

epicardium
which forms part of the pericardium, the sack that surrounds, protects, and lubricates the heart. Within the myocardium, there are several sheets of cardiac muscle cells or cardiomyocytes. The sheets of muscle that wrap around the left ventricle closest to the endocardium are oriented perpendicularly to those closest to the epicardium. When these sheets contract in a coordinated manner they allow the ventricle to squeeze in several directions simultaneously – longitudinally (becoming shorter from apex to base), radially (becoming narrower from side to side), and with a twisting motion (similar to wringing out a damp cloth) to squeeze the maximum possible amount of blood out of the heart with each heartbeat. Contracting heart muscle uses a lot of energy, and therefore requires a constant flow of blood to provide
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
and nutrients.
Blood Blood is a body fluid Body fluids, bodily fluids, or biofluids are liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isochoric flow) refers t ...

Blood
is brought to the myocardium by the
coronary arteries The coronary arteries are the arterial blood vessels of coronary circulation Coronary circulation is the circulation of blood in the blood vessels that supply the cardiac muscle, heart muscle (myocardium). Coronary arteries supply oxygen saturat ...

coronary arteries
. These originate from the
aortic root
aortic root
and lie on the outer or epicardial surface of the heart. Blood is then drained away by the coronary veins into the
right atrium The atrium (Latin ātrium, “entry hall”) is the upper chamber through which blood enters the Ventricle (heart), ventricles of the heart. There are two atria in the human heart – the left atrium receives blood from the pulmonary (lung) circul ...
.


Histology

When looked at microscopically, cardiac muscle can be likened to the wall of a house. Most of the wall is taken up by bricks, which in cardiac muscle are individual
cardiac muscle cell Cardiac muscle cells or cardiomyocytes (also known as myocardiocytes or cardiac myocytes) are the muscle cells (myocyte A muscle cell is also known as a myocyte when referring to either a cardiac muscle cell Cardiac muscle cells or cardiomyoc ...
s or cardiomyocytes. The mortar which surrounds the bricks is known as the
extracellular matrix In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...
, produced by supporting cells known as
fibroblast A fibroblast is a type of biological cell The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cells are the smallest units of life, and hence are often referre ...

fibroblast
s. In the same way that the walls of a house contain electrical wires and plumbing, cardiac muscle also contains specialized cells for conducting electrical signals rapidly (), and blood vessels to bring nutrients to the muscle cells and take away waste products (the
coronary arteries The coronary arteries are the arterial blood vessels of coronary circulation Coronary circulation is the circulation of blood in the blood vessels that supply the cardiac muscle, heart muscle (myocardium). Coronary arteries supply oxygen saturat ...

coronary arteries
,
veins Veins are blood vessels in humans, and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary vein, pulmonary and umbilical veins, both of which ca ...
and
capillary A capillary is a small blood vessel The blood vessels are the components of the circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an organ system that permits blood to circulate ...

capillary
network).


Cardiac muscle cells

Cardiac muscle cells or
cardiomyocytes Cardiac muscle cells or cardiomyocytes (also known as myocardiocytes or cardiac myocytes) are the muscle cells (myocytes) that make up the cardiac muscle (heart muscle). Each myocardial cell contains myofibrils, which are specialized organelles c ...
are the contracting cells that allow the heart to pump. Each cardiomyocyte needs to contract in coordination with its neighboring cells - known as a functional syncytium - working to efficiently pump blood from the heart, and if this coordination breaks down then – despite individual cells contracting – the heart may not pump at all, such as may occur during abnormal heart rhythms such as
ventricular fibrillation Ventricular fibrillation (V-fib or VF) is an abnormal heart rhythm in which the ventricles of the heart The heart is a cardiac muscle, muscular Organ (biology), organ in most animals, which pumps blood through the blood vessels of the ci ...

ventricular fibrillation
. Viewed through a microscope, cardiac muscle cells are roughly rectangular, measuring 100–150μm by 30–40μm. Individual cardiac muscle cells are joined together at their ends by
intercalated disks Intercalated discs(also called intercalated discs or glossy stripes) are microscopic identifying features of cardiac muscle Cardiac muscle (also called heart muscle or myocardium) is one of three types of vertebrate Vertebrates () comprise all ...

intercalated disks
to form long fibers. Each cell contains
myofibril s in parallel, and sarcomere A sarcomere (Greek σάρξ ''sarx'' "flesh", μέρος ''meros'' "part") is the complicated unit of striated muscle tissue. It is the repeating unit between two Z lines. Skeletal muscles are composed of tubular mu ...

myofibril
s, specialized protein fibers that slide past each other. These are organized into
sarcomere A sarcomere (Greek σάρξ ''sarx'' "flesh", μέρος ''meros'' "part") is the smallest functional unit of striated muscle tissue Striated muscle tissue is a muscle tissue Muscle tissue is a soft tissue that composes muscles in animal bodie ...

sarcomere
s, the fundamental contractile units of muscle cells. The regular organization of myofibrils into sarcomeres gives cardiac muscle cells a striped or appearance when looked at through a microscope, similar to skeletal muscle. These striations are caused by lighter
I bands A sarcomere (Greek σάρξ ''sarx'' "flesh", μέρος ''meros'' "part") is the smallest functional unit of striated muscle tissue Striated muscle tissue is a muscle tissue Muscle tissue is a soft tissue that composes muscles in animal bodi ...
composed mainly of a protein called actin, and darker composed mainly of myosin. Cardiomyocytes contain
T-tubule T-tubules (transverse tubules) are extensions of the sarcolemma, cell membrane that penetrate into the centre of Skeletal muscle#Skeletal muscle fibers, skeletal and cardiac muscle cells. With membranes that contain large concentrations of ion ...
s, pouches of membrane that run from the surface to the cell's interior which help to improve the efficiency of contraction. The majority of these cells contain only one
nucleus ''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...

nucleus
(although they may have as many as four), unlike skeletal muscle cells which typically contain many nuclei. Cardiac muscle cells contain many
mitochondria A mitochondrion (; ) is a double-membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Biological membranes include cell membranes ...

mitochondria
which provide the energy needed for the cell in the form of
adenosine triphosphate Adenosine triphosphate (ATP) is an organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properti ...

adenosine triphosphate
(ATP), making them highly resistant to fatigue.


= T-tubules

= T-tubules are microscopic tubes that run from the cell surface to deep within the cell. They are continuous with the cell membrane, are composed of the same
phospholipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane A polarized membrane is a lipid bilayer, lipid membrane that has a positive electrical charge on one side and a negative charge on another side, which produces the resting pote ...
, and are open at the cell surface to the
extracellular fluid In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, ...
that surrounds the cell.
T-tubules T-tubules (transverse tubules) are extensions of the cell membrane cell membrane vs. Prokaryotes A prokaryote is a typically unicellular organism that lacks a nuclear membrane-enclosed cell nucleus, nucleus. The word ''prokaryote'' comes fr ...
in cardiac muscle are bigger and wider than those in
skeletal muscle Skeletal muscles (commonly referred to as muscles) are Organ (biology), organs of the vertebrate muscular system that are mostly attached by tendons to bones of the skeleton. The muscle cells of skeletal muscles are much longer than in the other ...

skeletal muscle
, but fewer in number. In the centre of the cell they join together, running into and along the cell as a transverse-axial network. Inside the cell they lie close to the cell's internal calcium store, the
sarcoplasmic reticulum The sarcoplasmic reticulum (SR) is a membrane-bound structure found within muscle cells that is similar to the smooth endoplasmic reticulum in other Cell (biology), cells. The main function of the SR is to store calcium ions (Ca2+). Calcium ion le ...
. Here, a single tubule pairs with part of the sarcoplasmic reticulum called a terminal cisterna in a combination known as a
diad The diad is a structure in the cardiac myocyte A muscle cell is also known as a myocyte when referring to either a cardiac muscle cell Cardiac muscle cells or cardiomyocytes (also known as myocardiocytes or cardiac myocytes) are the muscle cel ...
. The functions of T-tubules include rapidly transmitting electrical impulses known as
action potentials In physiology Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology Biology is the natural science that studies life and living organisms, including their anatomy, phy ...

action potentials
from the cell surface to the cell's core, and helping to regulate the concentration of calcium within the cell in a process known as
excitation-contraction coupling Muscle contraction is the activation of tension Tension may refer to: Science * Psychological stress * Tension (physics), a force related to the stretching of an object (the opposite of compression) * Tension (geology), a stress which stretches r ...
.


= Intercalated discs

= The cardiac
syncytium A syncytium or symplasm (; plural syncytia; from Greek: σύν ''syn'' "together" and κύτος ''kytos'' "box, i.e. cell") is a multinucleate cell which can result from multiple cell fusions of uninuclear cells (i.e., cells with a single nucleu ...
is a network of cardiomyocytes connected by
intercalated disc Intercalated discs(also called intercalated discs or glossy stripes) are microscopic identifying features of cardiac muscle Cardiac muscle (also called heart muscle or myocardium) is one of three types of vertebrate Vertebrates () comprise all ...

intercalated disc
s 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. 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 syncytium obeys the all or none law. Intercalated discs are complex adhering structures that connect the single cardiomyocytes to an electrochemical
syncytium A syncytium or symplasm (; plural syncytia; from Greek: σύν ''syn'' "together" and κύτος ''kytos'' "box, i.e. cell") is a multinucleate cell which can result from multiple cell fusions of uninuclear cells (i.e., cells with a single nucleu ...
(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 consist of three different types of cell-cell junctions: the actin filament anchoring
adherens junctions Adherens junctions (or zonula adherens, intermediate junction, or "belt desmosome") are protein complexes that occur at cell–cell junctions in epithelial and endothelial tissues, usually more basal than tight junctions Tight junctions, also kn ...
, the intermediate filament anchoring
desmosomes A desmosome (; "binding body"), also known as a macula adherens (plural: maculae adherentes) (Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally s ...
, and
gap junctions Gap junctions are a specialized intercellular connection between a multitude of animal cell-types. They directly connect the cytoplasm In cell biology, the cytoplasm is all of the material within a Cell (biology), cell, enclosed by the cell mem ...
. 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 predominantly consist 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). The authors discuss the high importance of these findings for the understanding of inherited cardiomyopathies (such as
arrhythmogenic right ventricular cardiomyopathy Arrhythmogenic cardiomyopathy (ACM), arrhythmogenic right ventricular dysplasia (ARVD), or arrhythmogenic right ventricular cardiomyopathy (ARVC), is an inherited heart disease. ACM is caused by Genetic disorder, genetic defects of the parts of ...

arrhythmogenic right ventricular cardiomyopathy
). Under
light microscopy Light or visible light is electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies ...
, 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.


Fibroblasts

Cardiac fibroblasts are vital supporting cells within cardiac muscle. They are unable to provide forceful contractions like
cardiomyocytes Cardiac muscle cells or cardiomyocytes (also known as myocardiocytes or cardiac myocytes) are the muscle cells (myocytes) that make up the cardiac muscle (heart muscle). Each myocardial cell contains myofibrils, which are specialized organelles c ...
, but instead are largely responsible for creating and maintaining the extracellular matrix which forms the mortar in which cardiomyocyte bricks are embedded. Fibroblasts play a crucial role in responding to injury, such as a
myocardial infarction A myocardial infarction (MI), commonly known as a heart attack, occurs when blood flow Hemodynamics American and British English spelling differences#ae and oe, or haemodynamics are the Fluid dynamics, dynamics of blood flow. The circulatory sy ...

myocardial infarction
. Following injury, fibroblasts can become activated and turn into
myofibroblast A myofibroblast is a cell that is in between a fibroblast and a smooth muscle cell in phenotype right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of petal color in pea plants ...
s – cells which exhibit behaviour somewhere between a fibroblast (generating extracellular matrix) and a
smooth muscle cell Smooth muscle is an involuntary non-striated muscle. It is divided into two subgroups; the single-unit smooth muscle, single-unit (unitary) and multiunit smooth muscle. Within single-unit cells, the whole bundle or sheet Muscle contraction, contra ...

smooth muscle cell
(ability to contract). In this capacity, fibroblasts can repair an injury by creating collagen while gently contracting to pull the edges of the injured area together. Fibroblasts are smaller but more numerous than cardiomyocytes, and several fibroblasts can be attached to a cardiomyocyte at once. When attached to a cardiomyocyte they can influence the electrical currents passing across the muscle cell's surface membrane, and in the context are referred to as being electrically coupled. Other potential roles for fibroblasts include electrical insulation of the , and the ability to transform into other cell types including cardiomyocytes and
adipocyte Adipocytes, also known as lipocytes and fat cells, are the cells that primarily compose adipose tissue Adipose tissue, body fat, or simply fat is a loose connective tissue Connective tissue is one of the many basic types of animal An ...

adipocyte
s.


Extracellular matrix

Continuing the analogy of heart muscle as being like a wall, the extracellular matrix is the mortar that surrounds the cardiomyocyte and fibroblasts bricks. The matrix is composed of proteins such as
collagen Collagen () is the main structural protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowder ...

collagen
and
elastin Elastin is a key protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, f ...
along with
polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrate A carbohydrate () is a biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallograp ...
s (sugar chains) known as
glycosaminoglycan . For polysaccharide nomenclature see oligosaccharide nomenclature, here. R1, R2, R3 may have different values. Glycosaminoglycans (GAGs) or mucopolysaccharides are long linear polysaccharides consisting of repeating disaccharide units (i.e. two-su ...
s. Together, these substances give support and strength to the muscle cells, create elasticity in cardiac muscle, and keep the muscle cells hydrated by binding water molecules. The matrix in immediate contact with the muscle cells is referred to as the
basement membrane The basement membrane is a thin, pliable sheet-like type of extracellular matrix In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processe ...
, mainly composed of
type IV collagen Collagen IV (ColIV or Col4) is a type of collagen found primarily in the basal lamina. The type IV collagen C4 domain, collagen IV C4 domain at the C-terminus is not removed in post-translational processing, and the fibers link head-to-head, rather ...
and
laminin Laminins are high-molecular weight (~400 to ~900 kDa) proteins of the extracellular matrix. They are a major component of the basal lamina (one of the layers of the basement membrane), a protein network foundation for most cells and organs. The l ...
. Cardiomyocytes are linked to the basement membrane via specialised
glycoprotein Glycoproteins are protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within organisms, including , , , providing and , and from one location to another. Proteins dif ...
s called
integrin Integrins are transmembrane receptors Cell surface receptors (membrane receptors, transmembrane receptors) are receptor (biochemistry), receptors that are embedded in the cell membrane, plasma membrane of cell (biology), cells. They act in cell ...

integrin
s.


Physiology

The physiology of cardiac muscle shares many similarities with that of
skeletal muscle Skeletal muscles (commonly referred to as muscles) are Organ (biology), organs of the vertebrate muscular system that are mostly attached by tendons to bones of the skeleton. The muscle cells of skeletal muscles are much longer than in the other ...

skeletal muscle
. The primary function of both muscle types is to contract, and in both cases, a contraction begins with a characteristic flow of
ion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ...
s across the
cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane A biological membrane, biomembrane or cell membrane is a selectively permeable membra ...

cell membrane
known as an
action potential In physiology Physiology (; ) is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence ...

action potential
. The action potential subsequently triggers muscle contraction by increasing the concentration of
calcium Calcium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

calcium
within the cytosol. However, the mechanism by which calcium concentrations within the cytosol rise differ between skeletal and cardiac muscle. In cardiac muscle, the action potential comprises an inward flow of both sodium and calcium ions. The flow of sodium ions is rapid but very short-lived, while the flow of calcium is sustained and gives the plateau phase characteristic of cardiac muscle action potentials. The comparatively small flow of calcium through the
L-type calcium channel 308x308px, An L-type calcium channel with its subunits labeled along with some drugs known to inhibit the channel. The L-type calcium channel (also known as the dihydropyridine channel, or DHP receptor, DHP channel) is part of the high-voltage ac ...

L-type calcium channel
s triggers a much larger release of calcium from the sarcoplasmic reticulum in a phenomenon known as
calcium-induced calcium release Calcium-induced calcium release (CICR) describes a biological process whereby calcium Calcium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numb ...
. In contrast, in skeletal muscle, minimal calcium flows into the cell during action potential and instead the sarcoplasmic reticulum in these cells is directly coupled to the surface membrane. This difference can be illustrated by the observation that cardiac muscle fibers require calcium to be present in the solution surrounding the cell to contract, while skeletal muscle fibers will contract without extracellular calcium. During contraction of a cardiac muscle cell, the long protein
myofilament Myofilaments are the two protein filament In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular inte ...

myofilament
s oriented along the length of the cell slide over each other in what is known as the sliding filament hypothesis. There are two kinds of myofilaments, thick filaments composed of the protein
myosin Myosins () are a superfamily SUPERFAMILY is a database and search platform of structural and functional annotation for all proteins and genomes. It classifies amino acid sequences into known structural domains, especially into SCOP superfamilie ...

myosin
, and thin filaments composed of the proteins
actin Actin is a protein family, family of Globular protein, globular multi-functional proteins that form microfilaments. It is found in essentially all Eukaryote, eukaryotic cells, where it may be present at a concentration of over 100 Micromolar, μ ...
,
troponin image:Troponin Ribbon Diagram.png, 400px, Ribbon representation of the human cardiac troponin core complex (52 kDa core) in the calcium-saturated form. Blue = troponin C; green = troponin I; magenta = troponin T.; ; rendered with PyMOL Troponin, or ...
and
tropomyosin Tropomyosin is a two-stranded alpha-helical, coiled coil protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of ...

tropomyosin
. As the thick and thin filaments slide past each other the cell becomes shorter and fatter. In a mechanism known as crossbridge cycling, calcium ions bind to the protein troponin, which along with tropomyosin then uncover key binding sites on actin. Myosin, in the thick filament, can then bind to actin, pulling the thick filaments along the thin filaments. When the concentration of calcium within the cell falls, troponin and tropomyosin once again cover the binding sites on actin, causing the cell to relax.


Regeneration

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. Olaf Bergmann and his colleagues at the
Karolinska Institute The Karolinska Institute (KI; sv, Karolinska Institutet; sometimes known as the (Royal) Caroline Institute in English) is a research-led Medical school, medical university in Solna Municipality, Solna within the Stockholm urban area of Sweden. T ...
in
Stockholm Stockholm (; ) is the capital Capital most commonly refers to: * Capital letter Letter case (or just case) is the distinction between the letters that are in larger uppercase or capitals (or more formally ''majuscule'') and smalle ...

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. In the 2000s, the discovery of adult endogenous cardiac stem cells was reported, and studies were published that claimed that various stem cell lineages, including were able to differentiate into cardiomyocytes, and could be used to treat
heart failure Heart failure (HF), also known as congestive heart failure (CHF) and (congestive) cardiac failure (CCF), is a set of manifestations caused by the failure of the heart The heart is a cardiac muscle, muscular Organ (biology), organ in mo ...
. However, other teams were unable to replicate these findings, and many of the original studies were later retracted for scientific fraud.


Differences between atria and ventricles

Cardiac muscle forms both the atria and the ventricles of the heart. Although this muscle tissue is very similar between cardiac chambers, some differences exist. The myocardium found in the ventricles is thick to allow forceful contractions, while the myocardium in the atria is much thinner. The individual myocytes that make up the myocardium also differ between cardiac chambers. Ventricular cardiomyocytes are longer and wider, with a denser
T-tubule T-tubules (transverse tubules) are extensions of the sarcolemma, cell membrane that penetrate into the centre of Skeletal muscle#Skeletal muscle fibers, skeletal and cardiac muscle cells. With membranes that contain large concentrations of ion ...
network. Although the fundamental mechanisms of calcium handling are similar between ventricular and atrial cardiomyocytes, the calcium transient is smaller and decays more rapidly in atrial myocytes, with a corresponding increase in
calcium bufferingCalcium buffering describes the processes which help stabilise the concentration of free calcium ions within cells, in a similar manner to how pH buffers maintain a stable concentration of hydrogen ions. The majority of calcium ions within the cell ...
capacity. The complement of ion channels differs between chambers, leading to longer action potential durations and effective refractory periods in the ventricles. Certain ion currents such as ''I''K(UR) are highly specific to atrial cardiomyocytes, making them a potential target for treatments for
atrial fibrillation Atrial fibrillation (AF or A-fib) is an abnormal heart rhythm (arrhythmia) characterized by the rapid and irregular beating of the atrial chambers of the heart. It often begins as short periods of abnormal beating, which become longer or cont ...

atrial fibrillation
.


Clinical significance

Diseases affecting cardiac muscle are of immense clinical significance, and are the leading cause of death in developed nations. The most common condition affecting cardiac muscle is
ischaemic heart disease Coronary artery disease (CAD), also called coronary heart disease (CHD), ischemic heart disease (IHD), or simply heart disease, involves the reduction of blood flow to the heart muscle due to build-up of plaque (atherosclerosis Atheroscleros ...
, in which the blood supply to the heart is reduced. In ischaemic heart disease, the
coronary arteries The coronary arteries are the arterial blood vessels of coronary circulation Coronary circulation is the circulation of blood in the blood vessels that supply the cardiac muscle, heart muscle (myocardium). Coronary arteries supply oxygen saturat ...

coronary arteries
become narrowed by
atherosclerosis Atherosclerosis is a disease in which the wall of the artery An artery (plural arteries) () is a blood vessel The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels tr ...

atherosclerosis
. If these narrowings gradually become severe enough to partially restrict blood flow, the syndrome of
angina Angina, also known as angina pectoris, is chest pain Chest pain is pain or discomfort in the chest, typically the front of the chest. It may be described as sharp, dull, pressure, heaviness or squeezing. Associated symptoms may include pain in ...

angina
pectoris may occur. This typically causes chest pain during exertion that is relieved by rest. If a coronary artery suddenly becomes very narrowed or completely blocked, interrupting or severely reducing blood flow through the vessel, a
myocardial infarction A myocardial infarction (MI), commonly known as a heart attack, occurs when blood flow Hemodynamics American and British English spelling differences#ae and oe, or haemodynamics are the Fluid dynamics, dynamics of blood flow. The circulatory sy ...

myocardial infarction
or heart attack occurs. If the blockage is not relieved promptly by
medication A medication (also called medicament, medicine, pharmaceutical drug, medicinal drug or simply drug) is a drug used to medical diagnosis, diagnose, cure, therapy, treat, or preventive medicine, prevent disease. Drug therapy (pharmacotherapy) ...
,
percutaneous coronary intervention Percutaneous coronary intervention (PCI) is a non-surgical procedure used to treat narrowing of the coronary arteries The coronary arteries are the arterial blood vessels of coronary circulation, which transport oxygenated blood to the heart m ...
, or
surgery Surgery ''cheirourgikē'' (composed of χείρ, "hand", and ἔργον, "work"), via la, chirurgiae, meaning "hand work". is a medical or dental specialty that uses operative manual and instrumental techniques on a person to investigate or t ...
, then a heart muscle region may become permanently scarred and damaged. Heart muscle can also become damaged despite a normal blood supply. The heart muscle may become inflamed in a condition called
myocarditis Myocarditis, also known as inflammatory cardiomyopathy, is inflammation Inflammation (from la, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants, and is ...
, most commonly caused by a viral infection but sometimes caused by the body's own
immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as Tumor immunology, cancer cells and objects such ...
. Heart muscle can also be damaged by drugs such as alcohol, long standing high blood pressure or
hypertension Hypertension (HTN or HT), also known as high blood pressure (HBP), is a Chronic condition, long-term Disease, medical condition in which the blood pressure in the artery, arteries is persistently elevated. High blood pressure usually does not ...

hypertension
, or persistent abnormal heart racing. Specific diseases of heart muscle, called cardiomyopathies, can cause heart muscle to become abnormally thick (
hypertrophic cardiomyopathy Hypertrophic cardiomyopathy (HCM) is a condition in which the heart becomes hypertrophy, thickened without an obvious cause. The parts of the heart most commonly affected are the interventricular septum and the ventricles. This results in the hea ...
), abnormally large (
dilated cardiomyopathy Dilated cardiomyopathy (DCM) is a condition in which the Cardiomegaly, heart becomes enlarged and cannot pump blood effectively. Symptoms vary from none to feeling tired, pedal edema, leg swelling, and shortness of breath. It may also result in che ...
), or abnormally stiff (
restrictive cardiomyopathy Restrictive cardiomyopathy (RCM) is a form of cardiomyopathy Cardiomyopathy is a group of diseases that affect the heart muscle Cardiac muscle (also called heart muscle or myocardium) is one of three types of vertebrate Vertebrates () comp ...

restrictive cardiomyopathy
). Some of these conditions are caused by genetic mutations and can be inherited. Many of these conditions, if severe enough, can damage the heart so much that the pumping function of the heart is reduced. If the heart is no longer able to pump enough blood to meet the body's needs, this is described as
heart failure Heart failure (HF), also known as congestive heart failure (CHF) and (congestive) cardiac failure (CCF), is a set of manifestations caused by the failure of the heart The heart is a cardiac muscle, muscular Organ (biology), organ in mo ...
.


See also

* Frank–Starling law of the heart *
Regional function of the heart The assessment of regional function of the heart is a powerful tool for the detection of deterioration of a certain parts of the heart wall early on and before it diffuses further. One of the most accurate measures of changes in regional function is ...
* Nebulette


References


External links


Cardiac muscle histology
{{Authority control Cardiac anatomy Muscular system Cardiac electrophysiology Muscle tissue Articles containing video clips Histology