Cardiac muscle (also called heart muscle, myocardium, cardiomyocytes and cardiac myocytes) is one of three types of
vertebrate
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...
muscle tissues, with the other two being
skeletal muscle
Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...
and
smooth muscle
Smooth muscle is an involuntary non-striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit mus ...
. It is an involuntary,
striated muscle
Striations means a series of ridges, furrows or linear marks, and is used in several ways:
* Glacial striation
* Striation (fatigue), in material
* Striation (geology), a ''striation'' as a result of a geological fault
* Striation Valley, in Ant ...
that constitutes the main tissue of the
wall of the heart. The cardiac muscle (myocardium) forms a thick middle layer between the outer layer of the heart wall (the
pericardium
The pericardium, also called pericardial sac, is a double-walled sac containing the heart and the roots of the great vessels. It has two layers, an outer layer made of strong connective tissue (fibrous pericardium), and an inner layer made of ...
) and the inner layer (the
endocardium
The endocardium is the innermost layer of tissue that lines the chambers of the heart. Its cells are embryologically and biologically similar to the endothelial cells that line blood vessels. The endocardium also provides protection to the va ...
), with blood supplied via the
coronary circulation
Coronary circulation is the circulation of blood in the blood vessels that supply the heart muscle (myocardium).
Coronary arteries supply oxygenated blood to the heart muscle. Cardiac veins then drain away the blood after it has been deoxygenat ...
. It is composed of individual
cardiac muscle cells
Cardiac muscle (also called heart muscle, myocardium, cardiomyocytes and cardiac myocytes) is one of three types of vertebrate muscle tissues, with the other two being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that ...
joined by
intercalated disc
Intercalated discs or lines of Eberth are microscopic identifying features of cardiac muscle. Cardiac muscle consists of individual heart muscle cells (cardiomyocytes) connected by intercalated discs to work as a single functional syncytium. By con ...
s, and encased by
collagen fibers
Type I collagen is the most abundant collagen of the human body. It forms large, eosinophilic fibers known as collagen fibers.
It is present in scar tissue, the end product when tissue heals by repair, as well as tendons, ligaments, the endomys ...
and other substances that form the
extracellular matrix
In biology, the extracellular matrix (ECM), also called intercellular matrix, is a three-dimensional network consisting of extracellular macromolecules and minerals, such as collagen, enzymes, glycoproteins and hydroxyapatite that provide stru ...
.
Cardiac muscle
contracts
A contract is a legally enforceable agreement between two or more parties that creates, defines, and governs mutual rights and obligations between them. A contract typically involves the transfer of goods, services, money, or a promise to tran ...
in a similar manner to
skeletal muscle
Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...
, although with some important differences. Electrical stimulation in the form of a
cardiac action potential
The cardiac action potential is a brief change in voltage ( membrane potential) across the cell membrane of heart cells. This is caused by the movement of charged atoms (called ions) between the inside and outside of the cell, through proteins c ...
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 in bio ...
. The rise in calcium causes the cell's
myofilament
Myofilaments are the three protein filaments of myofibrils in muscle cells. The main proteins involved are myosin, actin, and titin. Myosin and actin are the ''contractile proteins'' and titin is an elastic protein. The myofilaments act togethe ...
s to slide past each other in a process called
excitation-contraction coupling
Muscle contraction is the activation of tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as w ...
.
Diseases of the heart muscle known as
cardiomyopathies
Cardiomyopathy is a group of diseases that affect the heart muscle. Early on there may be few or no symptoms. As the disease worsens, shortness of breath, feeling tired, and swelling of the legs may occur, due to the onset of heart failure. A ...
are of major importance. These include
ischemic
Ischemia or ischaemia is a restriction in blood supply to any tissue, muscle group, or organ of the body, causing a shortage of oxygen that is needed for cellular metabolism (to keep tissue alive). Ischemia is generally caused by problems wi ...
conditions caused by a restricted blood supply to the muscle such as
angina
Angina, also known as angina pectoris, is chest pain or pressure, usually caused by ischemia, insufficient blood flow to the Cardiac muscle, heart muscle (myocardium). It is most commonly a symptom of coronary artery disease.
Angina is typical ...
, and
myocardial infarction
A myocardial infarction (MI), commonly known as a heart attack, occurs when blood flow decreases or stops to the coronary artery of the heart, causing damage to the heart muscle. The most common symptom is chest pain or discomfort which may ...
.
Structure
Gross anatomy
![Cardiac Muscle](https://upload.wikimedia.org/wikipedia/commons/7/73/Cardiac_Muscle.png)
![Cardiac sarcomere structure](https://upload.wikimedia.org/wikipedia/commons/7/7f/Cardiac_sarcomere_structure.png)
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. Its cells are embryologically and biologically similar to the endothelial cells that line blood vessels. The endocardium also provides protection to the va ...
and the outer
epicardium
The pericardium, also called pericardial sac, is a double-walled sac containing the heart and the roots of the great vessels. It has two layers, an outer layer made of strong connective tissue (fibrous pericardium), and an inner layer made of ...
(also known as the visceral pericardium). The inner endocardium lines the cardiac chambers, covers the
cardiac valves
A heart valve is a Check valve, one-way valve that allows blood flow, blood to flow in one direction through the chambers of the heart. Four valves are usually present in a mammalian heart and together they determine the pathway of blood flow thro ...
, and joins with the
endothelium
The endothelium is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and the rest of the vessel ...
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 and the roots of the great vessels. It has two layers, an outer layer made of strong connective tissue (fibrous pericardium), and an inner layer made of ...
which forms part of the
pericardial sac
The pericardium, also called pericardial sac, is a double-walled sac containing the heart and the roots of the great vessels. It has two layers, an outer layer made of strong connective tissue (fibrous pericardium), and an inner layer made of ...
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 with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
and nutrients.
Blood
Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the c ...
is brought to the myocardium by the
coronary arteries
The coronary arteries are the arterial blood vessels of coronary circulation, which transport oxygenated blood to the heart muscle. The heart requires a continuous supply of oxygen to function and survive, much like any other tissue or organ of ...
. These originate from the
aortic root
The ascending aorta (AAo) is a portion of the aorta commencing at the upper part of the base of the left ventricle, on a level with the lower border of the third costal cartilage behind the left half of the sternum.
Structure
It passes obliqu ...
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 ( la, ātrium, , entry hall) is one of two upper chambers in the heart that receives blood from the circulatory system. The blood in the atria is pumped into the heart ventricles through the atrioventricular valves.
There are two atr ...
.
Microanatomy
![Myocardiocyte](https://upload.wikimedia.org/wikipedia/commons/c/c6/Myocardiocyte.png)
Cardiac muscle cells also called cardiomyocytes are the contractile
myocyte
A muscle cell is also known as a myocyte when referring to either a cardiac muscle cell (cardiomyocyte), or a smooth muscle cell as these are both small cells. A skeletal muscle cell is long and threadlike with many nuclei and is called a muscl ...
s of the cardiac muscle. The cells are surrounded by an
extracellular matrix
In biology, the extracellular matrix (ECM), also called intercellular matrix, is a three-dimensional network consisting of extracellular macromolecules and minerals, such as collagen, enzymes, glycoproteins and hydroxyapatite that provide stru ...
produced by supporting
fibroblast
A fibroblast is a type of cell (biology), biological cell that synthesizes the extracellular matrix and collagen, produces the structural framework (Stroma (tissue), stroma) for animal Tissue (biology), tissues, and plays a critical role in wound ...
cells. Specialised modified cardiomyocytes known as
pacemaker cell
350px, Image showing the cardiac pacemaker or SA node, the primary pacemaker within the electrical_conduction_system_of_the_heart">SA_node,_the_primary_pacemaker_within_the_electrical_conduction_system_of_the_heart.
The_muscle_contraction.htm ...
s, set the rhythm of the heart contractions. The pacemaker cells are only weakly contractile without sarcomeres, and are connected to neighboring contractile cells via
gap junction
Gap junctions are specialized intercellular connections between a multitude of animal cell-types. They directly connect the cytoplasm of two cells, which allows various molecules, ions and electrical impulses to directly pass through a regulate ...
s.
They are located in the
sinoatrial node
The sinoatrial node (also known as the sinuatrial node, SA node or sinus node) is an oval shaped region of special cardiac muscle in the upper back wall of the right atrium made up of cells known as pacemaker cells. The sinus node is approximat ...
(the primary pacemaker) positioned on the wall of the
right atrium
The atrium ( la, ātrium, , entry hall) is one of two upper chambers in the heart that receives blood from the circulatory system. The blood in the atria is pumped into the heart ventricles through the atrioventricular valves.
There are two atr ...
, near the entrance of the
superior vena cava
The superior vena cava (SVC) is the superior of the two venae cavae, the great venous trunks that return deoxygenated blood from the systemic circulation to the right atrium of the heart. It is a large-diameter (24 mm) short length vein th ...
. Other pacemaker cells are found in the atrioventricular node (secondary pacemaker).
Pacemaker cells carry the impulses that are responsible for the beating of the heart. They are distributed throughout the heart and are responsible for several functions. First, they are responsible for being able to
spontaneously generate and send out
electrical impulses. They also must be able to receive and respond to electrical impulses from the brain. Lastly, they must be able to transfer electrical impulses from cell to cell. Pacemaker cells in the sinoatrial node, and atrioventricular node are smaller and conduct at a relatively slow rate between the cells. Specialized conductive cells in the
bundle of His
The bundle of His (BH) or His bundle (HB) ( "hiss"Medical Terminology for Health Professions, Spiral bound Version'. Cengage Learning; 2016. . pp. 129–.) is a collection of heart muscle cells specialized for electrical conduction. As part of t ...
, and the
Purkinje fibers
The Purkinje fibers (; often incorrectly ; Purkinje tissue or subendocardial branches) are located in the inner ventricular walls of the heart, just beneath the endocardium in a space called the subendocardium. The Purkinje fibers are specia ...
are larger in diameter and conduct signals at a fast rate.
The Purkinje fibers rapidly conduct electrical signals;
coronary arteries
The coronary arteries are the arterial blood vessels of coronary circulation, which transport oxygenated blood to the heart muscle. The heart requires a continuous supply of oxygen to function and survive, much like any other tissue or organ of ...
to bring nutrients to the muscle cells, and
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 and umbilical veins, both of which carry oxygenated b ...
and a
capillary
A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the body: ...
network to take away waste products.
Cardiac muscle cells 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 quiver. It is due to disorganized electrical activity. Ventricular fibrillation results in cardiac arrest with loss of consciousness and no p ...
.
Viewed through a microscope, cardiac muscle cells are roughly rectangular, measuring 100–150μm by 30–40μm.
Individual cardiac muscle cells are joined at their ends by
intercalated disc
Intercalated discs or lines of Eberth are microscopic identifying features of cardiac muscle. Cardiac muscle consists of individual heart muscle cells (cardiomyocytes) connected by intercalated discs to work as a single functional syncytium. By con ...
s to form long fibers. Each cell contains
myofibril
A myofibril (also known as a muscle fibril or sarcostyle) is a basic rod-like organelle of a muscle cell. Skeletal muscles are composed of long, tubular cells known as muscle fibers, and these cells contain many chains of myofibrils. Each myofib ...
s, specialized protein contractile fibers of
actin
Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of over ...
and
myosin
Myosins () are a superfamily of motor proteins best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are ATP-dependent and responsible for actin-based motility.
The first myosin ...
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. It is the repeating unit between two Z-lines. Skeletal muscles are composed of tubular muscle cells (called musc ...
s, the fundamental contractile units of muscle cells. The regular organization of myofibrils into sarcomeres gives cardiac muscle cells a striped or
striated appearance when looked at through a microscope, similar to skeletal muscle. These striations are caused by lighter
I bands
I, or i, is the ninth letter and the third vowel letter of the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''i'' (pronounced ), plural ...
composed mainly of actin, and darker
A bands composed mainly of myosin.
Cardiomyocytes contain
T-tubule
T-tubules (transverse tubules) are extensions of the cell membrane that penetrate into the center of skeletal and cardiac muscle cells. With membranes that contain large concentrations of ion channels, transporters, and pumps, T-tubules permit ...
s, pouches of
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 that separates and protects the interior of all cells from the outside environment ( ...
that run from the cell surface to the cell's interior which help to improve the efficiency of contraction. The majority of these cells contain only one
nucleus
Nucleus ( : 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
Nucle ...
(some may have two central nuclei), unlike skeletal muscle cells which contain
many nuclei. Cardiac muscle cells contain many
mitochondria
A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...
which provide the energy needed for the cell in the form of
adenosine triphosphate
Adenosine triphosphate (ATP) is an organic compound that provides energy to drive many processes in living cells, such as muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis. Found in all known forms of ...
(ATP), making them highly resistant to fatigue.
T-tubules
T-tubule
T-tubules (transverse tubules) are extensions of the cell membrane that penetrate into the center of skeletal and cardiac muscle cells. With membranes that contain large concentrations of ion channels, transporters, and pumps, T-tubules permit ...
s 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 made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cells. The cell membranes of almost all organisms and many vir ...
, and are open at the cell surface to the
extracellular fluid
In cell biology, extracellular fluid (ECF) denotes all body fluid outside the cells of any multicellular organism. Total body water in healthy adults is about 60% (range 45 to 75%) of total body weight; women and the obese typically have a lower ...
that surrounds the cell. T-tubules in cardiac muscle are bigger and wider than those in
skeletal muscle
Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...
, but fewer in number.
In the centre of the cell they join, 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 in bio ...
. Here, a single tubule pairs with part of the sarcoplasmic reticulum called a terminal cisterna in a combination known as a
diad
Within the muscle tissue of animals and humans, contraction and relaxation of the muscle cells (myocytes) is a highly regulated and rhythmic process. In cardiomyocytes, or cardiac muscle cells, muscular contraction takes place due to movement at a ...
.
The functions of T-tubules include rapidly transmitting electrical impulses known as
action potentials
An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, c ...
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-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as w ...
.
They are also involved in mechano-electric feedback, as evident from cell contraction induced T-tubular content exchange (advection-assisted diffusion), which was confirmed by confocal and 3D electron tomography observations.
Intercalated discs
![1020 Cardiac Muscle](https://upload.wikimedia.org/wikipedia/commons/7/7e/1020_Cardiac_Muscle.jpg)
The
cardiac syncytium is a network of cardiomyocytes connected by
intercalated disc
Intercalated discs or lines of Eberth are microscopic identifying features of cardiac muscle. Cardiac muscle consists of individual heart muscle cells (cardiomyocytes) connected by intercalated discs to work as a single functional syncytium. By con ...
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
In physiology, the all-or-none law (sometimes the all-or-none principle or all-or-nothing law) is the principle that if a single nerve fibre is stimulated, it will always give a maximal response and produce an electrical impulse of a single ampl ...
.
Intercalated discs are complex adhering structures that connect the single cardiomyocytes to an electrochemical
syncytium
A syncytium (; plural syncytia; from Greek: σύν ''syn'' "together" and κύτος ''kytos'' "box, i.e. cell") or symplasm is a multinucleate cell which can result from multiple cell fusions of uninuclear cells (i.e., cells with a single nucleus) ...
(in contrast to the skeletal muscle, which becomes a multicellular syncytium during
embryonic development
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...
). 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
fascia adherens junctions, the intermediate filament anchoring
desmosomes
A desmosome (; "binding body"), also known as a macula adherens (plural: maculae adherentes) (Latin for ''adhering spot''), is a cell structure specialized for cell-to-cell adhesion. A type of junctional complex, they are localized spot-like adh ...
, and
gap junctions
Gap junctions are specialized intercellular connections between a multitude of animal cell-types. They directly connect the cytoplasm of two cells, which allows various molecules, ions and electrical impulses to directly pass through a regulate ...
.
They allow action potentials to spread between cardiac cells by permitting the passage of ions between cells, producing depolarization of the heart muscle. The three types of junction act together as a single
area composita
Intercalated discs or lines of Eberth are microscopic identifying features of cardiac muscle. Cardiac muscle consists of individual heart muscle cells (cardiomyocytes) connected by intercalated discs to work as a single functional syncytium. By con ...
.
Under
light microscopy
Microscopy is the technical field of using microscopes to view objects and areas of objects that cannot be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of micr ...
, 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 (also called heart muscle, myocardium, cardiomyocytes and cardiac myocytes) is one of three types of vertebrate muscle tissues, with the other two being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that ...
, but instead are largely responsible for creating and maintaining the extracellular matrix which surrounds the cardiomyocytes.
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 decreases or stops to the coronary artery of the heart, causing damage to the heart muscle. The most common symptom is chest pain or discomfort which may ...
. Following injury, fibroblasts can become activated and turn into
myofibroblast
A myofibroblast is a cell phenotype that was first described as being in a state between a fibroblast and a smooth muscle cell.
Structure
Myofibroblasts are contractile web-like fusiform cells that are identifiable by their expression of α-sm ...
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, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit mus ...
(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, as originally shown in vitro in the 1960s, and ultimately confirmed in native cardiac tissue with the help of optogenetic techniques. Other potential roles for fibroblasts include electrical insulation of the
cardiac conduction system
The cardiac conduction system (CCS) (also called the electrical conduction system of the heart) transmits the signals generated by the sinoatrial node – the heart's pacemaker, to cause the heart muscle to contract, and pump blood through 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, specialized in storing energy as fat. Adipocytes are derived from mesenchymal stem cells which give rise to adipocytes through adipogenesis. I ...
s.
Extracellular matrix
The
extracellular matrix
In biology, the extracellular matrix (ECM), also called intercellular matrix, is a three-dimensional network consisting of extracellular macromolecules and minerals, such as collagen, enzymes, glycoproteins and hydroxyapatite that provide stru ...
(ECM) surrounds the cardiomyocyte and fibroblasts. The ECM is composed of proteins including
collagen
Collagen () is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole ...
and
elastin
Elastin is a protein that in humans is encoded by the ''ELN'' gene. Elastin is a key component of the extracellular matrix in gnathostomes (jawed vertebrates). It is highly elastic and present in connective tissue allowing many tissues in the bod ...
along with
polysaccharide
Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wa ...
s (sugar chains) known as
glycosaminoglycan
Glycosaminoglycans (GAGs) or mucopolysaccharides are long, linear polysaccharides consisting of repeating disaccharide units (i.e. two-sugar units). The repeating two-sugar unit consists of a uronic sugar and an amino sugar, except in the case ...
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 that provides cell and tissue support and acts as a platform for complex signalling. The basement membrane sits between Epithelium, epithelial tissues including mesot ...
, mainly composed of
type IV collagen
Collagen IV (ColIV or Col4) is a type of collagen found primarily in the basal lamina. The collagen IV C4 domain at the C-terminus is not removed in post-translational processing, and the fibers link head-to-head, rather than in parallel. Also, ...
and
laminin
Laminins are a family of glycoproteins of the extracellular matrix of all animals. They are major components of the basal lamina (one of the layers of the basement membrane), the protein network foundation for most cells and organs. The laminins ...
. Cardiomyocytes are linked to the basement membrane via specialised
glycoprotein
Glycoproteins are proteins which contain oligosaccharide chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycos ...
s called
integrin
Integrins are transmembrane receptors that facilitate cell-cell and cell-extracellular matrix (ECM) adhesion. Upon ligand binding, integrins activate signal transduction pathways that mediate cellular signals such as regulation of the cell cycle, ...
s.
Development
Humans are born with a set number of heart muscle cells, or cardiomyocytes, which increase in size as the heart grows larger during childhood development. Evidence suggests that cardiomyocytes are slowly turned over during aging, but less than 50% of the cardiomyocytes present at birth are replaced during a normal life span. The growth of individual cardiomyocytes not only occurs during normal heart development, it also occurs in response to extensive exercise (
athletic heart syndrome
Athletic heart syndrome (AHS) is a non-disease, pathological condition commonly seen in sports medicine in which the human heart is cardiomegaly, enlarged, and the resting heart rate is bradycardia, lower than normal.
The athlete's heart is asso ...
), heart disease, or heart muscle injury such as after a myocardial infarction. A healthy adult cardiomyocyte has a cylindrical shape that is approximately 100μm long and 10–25μm in diameter. Cardiomyocyte hypertrophy occurs through sarcomerogenesis, the creation of new sarcomere units in the cell. During heart volume overload, cardiomyocytes grow through eccentric hypertrophy.
The cardiomyocytes extend lengthwise but have the same diameter, resulting in ventricular dilation. During heart pressure overload, cardiomyocytes grow through concentric hypertrophy.
The cardiomyocytes grow larger in diameter but have the same length, resulting in heart wall thickening.
Physiology
The physiology of cardiac muscle shares many similarities with that of
skeletal muscle
Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...
. 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 or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
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 that separates and protects the interior of all cells from the outside environment ( ...
known as an
action potential
An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...
. The
cardiac action potential
The cardiac action potential is a brief change in voltage ( membrane potential) across the cell membrane of heart cells. This is caused by the movement of charged atoms (called ions) between the inside and outside of the cell, through proteins c ...
subsequently triggers muscle contraction by increasing the concentration of
calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to ...
within the cytosol.
Cardiac cycle
The
cardiac cycle
The cardiac cycle is the performance of the human heart from the beginning of one heartbeat to the beginning of the next. It consists of two periods: one during which the heart muscle relaxes and refills with blood, called diastole, following ...
is the performance of the human heart from the beginning of one heartbeat to the beginning of the next. It consists of two periods: one during which the heart muscle relaxes and refills with blood, called
diastole
Diastole ( ) is the relaxed phase of the cardiac cycle when the chambers of the heart are re-filling with blood. The contrasting phase is systole when the heart chambers are contracting. Atrial diastole is the relaxing of the atria, and ventric ...
, following a period of robust contraction and pumping of blood, dubbed
systole
Systole ( ) is the part of the cardiac cycle during which some chambers of the heart contract after refilling with blood. The term originates, via New Latin, from Ancient Greek (''sustolē''), from (''sustéllein'' 'to contract'; from ''sun ...
. After emptying, the heart immediately relaxes and expands to receive another influx of blood returning from the lungs and other systems of the body, before again contracting to pump blood to the lungs and those systems. A normally performing heart must be fully expanded before it can efficiently pump again.
The rest phase is considered polarized. The
resting potential A relatively static membrane potential which is usually referred to as the ground value for trans-membrane voltage.
The relatively static membrane potential of quiescent cells is called the resting membrane potential (or resting voltage), as oppo ...
during this phase of the beat separates the ions such as sodium, potassium, and calcium. Myocardial cells possess the property of automaticity or spontaneous
depolarization
In biology, depolarization or hypopolarization is a change within a cell, during which the cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell compared to the outside. Depolarization is esse ...
. This is the direct result of a membrane which allows sodium ions to slowly enter the cell until the threshold is reached for depolarization. Calcium ions follow and extend the depolarization even further. Once calcium stops moving inward, potassium ions move out slowly to produce repolarization. The very slow repolarization of the CMC membrane is responsible for the long refractory period.
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
The L-type calcium channel (also known as the dihydropyridine channel, or DHP channel) is part of the high-voltage activated family of voltage-dependent calcium channel.
"L" stands for long-lasting referring to the length of activation. This c ...
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 is able to activate calcium release from intracellular Ca2+ stores (e.g., endoplasmic reticulum or sarcoplasmic reticulum). Although CICR was first proposed for ...
. 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 three protein filaments of myofibrils in muscle cells. The main proteins involved are myosin, actin, and titin. Myosin and actin are the ''contractile proteins'' and titin is an elastic protein. The myofilaments act togethe ...
s oriented along the length of the cell slide over each other in what is known as the
sliding filament theory
The sliding filament theory explains the mechanism of muscle contraction based on muscle proteins that slide past each other to generate movement. According to the sliding filament theory, the myosin (thick filaments) of muscle fibers slide past ...
. There are two kinds of myofilaments, thick filaments composed of the protein
myosin
Myosins () are a superfamily of motor proteins best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are ATP-dependent and responsible for actin-based motility.
The first myosin ...
, and thin filaments composed of the proteins
actin
Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of over ...
,
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, ...
and
tropomyosin
Tropomyosin is a two-stranded alpha-helical, coiled coil protein found in actin-based cytoskeletons.
Tropomyosin and the actin skeleton
All organisms contain organelles that provide physical integrity to their cells. These type of organelles ar ...
. As the thick and thin filaments slide past each other the cell becomes shorter and fatter. In a mechanism known as
cross-bridge cycling
Muscle contraction is the activation of Tension (physics), tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscl ...
, 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
![Dogcardiacmuscle400](https://upload.wikimedia.org/wikipedia/commons/b/b3/Dogcardiacmuscle400.jpg)
It was commonly believed that cardiac muscle cells could not be regenerated. However, this was contradicted by a report published in 2009.
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 university in Solna within the Stockholm urban area of Sweden. The Karolinska Institute is consist ...
in
Stockholm
Stockholm () is the Capital city, capital and List of urban areas in Sweden by population, largest city of Sweden as well as the List of urban areas in the Nordic countries, largest urban area in Scandinavia. Approximately 980,000 people liv ...
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
bone marrow stem cells were able to differentiate into cardiomyocytes, and could be used to treat
heart failure
Heart failure (HF), also known as congestive heart failure (CHF), is a syndrome, a group of signs and symptoms caused by an impairment of the heart's blood pumping function. Symptoms typically include shortness of breath, excessive fatigue, a ...
.
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
![2006 Heart Musculature](https://upload.wikimedia.org/wikipedia/commons/0/0e/2006_Heart_Musculature.jpg)
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 cell membrane that penetrate into the center of skeletal and cardiac muscle cells. With membranes that contain large concentrations of ion channels, transporters, and pumps, T-tubules permit ...
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 buffering Calcium buffering describes the processes which help stabilise the concentration of free calcium ions within cells, in a similar manner to how Buffer solution, pH buffers maintain a stable concentration of hydrogen ions. The majority of calcium ion ...
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.
Clinical significance
Diseases affecting cardiac muscle, known as
cardiomyopathies
Cardiomyopathy is a group of diseases that affect the heart muscle. Early on there may be few or no symptoms. As the disease worsens, shortness of breath, feeling tired, and swelling of the legs may occur, due to the onset of heart failure. A ...
, are the leading cause of death in
developed countries
A developed country (or industrialized country, high-income country, more economically developed country (MEDC), advanced country) is a sovereign state that has a high quality of life, developed economy and advanced technological infrastruct ...
. The most common condition is
coronary artery disease
Coronary artery disease (CAD), also called coronary heart disease (CHD), ischemic heart disease (IHD), myocardial ischemia, or simply heart disease, involves the reduction of blood flow to the heart muscle due to build-up of atherosclerotic pla ...
, in which the
blood supply to the heart is reduced. The
coronary arteries
The coronary arteries are the arterial blood vessels of coronary circulation, which transport oxygenated blood to the heart muscle. The heart requires a continuous supply of oxygen to function and survive, much like any other tissue or organ of ...
become narrowed by
the formation of atherosclerotic plaques.
If these narrowings become severe enough to partially restrict blood flow, the syndrome of
angina
Angina, also known as angina pectoris, is chest pain or pressure, usually caused by ischemia, insufficient blood flow to the Cardiac muscle, heart muscle (myocardium). It is most commonly a symptom of coronary artery disease.
Angina is typical ...
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 decreases or stops to the coronary artery of the heart, causing damage to the heart muscle. The most common symptom is chest pain or discomfort which may ...
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 diagnose, cure, treat, or prevent disease. Drug therapy (pharmacotherapy) is an important part of the medical field and re ...
,
percutaneous coronary intervention
Percutaneous coronary intervention (PCI) is a non-surgical procedure used to treat narrowing of the coronary arteries of the heart found in coronary artery disease. The process involves combining coronary angioplasty with stenting, which is the ...
, or
surgery
Surgery ''cheirourgikē'' (composed of χείρ, "hand", and ἔργον, "work"), via la, chirurgiae, meaning "hand work". is a medical specialty that uses operative manual and instrumental techniques on a person to investigate or treat a pat ...
, then a heart muscle region may become permanently scarred and damaged. A specific cardiomyopathy, can cause heart muscle to become abnormally thick (
hypertrophic cardiomyopathy
Hypertrophic cardiomyopathy (HCM, or HOCM when obstructive) is a condition in which the heart becomes thickened without an obvious cause. The parts of the heart most commonly affected are the interventricular septum and the ventricles. This r ...
), abnormally large (
dilated cardiomyopathy
Dilated cardiomyopathy (DCM) is a condition in which the heart becomes enlarged and cannot pump blood effectively. Symptoms vary from none to feeling tired, leg swelling, and shortness of breath. It may also result in chest pain or fainting. Co ...
), or abnormally stiff (
restrictive cardiomyopathy
Restrictive cardiomyopathy (RCM) is a form of cardiomyopathy in which the walls of the heart are rigid (but not thickened). Thus the heart is restricted from stretching and filling with blood properly. It is the least common of the three original s ...
). Some of these conditions are caused by genetic mutations and can be inherited.
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 an acquired cardiomyopathy due to inflammation of the heart muscle. Symptoms can include shortness of breath, chest pain, decreased ability to exercise, and an irregular heartbeat. The ...
, 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 cancer cells and objects such as wood splinte ...
.
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 long-term medical condition in which the blood pressure in the arteries is persistently elevated. High blood pressure usually does not cause symptoms. Long-term high bl ...
, or persistent abnormal
heart racing.
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), is a syndrome, a group of signs and symptoms caused by an impairment of the heart's blood pumping function. Symptoms typically include shortness of breath, excessive fatigue, a ...
.
Significant damage to cardiac muscle cells is referred to as
myocytolysis Myocytolysis refers to a state of significant damage to cardiac myocytes, muscle cells of the heart, caused by myocardial strain. It was first described in medical literature by Schlesinger and Reiner in 1955. It is considered a type of cellular nec ...
which is considered a type of cellular
necrosis
Necrosis () is a form of cell injury which results in the premature death of cells in living tissue by autolysis. Necrosis is caused by factors external to the cell or tissue, such as infection, or trauma which result in the unregulated dige ...
defined as either coagulative or colliquative.
See also
*
Frank–Starling law of the heart
*
Nebulette
Nebulette is a cardiac-specific isoform belonging to the nebulin family of proteins. It is encoded by the ''NEBL'' gene. This family is composed of 5 members: nebulette, nebulin, N-RAP, LASP-1 and LASP-2. Nebulette localizes to Z-discs of cardi ...
*
Protein S100-A1
Protein S100-A1, also known as S100 calcium-binding protein A1 is a protein which in humans is encoded by the ''S100A1'' gene. S100A1 is highly expressed in cardiac muscle, cardiac and skeletal muscle, and localizes to sarcomere, Z-discs and sarc ...
*
Regional function of the heart
The assessment of regional function of the heart is a powerful tool for early detection of deterioration in certain parts of the heart wall before a cardiac arrest is diagnosed. One of the most accurate measures of changes in regional function is ...
References
External links
Cardiac muscle histology
{{Authority control
Cardiac anatomy
Muscular system
Cardiac electrophysiology
Muscle tissue
Articles containing video clips
Histology