The Info List - Cardiac Muscle

Cardiac muscle
Cardiac muscle
(heart muscle) is one of the three major types of muscle, the others being skeletal and smooth muscle. It is an involuntary, striated muscle that is found in the walls of the heart. This muscle tissue is known as myocardium, and forms a thick middle layer between the outer layer of the heart wall (the epicardium) and the inner layer (the endocardium). Myocardium
is composed of individual heart muscle cells (cardiomyocytes) joined together by intercalated disks, encased by collagen fibres and other substances forming the extracellular matrix. Cardiac muscle
Cardiac muscle
contracts in a similar manner to skeletal muscle, although with some important differences. An electrical stimulation in the form of an action potential triggers the release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum. The rise in calcium causes the cell's myofilaments to slide past each other in a process called excitation contraction coupling. Diseases of heart muscle are of major importance. These include conditions caused by a restricted blood supply to the muscle including angina pectoris and myocardial infarction, and other heart muscle disease known as cardiomyopathies.


1 Structure

1.1 Gross anatomy 1.2 Histology

1.2.1 Cardiac muscle
Cardiac muscle
cells T-tubules Intercalated discs

1.2.2 Fibroblasts 1.2.3 Extracellular matrix

2 Physiology

2.1 Regeneration 2.2 Differences between atria and ventricles

3 Clinical significance 4 See also 5 References 6 External links

Structure[edit] Gross anatomy[edit] Main article: Heart
§ Structure

3D rendering showing thick myocardium within heart wall.

Cardiac muscle

Cardiac muscle
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 and the outer epicardium (also known as the visceral pericardium). The inner endocardium lines the cardiac chambers, covers the cardiac valves, and joins with the endothelium that lines the blood vessels that connect to the heart. On the outer aspect of the myocardium is the epicardium which forms part of the pericardium, the sack that surrounds, protects, and lubricates the heart.[1] 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 direction 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 out the maximum amount of blood with each heartbeat.[2] Histology[edit] 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 cells or cardiomyocytes. The mortar which surrounds the bricks is known as the extracellular matrix, produced by supporting cells known as fibroblasts. In the same way that the walls of a house contain electrical wires and plumbing, cardiac muscle also contains specialised cells for conducting electrical signals rapidly (the cardiac conduction system), and blood vessels to bring nutrients to the muscle cells and take away waste products (the coronary arteries, veins and capillary network).[3] Cardiac muscle
Cardiac muscle
cells[edit] Main article: Cardiac muscle
Cardiac muscle
cells Cardiac muscle cells
Cardiac muscle cells
or cardiomyocytes are the contracting cells which allow the heart to pump. Each cardiomyocyte needs to contract in coordination with its neighbouring cells 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.[4] Viewed through a microscope, cardiac muscle cells are roughly rectangular, measuring 100–150μm by 30–40μm.[5] Individual cardiac muscle cells are joined together at their ends by intercalated disks to form long fibres. Each cell contains myofibrils, specialised protein fibres that slide past each other. These are organised into sarcomeres, the fundamental contractile units of muscle cells. The regular organisation 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 bands
composed mainly of a protein called actin, and darker A bands composed mainly of myosin.[3] Cardiomyocytes contain T-tubules, pouches of membrane that run from the surface to the cell's interior which help to which improve the efficiency of contraction. The majority of these cells contain only one nucleus (although they may have as many as four), unlike skeletal muscle cells which typically contain many nuclei. Cardiac muscle
Cardiac muscle
cells contain many mitochondria which provide the energy needed for the cell in the form of adenosine triphosphate (ATP), making them highly resistant to fatigue.[5][3] T-tubules[edit] Main article: 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, and are open at the cell surface to the extracellular fluid that surrounds the cell. T-tubules in cardiac muscle are bigger and wider than those in skeletal muscle, but fewer in number.[5] 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. Here, a single tubule pairs with part of the sarcoplasmic reticulum called a terminal cisterna in a combination known as a diad.[6] The functions of T-tubules
include rapidly transmitting electrical impulses known as 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.[5] Intercalated discs[edit] Main article: Intercalated disc

Intercalated discs are part of the cardiac muscle sarcolemma and they contain gap junctions and desmosomes.

The cardiac syncytium is a network of cardiomyocytes connected to each other by intercalated discs that enable the rapid transmission of electrical impulses through the network, enabling the syncytium to act in a coordinated contraction of the myocardium. There is an atrial syncytium and a ventricular syncytium that are connected by cardiac connection fibres.[7] 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.[8] Intercalated discs are complex adhering structures that connect the single cardiomyocytes to an electrochemical syncytium (in contrast to the skeletal muscle, which becomes a multicellular syncytium during mammalian embryonic development). The discs are responsible mainly for force transmission during muscle contraction. Intercalated discs consist of three different types of cell-cell junctions: the actin filament anchoring adherens junctions, the intermediate filament anchoring desmosomes, and gap junctions. They allow action potentials to spread between cardiac cells by permitting the passage of ions between cells, producing depolarization of the heart muscle. However, novel molecular biological and comprehensive studies unequivocally showed that intercalated discs consist for the most part of mixed-type adhering junctions named area composita (pl. areae compositae) representing an amalgamation of typical desmosomal and fascia adhaerens proteins (in contrast to various epithelia).[9][10][11] The authors discuss the high importance of these findings for the understanding of inherited cardiomyopathies (such as arrhythmogenic right ventricular cardiomyopathy). Under light microscopy, intercalated discs appear as thin, typically dark-staining lines dividing adjacent cardiac muscle cells. The intercalated discs run perpendicular to the direction of muscle fibers. Under electron microscopy, an intercalated disc's path appears more complex. At low magnification, this may appear as a convoluted electron dense structure overlying the location of the obscured Z-line. At high magnification, the intercalated disc's path appears even more convoluted, with both longitudinal and transverse areas appearing in longitudinal section.[12] Fibroblasts[edit] Main article: Fibroblasts Cardiac fibroblasts are vital supporting cells within cardiac muscle. They are unable to provide forceful contractions like cardiomyocytes, but instead are largely responsible for creating and maintaining the extracellular matrix which forms the mortar in which cardiomyocyte bricks are embedded.[3] Fibroblasts
play a crucial role in responding to injury, such as a myocardial infarction. Following injury, fibroblasts can become activated and turn into myofibroblasts – cells which exhibit behaviour somewhere between a fibroblast (generating extracellular matrix) and a 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.[13] 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.[14] Other potential roles for fibroblasts include electrical insulation of the cardiac conduction system, and the ability to transform into other cell types including cardiomyocytes and adipocytes.[13] Extracellular matrix[edit] Main article: Extracellular matrix Continuing the analogy of heart muscle as being like a wall, the extracellular matrix is the mortar which surrounds the cardiomyocyte and fibroblasts bricks. The matrix is composed of proteins such as collagen and elastin along with polysaccharides(sugar chains) known as glycosaminoglycans.[3] 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, mainly composed of type IV collagen and laminin. Cardiomyocytes are linked to the basement membrane via specialised glycoproteins called integrins.[15] Physiology[edit]

Play media

An isolated cardiac muscle cell, beating

Main article: Excitation-contraction coupling The physiology of cardiac muscle shares many similarities with that of skeletal muscle. The primary function of both muscle types is to contract, and in both cases a contraction begins with a characteristic flow of ions across the cell membrane known as an action potential. The action potential subsequently triggers muscle contraction by increasing the concentration of 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 channels triggers a much larger release of calcium from the sarcoplasmic reticulum in a phenomenon known as calcium-induced calcium release. 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 fibres require calcium to be present in the solution surrounding the cell in order to contract, while skeletal muscle fibres will contract without extracellular calcium. During contraction of a cardiac muscle cell, the long protein myofilaments oriented along the length of the 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, and thin filaments composed of the proteins actin, troponin and 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[edit]

Dog cardiac muscle (400X)

Until recently, it was commonly believed that cardiac muscle cells could not be regenerated. However, a study reported in the April 3, 2009 issue of Science contradicts that belief.[16] Olaf Bergmann and his colleagues at the Karolinska Institute
Karolinska Institute
in Stockholm
tested samples of heart muscle from people born before 1955 who had very little cardiac muscle around their heart, many showing with disabilities from this abnormality. By using DNA samples from many hearts, the researchers estimated that a 4-year-old renews about 20% of heart muscle cells per year, and about 69 percent of the heart muscle cells of a 50-year-old were generated after he or she was born. One way that cardiomyocyte regeneration occurs is through the division of pre-existing cardiomyocytes during the normal aging process.[17] The division process of pre-existing cardiomyocytes has also been shown to increase in areas adjacent to sites of myocardial injury. In addition, certain growth factors promote the self-renewal of endogenous cardiomyocytes and cardiac stem cells. For example, insulin-like growth factor 1, hepatocyte growth factor, and high-mobility group protein B1 increase cardiac stem cell migration to the affected area, as well as the proliferation and survival of these cells.[18] Some members of the fibroblast growth factor family also induce cell-cycle re-entry of small cardiomyocytes. Vascular endothelial growth factor also plays an important role in the recruitment of native cardiac cells to an infarct site in addition to its angiogenic effect. Based on the natural role of stem cells in cardiomyocyte regeneration, researchers and clinicians are increasingly interested in using these cells to induce regeneration of damaged tissue. Various stem cell lineages have been shown to be able to differentiate into cardiomyocytes, including bone marrow stem cells. For example, in one study, researchers transplanted bone marrow cells, which included a population of stem cells, adjacent to an infarct site in a mouse model. Nine days after surgery, the researchers found a new band of regenerating myocardium.[19] However, this regeneration was not observed when the injected population of cells was devoid of stem cells, which strongly suggests that it was the stem cell population that contributed to the myocardium regeneration. Other clinical trials have shown that autologous bone marrow cell transplants delivered via the infarct-related artery decreases the infarct area compared to patients not given the cell therapy.[20] Differences between atria and ventricles[edit] Cardiac muscle
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
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 capacity.[21] 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 IK(UR) are highly specific to atrial cardiomyocytes, making them a potential target for treatments for atrial fibrillation.[22] Clinical significance[edit] Diseases affecting cardiac muscle are of immense clinical significance, and are the leading cause of death in developed nations.[23] The most common condition affecting cardiac muscle is ischaemic heart disease, in which the blood supply to the heart is reduced. In ischaemic heart disease, the coronary arteries become narrowed by atherosclerosis.[24] If these narrowings gradually become severe enough to partially restrict blood flow, the syndrome of angina pectoris may occur.[24] This typically causes chest pain during exertion that is relieved by rest. If a coronary artery suddenly becomes very narrowed or completed blocked, interrupting or severely reducing blood flow through the vessel, a myocardial infarction or heart attack occurs.[25] If the blockage is not relieved promptly by medication, percutaneous coronary intervention, or surgery, then a region of heart muscle may become permanently scarred and damaged.[26] Heart
muscle can also become damaged despite a normal blood supply. The heart muscle may become inflamed in a condition called myocarditis,[27] most commonly caused by a viral infection[28] but sometimes caused by the body's own immune system.[29] Heart
muscle can also be damaged by drugs such as alcohol, long standing high blood pressure or hypertension, or persistent abnormal heart racing.[30] Specific diseases of heart muscle called cardiomyopathies can cause heart muscle to become abnormally thick (hypertrophic cardiomyopathy),[31] abnormally large (dilated cardiomyopathy),[32] or abnormally stiff (restrictive cardiomyopathy).[33] Some of these conditions are caused by genetic mutations and can be inherited.[34] 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.[30] See also[edit]

Wikimedia Commons has media related to Cardiac muscle.

This article uses anatomical terminology; for an overview, see Anatomical terminology.

Frank–Starling law of the heart Regional function of the heart Nebulette


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External links[edit]

Cardiac muscle
Cardiac muscle

v t e

Muscular system


Muscle tissue

Cardiac muscle Skeletal muscle Smooth muscle


Superficial Deep Visceral

Fascial compartment Tendon/Aponeurosis


Fusiform Pennate muscle

Unipennate Bipennate


Anatomical terms of muscle

Origin Insertion

List of muscles of the human body Composite muscle

v t e

Muscle tissue

Smooth muscle

Calmodulin Vascular smooth muscle

Striated muscle

Skeletal muscle

Costamere/ DAPC

Membrane/ extracellular





Sarcospan Laminin, alpha 2


Dystrophin Dystrobrevin



A B1 B2 G1 G2

Syncoilin Dysbindin Synemin/desmuslin


NOS1 Caveolin 3

Sarcomere/ (a, i, and h bands; z and m lines)


thin filament/actin thick filament/myosin elastic filament/titin nebulin




Connective tissue

Epimysium Fascicle Perimysium Endomysium Connective tissue
Connective tissue
in skeletal muscle


Neuromuscular junction Motor unit Muscle spindle Excitation–contraction coupling Sliding filament mechanism

Cardiac muscle

Myocardium Intercalated disc Nebulette



Muscle fiber

intrafusal extrafusal

Myofibril Microfilament/Myofilament Sarcomere


Myoblast/Myocyte Myosatellite cell


Desmin Sarcoplasm Sarcolemma


Sarcoplasmic reticulum

Other/ ungrouped

Myotilin Telethonin Dysferlin Fukutin Fukutin-related protein

v t e

Anatomy of the heart



base apex sulci

coronary interatrial anterior interventricular posterior interventricular


right left



interatrial septum pectinate muscles terminal sulcus


interventricular septum trabeculae carneae chordae tendineae papillary muscle

valves cusps atrioventricular septum

cardiac skeleton intervenous tubercle


Right heart

(venae cavae, coronary sinus) → right atrium (atrial appendage, fossa ovalis, limbus of fossa ovalis, crista terminalis, valve of inferior vena cava, valve of coronary sinus) → tricuspid valve → right ventricle (infundibulum, moderator band/septomarginal trabecula) → pulmonary valve → (pulmonary artery and pulmonary circulation)

Left heart

(pulmonary veins) → left atrium (atrial appendage) → mitral valve → left ventricle → aortic valve (aortic sinus) → (aorta and systemic circulation)



heart valves


Conduction system

cardiac pacemaker SA node Bachmann's bundle AV node bundle of His bundle branches Purkinje fibers

Pericardial cavity

pericardial sinus


fibrous pericardium

sternopericardial ligaments

serous pericardium

epicardium/visceral layer

fold of left vena cava

Blood supply

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