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Cardiac muscle, or heart muscle, is a layer of muscle tissue that lies between the inner and outer layers of the heart. It is one of three types of muscle tissue in the body, the other two being skeletal muscle tissue and smooth muscle tissue. Cardiac muscle is unique in that it is the only muscle tissue that is found in the heart, and it is responsible for the heart's pumping action through involuntary, coordinated contractions. The pumping action of the heart is made possible by the contraction of individual cardiac muscle cells, which are connected by intercalated discs. These discs allow the muscle cells to beat in synchrony and pass along electrical signals, resulting in a wave of contractions that create the heartbeat.
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What You'll Learn
Cardiac muscle cells
Cardiac muscle, also called myocardium or heart muscle, is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that constitutes the main tissue of the wall of the heart. The cardiac muscle forms a thick middle layer between the outer layer of the heart wall (the pericardium or epicardium) and the inner layer (the endocardium).
The individual cardiac muscle cell (cardiomyocyte) is a tubular structure composed of chains of myofibrils, which are rod-like units within the cell. The myofibrils consist of repeating sections of sarcomeres, which are the fundamental contractile units of the muscle cells. Sarcomeres are composed of long proteins that organise into thick and thin filaments, called myofilaments. Thin myofilaments contain the protein actin, and thick myofilaments contain the protein myosin. The myofilaments slide past each other as the muscle contracts and relaxes. This process is activated by the release of calcium from the sarcoplasmic reticulum (SR) when delivering an action potential to the muscle, in a process called excitation-contraction coupling. The sliding of actin and myosin past each other produces the formation of “cross-bridges”, which causes contraction of the heart and generation of force.
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Pacemaker cells
The action potential generated by the SA node passes down the electrical conduction system of the heart, depolarising the other potential pacemaker cells, such as those in the atrioventricular (AV) node, which acts as the secondary pacemaker. This normal conduction of electrical activity in the heart allows all contractile cells of the heart to act in a coordinated fashion and contract as a unit. The pacemaker cells are connected to neighbouring contractile cells via gap junctions, which enable them to locally depolarise adjacent cells and ensure the wave of contractions occurs in a synchronised manner.
In addition to the SA and AV nodes, pacemaker cells can also be found in the bundle of His and Purkinje fibres. These cells can become pacemakers if the SA and AV nodes fail to function. An ectopic pacemaker, or ectopic focus, refers to an excitable group of cells that cause a premature heartbeat outside the normally functioning SA node. If chronic, this can result in arrhythmias, which can be treated with an artificial pacemaker.
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Intercalated discs
The cardiac muscle is a unique type of muscle tissue that is only found in the heart. It is responsible for the heart's involuntary, coordinated contractions, which enable the pumping of blood through the circulatory system. To identify cardiac muscle, it is important to understand its structure, which includes specialised cells called pacemaker cells, and the presence of intercalated discs.
Adherens junctions are specialised structures that provide mechanical coupling between neighbouring cells. There are three morphologically distinct forms of adherens junctions: puncta adherentia, zonula adherens, and fascia adherens. The fascia adherens morphology is specifically found in cardiac intercalated discs. Adherens junctions facilitate cell-cell mechanical anchoring at two crucial points: the extracellular space and the intracellular space. In the extracellular space, cadherins tightly bind to each other, while in the intracellular space, the cadherins are indirectly attached to the actin cytoskeleton through a string of intermolecular interactions.
Gap junctions, another component of intercalated discs, form intercellular channels that allow the direct passage of electrical charges between cardiomyocytes. These channels are composed of two hexameric structures called connexons, which dock across the extracellular space, forming a permeable pore. The presence of gap junctions in intercalated discs enables the transfer of stimulation from one cardiac muscle cell to the next, resulting in a coordinated contraction of the cardiac muscle.
In summary, intercalated discs are essential structures in the cardiac muscle, facilitating mechanical and electrical connections between cardiomyocytes. They play a critical role in maintaining the coordinated contractions of the heart and ensuring the efficient pumping of blood. By understanding the structure and function of intercalated discs, we can better comprehend the remarkable synchrony and pumping performance of the cardiac muscle.
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Cardiomyopathies
Hypertrophic Cardiomyopathy
This type of cardiomyopathy is characterised by the enlargement and thickening of the heart muscle, particularly in the lower chambers or ventricles. It is usually inherited and affects about 1 in 500 people worldwide.
Dilated Cardiomyopathy
Here, the ventricles become enlarged and weaker, making it difficult for them to pump blood effectively. This type of cardiomyopathy can be caused by a range of factors, including alcohol abuse, heavy metal exposure, coronary artery disease, cocaine use, and viral infections. It is inherited in about one-third of cases and affects 1 in 2,500 people.
Restrictive Cardiomyopathy
This type of cardiomyopathy causes the ventricles to stiffen, preventing them from filling to their full volume. It may be caused by amyloidosis, hemochromatosis, and certain cancer treatments.
Arrhythmogenic Right Ventricular Dysplasia
This form of cardiomyopathy involves the replacement of the cardiac muscle tissue of the right ventricle with fatty or fibre-rich tissue. It can lead to arrhythmia, an abnormal heart rate or rhythm, and is more commonly seen in young people.
Takotsubo Cardiomyopathy (Broken Heart Syndrome)
This type of cardiomyopathy is caused by extreme emotional or physical stress and can lead to a range of symptoms, including chest pain, arrhythmia, shortness of breath, and swelling.
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Microscopic features
When viewed under a microscope, cardiac muscle tissue appears striated or striped, with a central nucleus. This is due to the regular organization of myofibrils into sarcomeres, which are composed of lighter I bands (actin) and darker A bands (myosin). The intercalated discs that connect the cardiomyocytes appear as thin, dark-staining lines dividing adjacent cardiac muscle cells.
Cardiac muscle tissue is located in the walls of the heart and is composed of branching fibres with intercalated discs that join the cells together. These discs allow for the synchronized contraction of the cells, and the branching fibres allow for the force to be transmitted in all directions. The 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.
Cardiac muscle cells also contain T-tubules, which are microscopic tubes that run from the cell surface to deep within the cell. These pouches of cell membrane improve the efficiency of contraction. The majority of these cells contain only one nucleus, although some may have two central nuclei.
Cardiac muscle tissue is unique compared to skeletal muscle tissue, which is found attached to bones and is responsible for voluntary movements. Skeletal muscle tissue is made up of long, cylindrical fibres that are unbranched and multinucleated. While both types of muscle tissue appear striated under a microscope, the nuclei of skeletal muscle tissue are located at the periphery of the fibres rather than centrally.
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Frequently asked questions
Cardiac muscle, also known as heart muscle, is the layer of muscle tissue that lies between the inner and outer layers of the heart. It is one of the three types of muscle tissue in the body, the other two being skeletal muscle tissue and smooth muscle tissue.
Cardiac muscle contracts and relaxes in a coordinated manner, allowing the ventricles to squeeze in several directions at the same time. This process is controlled by specialised cells called pacemaker cells, which are connected to other cardiac muscle cells.
Unlike skeletal muscle, cardiac muscle is not controlled by the somatic nervous system. Instead, it is regulated by the autonomous nervous system. Cardiac muscle cells typically have only one nucleus, whereas skeletal muscle cells can have multiple nuclei.
