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The heart is the only organ that is also a muscle. This muscle, also known as cardiac muscle, 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 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 cardiac muscle is responsible for keeping the heart pumping blood around the body.
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What You'll Learn
Cardiac muscle cells
Intercalated discs are complex adhering structures that provide force transmission during muscle contraction. Under light microscopy, they appear as thin, dark-staining lines dividing adjacent cardiac muscle cells. Electrical resistance through these discs is very low, allowing free diffusion of ions and facilitating the transmission of action potentials from one cardiac muscle cell to the next. The intercalated discs run perpendicular to the direction of muscle fibres, and their path appears convoluted under electron microscopy.
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Intercalated discs
Under light microscopy, intercalated discs appear as thin, dark-staining lines that run perpendicular to the direction of muscle fibres, dividing adjacent cardiac muscle cells. However, when viewed under electron microscopy, their structure becomes more intricate and convoluted.
The intercalated disc is composed of three types of cell-cell junctions: adherens junctions, desmosomes, and gap junctions. Adherens junctions, such as fascia adherens, provide anchoring sites for actin filaments and connect to the nearest sarcomere. Desmosomes, on the other hand, anchor intermediate filaments, while gap junctions facilitate intercellular communication and the passage of electrical charges between cells.
These three types of junctions work together to enable force transmission during muscle contraction and ensure the synchronized contraction of cardiac tissue in a wave-like pattern, allowing the heart to function effectively as a pump.
The intercalated disc plays a crucial role in maintaining cardiac function, and mutations in the intercalated disc gene can lead to various cardiomyopathies and heart failure. Thus, understanding the structure and function of the intercalated disc is essential for advancing our knowledge of cardiac function and its associated pathologies.
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Cardiomyopathy
Cardiac muscle, also known as myocardium, 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 heart wall. The myocardium forms a thick middle layer of the heart wall, with the outer layer being the pericardium (or epicardium) and the inner layer being the endocardium. The heart consists mostly of myocardium, or cardiac muscle cells.
In most cases of cardiomyopathy, the heart muscle becomes enlarged, thick, or rigid, and as the disease worsens, the heart becomes weaker. This can lead to heart failure or irregular heartbeats called arrhythmias. Cardiomyopathy may also cause other complications such as heart valve problems. Some people with cardiomyopathy experience symptoms such as shortness of breath, fatigue, dizziness, fainting, swelling in the ankles or legs, or chest pain, while others may have no symptoms at all. Treatment options include lifestyle changes, medications, surgery, implanted devices, and therapy to lower stress.
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Cardiac muscle layers
The heart wall is made up of three layers of tissue: the endocardium, the myocardium, and the epicardium.
The endocardium is the innermost layer of the heart. It lines the inner surfaces of the heart chambers, including the heart valves. The endocardium has two layers. The inner layer lines the heart chambers and is made of endothelial cells. The second layer is a subendocardial connective tissue that is continuous with the connective tissue of the myocardium. The subendocardial layer is thin and houses the Purkinje fibres and other components of the heart's conducting system.
The myocardium is the thickest layer and is the main constituent of the heart. It is a muscle layer that enables heart contractions. Histologically, the myocardium is made up of cardiomyocytes, which are cardiac muscle cells. Cardiomyocytes have a single nucleus in the centre of the cell, which distinguishes them from skeletal muscle cells, which have multiple nuclei. Cardiomyocytes are rich in glycogen deposits and mitochondria, which provide the energy needed for the cell in the form of adenosine triphosphate (ATP).
The outermost layer of the heart wall is the epicardium, which is also known as the visceral pericardium. It is composed of loose connective tissue and fat, and is lined externally by simple squamous epithelial cells (mesothelium). The subepicardial layer lies between the myocardium and the epicardium and contains coronary vessels, nerves, and connective tissue.
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Cardiac muscle tissue
The heart is made up of three layers, with the myocardium forming the thick middle layer. The outer layer is called the pericardium, and the inner layer is the endocardium. The endocardium is not cardiac muscle tissue; instead, it is made up of simple squamous epithelial cells and forms the inner lining of the heart chambers and valves. The pericardium is a fibrous sac surrounding the heart, consisting of the epicardium, pericardial space, parietal pericardium, and fibrous pericardium. The myocardium itself is made up of cardiac muscle cells, also known as cardiomyocytes, which are joined by intercalated discs. These discs are complex adhering structures that connect the single cardiomyocytes to an electrochemical syncytium. They consist of three different types of cell-cell junctions: the actin filament anchoring fascia adherens junctions, the intermediate filament anchoring desmosomes, and gap junctions.
Cardiomyopathy is a group of medical conditions that affect cardiac muscle tissue and impair the heart's ability to pump blood or relax. This can lead to conditions such as hypertrophic cardiomyopathy, where the cardiac muscles enlarge and thicken for no apparent reason, or dilated cardiomyopathy, where the cardiac muscle tissue of the left ventricle stretches and the heart's chambers dilate.
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Frequently asked questions
Cardiac muscle, also called heart muscle or myocardium, is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle.
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).
Cardiac muscle tissue contracts and releases involuntarily to keep the heart pumping blood around the body.
Unlike skeletal muscle, cardiac muscle is under involuntary control. Cardiac muscle also contracts in a similar manner to skeletal muscle but with some important differences.
Intercalated discs are complex adhering structures that connect the single cardiomyocytes to an electrochemical syncytium. They help hold the cardiac muscle fibres together during a contraction.
