Victor Forge
Cardiac muscle, also called heart muscle or myocardium, is one of three types of muscle tissues in the body, the others being skeletal and smooth muscle. It is an involuntary, striated muscle that constitutes the main tissue of the wall of the heart. The heart wall has three layers: the outer layer or epicardium, the myocardium, and the inner layer or endocardium. The myocardium is a thin yet strong membrane that surrounds and protects the heart and its associated structures. Purkinje fibres are additional myocardial conductive fibres that spread the impulse to the myocardial contractile cells in the ventricles. These fibres are also bundles of heart muscle.
| Characteristics | Values |
|---|---|
| Type of muscle | Cardiac muscle or myocardium is one of three types of muscle tissues in the body, the others being skeletal muscle and smooth muscle |
| Location | Forms the thick middle layer of the heart wall, between the outer layer (pericardium/epicardium) and the inner layer (endocardium) |
| Composition | Individual cardiac muscle cells (cardiomyocytes) joined by intercalated discs, encased by collagen fibres and other substances that form the extracellular matrix |
| Function | Contracts to pump blood through the heart and into the rest of the body |
| Control | Controlled by the autonomic nervous system |
| Appearance | Striated, owing to the arrangement of thick and thin filaments that make up the contractile apparatus |
| Energy requirements | Requires a constant flow of blood to provide oxygen and nutrients |
| Diseases | Cardiomyopathies, including ischemic conditions such as angina and myocardial infarction |
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What You'll Learn
Cardiac muscle cells (cardiomyocytes)
Cardiac muscle cells, also called cardiomyocytes, are the contractile cells of the cardiac muscle that allow the heart to pump. They are involuntary, striated, branched, and contain many mitochondria. Each cardiomyocyte contains a single, centrally located nucleus surrounded by a cell membrane called the sarcolemma. The sarcolemma contains voltage-gated calcium channels, which are specialized ion channels that skeletal muscles do not possess.
Cardiac muscle cells are roughly rectangular in shape and are joined at their ends by intercalated discs to form long fibers. These intercalated 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. The desmosomes provide a tight mechanical connection between cells, while the gap junctions allow action potentials to propagate between cells.
Each cardiac muscle cell contains myofibrils, specialized protein contractile fibers of actin and myosin that slide past each other. These are organized into sarcomeres, the fundamental contractile units of muscle cells. The regular organization of myofibrils into sarcomeres gives cardiac muscle cells a striped or striated appearance when viewed under a microscope, similar to skeletal muscle.
Cardiomyocytes contain T-tubules, pouches of cell membrane that run from the cell surface to the cell's interior, which help to improve the efficiency of contraction. They are also involved in mechano-electric feedback and help regulate the concentration of calcium within the cell in a process known as excitation-contraction coupling.
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Intercalated discs
The actin filament anchoring fascia adherens junctions and the intermediate filament anchoring desmosomes provide mechanical coupling between neighbouring cells. The fascia adherens junctions connect the actin cytoskeleton of adjacent cells, while the desmosomes anchor the cell membrane to the intermediate filament network.
The gap junctions electrically connect the cytoplasm of adjacent cardiomyocytes, allowing action potentials to spread between cardiac cells by permitting the passage of ions between them. This produces depolarization of the heart muscle and enables the heart to contract and relax as a single unit, or functional syncytium.
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Calcium and contraction
Calcium plays a crucial role in the contraction of the heart muscle, or myocardium, which forms the bulk of the heart wall. The process by which electrical excitation leads to contraction is known as excitation-contraction (E-C) coupling.
During E-C coupling, an increase in the calcium concentration in the cytoplasm of cardiac muscle cells, known as the systolic Ca2+ transient, triggers the contraction process. This increase in calcium concentration is primarily due to the release of calcium from the sarcoplasmic reticulum (SR), a calcium store within the cell. The release of calcium from the SR occurs through a channel called the ryanodine receptor (RyR). The probability of the RyR being open, and thus allowing calcium to exit the SR, is influenced by the amount of calcium in the cytoplasm and SR. This relationship results in a steep dependence of calcium release on SR calcium content.
The release of calcium from the SR leads to a rise in calcium concentration in the cytoplasm. This increase in calcium causes the cell's myofilaments to slide past each other, resulting in cell shortening and the development of pressure within the ventricles. This pressure leads to the ejection of blood from the heart. The force of contraction depends on the amount of calcium bound to troponin, which is influenced by factors such as the magnitude and duration of the calcium transient, as well as the strength of calcium binding, which can be altered genetically or through factors like phosphorylation.
Additionally, calcium entry into the cell through L-type Ca2+ channels also contributes to the calcium concentration in the cytoplasm. While this is not the primary source of calcium, it triggers the release of additional calcium from the SR in a process known as calcium-induced calcium release. To ensure proper relaxation of the heart muscle, the calcium concentration in the cytoplasm must be lowered back to resting levels through the reuptake of calcium into the SR and its removal from the cell via the Na+-Ca2+ exchange.
Disturbances in calcium signaling can have significant implications for heart function, potentially leading to heart failure and arrhythmias. Therefore, understanding the role of calcium in cardiac contraction is crucial for maintaining cardiovascular health and developing therapeutic interventions.
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Electrical impulses
The heart's pumping action is regulated by an electrical conduction system that coordinates the contraction of the various chambers of the heart. This electrical conduction system is the network of nodes, cells, and signals that controls the heartbeat. The heart's electrical conduction system sends out thousands of signals per day to keep the heart beating.
The electrical stimulus is generated by the sinus node (also called the sinoatrial node, or SA node). This is a small mass of specialized tissue located in the right upper chamber (atria) of the heart. The sinus node generates an electrical stimulus regularly, 60 to 100 times per minute under normal conditions. The atria are then activated. The electrical stimulus travels down through the conduction pathways and causes the heart's ventricles to contract and pump out blood. The 2 upper chambers of the heart (atria) are stimulated first and contract for a short period of time before the 2 lower chambers of the heart (ventricles). The electrical impulse travels from the sinus node to the atrioventricular node (also called the AV node).
The myocardial conducting cells (1 percent of the cells) form the conduction system of the heart. These cells initiate and propagate the action potential (the electrical impulse) that travels throughout the heart and triggers the contractions that propel the blood. Some of the muscle cells within the heart, the Purkinje fibers, are specialized to carry electrical impulses. These cells are grouped into bundles that form two branches, one to each ventricle. These special conducting fibers are responsible for the final distribution of electrical stimulus to the myocardium.
T (transverse) tubules penetrate from the surface plasma membrane, the sarcolemma, to the interior of the cell, allowing the electrical impulse to reach the interior. The T tubules are only found at the Z discs, whereas in skeletal muscle, they are found at the junction of the A and I bands.
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Cardiac muscle fibres
Cardiac muscle, also called myocardium, is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. It is composed of individual cardiac muscle cells, or cardiomyocytes, joined by intercalated discs and encased by collagen fibres and other substances that form the extracellular matrix.
Cardiac muscle cells are the contracting cells that allow the heart to pump. Each cardiomyocyte needs to contract in coordination with its neighbouring cells, working together to efficiently pump blood from the heart. If this coordination breaks down, the heart may not pump at all, as can occur during abnormal heart rhythms such as ventricular fibrillation.
Cardiac muscle cells are roughly rectangular in shape, measuring 100-150μm by 30-40μm. They are joined at their ends by intercalated discs to form long fibres. Each cell contains myofibrils, specialised protein contractile fibres of actin and myosin that slide past each other. These are organised into sarcomeres, the fundamental contractile units of muscle cells. The sarcomeres allow for contractility, and the rise in calcium causes the cell's myofilaments to slide past each other in a process called excitation-contraction coupling.
The intercalated 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. The discs are responsible for force transmission during muscle contraction, allowing action potentials to spread between cardiac cells by permitting the passage of ions between cells, producing depolarisation of the heart muscle.
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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. It is an involuntary, striated muscle that constitutes the main tissue of the wall of the heart.
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 organize into thick and thin filaments, called myofilaments. These myofilaments slide past each other as the muscle contracts and relaxes.
Diseases of the heart muscle are known as cardiomyopathies. These include ischemic conditions caused by a restricted blood supply to the muscle such as angina and myocardial infarction. Cardiomyopathic diseases affect the heart's ability to function properly. Some of these conditions can be treated if detected early on, but there are some fatal diagnoses as well.
