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Cardiac muscle, also known as myocardium or heart muscle, is a type of muscle tissue that forms the heart. It is one of three types of muscle tissues, the others being skeletal muscle and smooth muscle. Cardiac muscle is striated, involuntary, and constitutes the main tissue of the wall of the heart. It is composed of individual cardiac muscle cells, known as cardiomyocytes, which are joined by intercalated discs and encased by collagen fibres and other substances that form the extracellular matrix.
| Characteristics | Values |
|---|---|
| What is cardiac muscle? | One of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. |
| Location | The walls of the heart. |
| Appearance | Striated or striped. |
| Control | Involuntary. |
| Composition | Individual cardiac muscle cells or cardiomyocytes joined by intercalated discs. |
| Blood supply | Coronary circulation. |
| Contraction | In a similar manner to skeletal muscle, but with some differences. Electrical stimulation triggers the release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum. |
| Calcium | The rise in calcium causes the cell's myofilaments to slide past each other in a process called excitation-contraction coupling. |
| Pacemaker cells | Carry the impulses that are responsible for the beating of the heart. |
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What You'll Learn
Cardiac muscle forms the atria and ventricles of the heart
The human body has three types of muscle tissue: skeletal, smooth, and cardiac. Cardiac muscle, also known as myocardium or heart muscle, is found only in the heart. It is an involuntary, striated muscle that forms the thick middle layer of the heart wall, between the outer layer (the pericardium or epicardium) and the inner layer (the endocardium).
Cardiac muscle forms both the atria and the ventricles of the heart. The myocardium found in the ventricles is thicker to allow for forceful contractions, while the myocardium in the atria is much thinner. The individual myocytes that make up the myocardium differ between the cardiac chambers. Ventricular cardiomyocytes are longer and wider, with a denser T-tubule network. The calcium transient is smaller and decays more rapidly in atrial myocytes, with a corresponding increase in calcium buffering capacity.
The cardiac muscle cells or cardiomyocytes are the contractile units of the cardiac muscle. They are surrounded by an extracellular matrix produced by supporting fibroblast cells. The primary function of cardiomyocytes is to contract, which generates the pressure needed to pump blood through the circulatory system. These contractions are involuntary and occur in response to electrical impulses from the nervous system. The contractile forces of the cardiac muscle and the frequency at which they are activated determine the cardiac output, which is defined as heart rate multiplied by stroke volume.
Cardiac muscle cells contain branched fibres connected by intercalated discs that contain gap junctions and desmosomes. These interconnections allow the cardiomyocytes to contract together synchronously, enabling the heart to work as a pump. The contractility of the cardiac muscle is influenced by the concentration of calcium in the myocyte, which determines the force generated with each contraction.
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Cardiac muscle is striated and involuntary
The human body contains three kinds of muscle tissue: skeletal, smooth, and cardiac. Cardiac muscle, also known as myocardium or heart muscle, is a specialized type of tissue that exists only in the heart. It is one of the three major categories of muscles found within the human body. The other two are smooth muscle and skeletal muscle.
Cardiac muscle forms the bulk of the heart, constituting the main tissue of the heart wall. It forms a thick middle layer between the outer layer of the heart wall (the pericardium or epicardium) and the inner layer (the endocardium). The endocardium lines the cardiac chambers, covers the cardiac valves, and joins with the endothelium that lines the blood vessels connected to the heart. The outer layer, or epicardium, forms part of the pericardial sac that surrounds, protects, and lubricates the heart.
Cardiac muscle cells contain a single, centrally located nucleus surrounded by a cell membrane called the sarcolemma. The sarcolemma contains voltage-gated calcium channels, specialized ion channels that skeletal muscle does not possess. The concentration of calcium in the myocyte determines how much force is generated with each contraction. Electrical stimulation in the form of a cardiac action potential triggers the release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum, leading to muscle contraction.
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Pacemaker cells are a specialised type of cardiac tissue
The human body contains three major categories of muscles: cardiac muscle, smooth muscle, and skeletal muscle. Cardiac muscle, also known as myocardium or heart muscle, is an involuntary, striated muscle that forms the heart wall's main tissue. It is responsible for the contractility of the heart, which results in the pumping action.
Cardiac muscle cells, also known as cardiomyocytes, are the contractile myocytes of the cardiac muscle. These cells are surrounded by an extracellular matrix produced by supporting fibroblast cells. Some cardiomyocytes are specialised and modified, known as pacemaker cells, which set the rhythm of the heart contractions.
Pacemaker cells are also found in the atrioventricular node, which acts as a secondary pacemaker. These cells carry the impulses that cause the heart to beat and are distributed throughout the heart. The Purkinje fibres, a network of specialised cells, are also involved in the rapid transmission of electrical signals to the heart's ventricles, allowing for coordinated ventricular contraction.
The cardiac conduction system is a complex network of nodes, cells, and signals that controls the heartbeat. It consists of specialised cells and nodes that regulate the heartbeat, including pacemaker cells. The similarities in pacemaker cell physiology and tissue architecture in the SAN are observed across diverse mammalian species, indicating a conserved evolutionary program.
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Cardiac muscle is one of three types of muscle tissues
Cardiac muscle, also called myocardium or heart muscle, is one of three types of muscle tissues found within the human body, the other two being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that forms 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 endocardium is not cardiac muscle and is made up of simple squamous epithelial cells that form the inner lining of the heart chambers and valves. The cardiac muscle forms both the atria and the ventricles of the heart.
Cardiac muscle is responsible for the contractility of the heart and, therefore, the pumping action. It must contract with enough force and supply enough blood to meet the metabolic demands of the entire body. 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. The myofilaments slide past each other as the muscle contracts and relaxes, producing the formation of "cross-bridges", which cause the contraction of the heart and the generation of force.
Cardiac muscle cells contain mitochondria, which many people refer to as the powerhouses of the cells. They also contain voltage-gated calcium channels, specialized ion channels that skeletal muscle does not possess. Calcium channels are the critical factor that determines how much force is generated with each contraction. Electrical stimulation in the form of a cardiac action potential triggers the release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum. The release of calcium from the sarcoplasmic reticulum causes the cell's myofilaments to slide past each other in a process called excitation-contraction coupling.
Cardiac muscle is highly organized and contains many types of cells, including fibroblasts, smooth muscle cells, and cardiomyocytes. It is the only type of muscle tissue that develops in the heart. It contains cardiac muscle cells, which perform highly coordinated actions that keep the heart pumping and blood circulating throughout the body.
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Calcium is critical for the force of cardiac muscle contractions
The contractile forces of the cardiac muscle and the frequency at which they are activated determine the cardiac output, which is defined as heart rate multiplied by stroke volume. The contractility of the heart, or its ability to contract with sufficient force, is essential to meet the metabolic demands of the entire body. The cardiac muscle cells contain voltage-gated calcium channels, which play a crucial role in the excitation-contraction coupling process.
During electrical stimulation, or the cardiac action potential, calcium is released from the cell's internal calcium store, the sarcoplasmic reticulum. This release of calcium triggers a process called excitation-contraction coupling, which is fundamental to the contraction of the myocardium. The rise in calcium causes the cell's myofilaments to slide past each other, resulting in cell shortening and the development of pressure within the ventricle, leading to the ejection of blood.
The force of contraction depends on the amount of calcium bound to troponin, which is influenced by the magnitude and duration of the rise in intracellular calcium concentration. Additionally, the strength of calcium binding can be altered genetically or through factors such as phosphorylation. The regulation of intracellular calcium concentration is essential for maintaining the balance between calcium influx and efflux during each cardiac cycle.
In summary, calcium plays a critical role in the force of cardiac muscle contractions by regulating the contractility of the myocardium and modulating the binding to troponin, which ultimately determines the force of contraction. The intricate balance of calcium concentration and its interaction with other physiological mechanisms contribute to the overall function of the cardiac muscle.
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Frequently asked questions
Cardiac muscle is found in the heart.
Cardiac muscle is one of three types of muscle tissues found within the human body, the others being smooth muscle and skeletal muscle.
Cardiac muscle is made up of cardiac muscle cells, also called cardiomyocytes.
Cardiac muscle cells are striated, branched, and contain many mitochondria.
Cardiac muscle contracts and relaxes involuntarily to keep the heart pumping and blood circulating around the body.
