Victor Forge
The human body is made up of three types of muscle tissue: skeletal, smooth, and cardiac. The heart is the only organ in the body that is also a muscle, and it is made of a special type of muscle tissue called cardiac muscle. This tissue is also known as myocardium, and it forms the thick middle layer of the heart. Cardiac muscle tissue is composed of cells called cardiomyocytes, which are tubular structures composed of chains of myofibrils. These myofibrils contract and relax involuntarily, creating the rhythmic, wave-like contractions known as the heartbeat. Cardiomyocytes are the only muscle cells in the body that have their own pacemaker cells, which cause them to contract and expand in response to electrical impulses from the nervous system.
| Characteristics | Values |
|---|---|
| Type | Cardiac muscle, also called heart muscle or myocardium |
| Involuntariness | Yes |
| Muscle Tissue | One of three types of vertebrate muscle tissues, along with skeletal and smooth muscle |
| Function | Makes the heart beat and pump blood to the rest of the body |
| Structure | Composed of individual cardiac muscle cells (cardiomyocytes) joined by intercalated discs, and encased by collagen fibers and other substances that form the extracellular matrix |
| Shape | Tubular |
| Composition | Chains of myofibrils, which are rod-like units within the cell |
| Contraction | Requires a constant supply of oxygen and nutrients; contraction of cardiomyocytes in a coordinated manner allows the ventricle to squeeze in several directions simultaneously |
| Diseases | Cardiomyopathies, ischemic syndromes, coronary artery disease |
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What You'll Learn
Cardiac muscle cells
The heart is an involuntary muscle, and the cardiac muscle, also called myocardium, constitutes the main tissue of the wall of the heart. It is one of three types of muscle in the body, the others being skeletal and smooth muscle. The myocardium forms a thick middle layer between the outer layer of the heart wall (the pericardium or epicardium) and the inner layer (the endocardium).
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Intercalated discs
The heart is an involuntary muscle, beating on its own without any conscious input. The cardiac muscle, or myocardium, is one of three types of muscle in the body, the other two being skeletal and smooth muscle.
Cardiac muscle cells (cardiomyocytes) are connected by intercalated discs, which are small connections that allow the muscle cells to work as a single functional syncytium. Intercalated discs support the synchronized contraction of cardiac tissue in a wave-like pattern, enabling the heart to work as a pump.
Mutations in the intercalated disc gene can lead to various cardiomyopathies, which are diseases of the heart muscle, and can eventually cause heart failure.
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Pacemaker cells
The heart is composed of many cell types, including cardiomyocytes, fibroblasts, endothelial cells, and perivascular cells. The cardiomyocytes can be further classified into general cardiomyocytes and pacemaker cells.
In the heart, action potentials are generated by pacemaker cells, which use them to control the rhythmic contraction of muscles. Action potentials occur when specialised channels in the cell membrane open and allow ions to flow into or out of the cell. This change in electric charge makes the cell more positive on the inside, attracting more ions from neighbouring cells and triggering a chain reaction that propagates the action potential along the heart muscle. This eventually leads to the contraction of the heart and pumps blood around our bodies. The pacemaker cells are connected to neighbouring contractile cells via gap junctions, which enable them to locally depolarize adjacent cells.
In the case of SA node dysfunction or blockage of the impulse generated, a group of cells further down the heart, typically the AV node, will become the pacemaker. If the AV node also fails, Purkinje fibres can act as the default pacemaker. If the body's intrinsic conduction system is damaged, a mechanical device called an artificial pacemaker may be used to produce these impulses synthetically.
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Cardiomyopathy
The heart is an involuntary muscle, beating and pumping blood around the body without conscious input. The cardiac muscle, or myocardium, is the thick middle layer of the heart, surrounded by the epicardium and an inner endocardium. The individual cardiac muscle cell (cardiomyocyte) is a tubular structure composed of chains of myofibrils, which are rod-like units within the cell.
There are several types of cardiomyopathy, including hypertrophic cardiomyopathy, which affects young adults and athletes, and peripartum cardiomyopathy, which affects some women before, during, or after pregnancy. Another type is stress-induced cardiomyopathy, also known as broken heart syndrome. Cardiomyopathy can be treated with medicines, procedures, healthy lifestyle changes, implanted devices, and therapy to lower stress.
Cardiac surgery can be required to treat cardiomyopathy, and cardiopulmonary bypass is a technique used to suspend contractions of the cardiac muscle while maintaining perfusion of blood and oxygen to the organs and tissues of the body.
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Coronary circulation
The heart is an involuntary muscle that beats on its own without external input. It is made up of cardiac muscle cells called cardiomyocytes, which are rectangular, branching tubular structures composed of chains of myofibrils. These myofibrils are the contractile units of the muscle cells, allowing the heart to work as a pump.
The coronary arteries arise from the sinuses of Valsalva, with the right coronary artery (RCA) supplying blood to the right side of the heart, including the right atrium, right ventricle, and the SA and AV nodes, which control heart rhythm. The left coronary artery (LCA) supplies blood to the left side of the heart, including the left atrium and left ventricle. These arteries have smaller branches that interconnect, forming anastomoses, which allow blood to circulate even with partial blockages.
Any interruption in coronary circulation can quickly lead to a heart attack (myocardial infarction) due to oxygen starvation of the heart muscle. The most common cause of such interruptions is coronary artery disease (CAD), which is often a result of atherosclerosis, or plaque buildup in the arteries. Coronary circulation is also unique in its pharmacologic characteristics, such as reactivity to adrenergic stimulation and sensitivity to endothelin, a potent vasoconstrictor.
Understanding coronary circulation physiology is essential for clinicians to develop preventive measures and treatments for heart-related issues. For example, regular exercise training can decrease CAD morbidity and mortality by increasing arteriolar diameters and densities and improving vasoreactivity.
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Frequently asked questions
The heart is a muscular organ that pumps blood to the rest of the body. It is made up of cardiac muscle, which is a special type of muscle tissue found only in the heart.
Cardiac muscles are striated, branched, and contain many mitochondria. They are under involuntary control and contract together in a fluid, synchronized fashion to enable the heart to work as a pump.
The heart has three layers: the pericardium, myocardium, and endocardium. The myocardium is the muscular middle layer.
The primary function of the heart is to pump blood into circulation by generating enough force. The heart is the main organ of the cardiovascular system, a network of blood vessels that pumps blood throughout the body.
The heart contracts (squeeze) and relaxes to send blood throughout the body. This process requires a constant supply of oxygen and nutrients to meet the energy demands of the cardiac muscle.
