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Cardiac muscle, also known as myocardium or heart muscle, is one of three types of vertebrate muscle tissues, the others being skeletal and smooth muscle. It is an involuntary, striated muscle that constitutes the main tissue of the heart wall. 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. The contractile stimuli propagate from one cell to the next, resulting in a synchronous contraction of the entire tissue section.
| Characteristics | Values |
|---|---|
| Appearance | Striated or striped |
| Cell structure | Chains of myofibrils |
| Myofibrils | Composed of repeating sections of sarcomeres |
| Sarcomeres | Composed of long proteins that organize into thick and thin filaments |
| Thick filaments | Contain the protein myosin |
| Thin filaments | Contain the proteins actin, troponin, and tropomyosin |
| Contraction mechanism | Sliding filament theory |
| Intercalated discs | Thin, dark-staining lines dividing adjacent cardiac muscle cells |
| Nucleus | Elongated and located in the centre of the cell |
| Capillaries | Extensive network in the endomysium |
| Autonomic nervous system input | Receives input to increase or decrease heart rate |
| Calcium | Critical factor determining force generated with each contraction |
| Contraction | Involuntary |
| Type | Involuntary, striated muscle |
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Cardiac muscle cells
Cardiac muscle, also called myocardium or heart muscle, is one of the three types of vertebrate muscle tissues, the other two being skeletal muscle and smooth muscle. It forms the thick middle layer of the heart, sandwiched between the outer layer of the heart wall (the pericardium or epicardium) and the inner layer (the endocardium). The cardiac muscle is composed of individual cardiac muscle cells or cardiomyocytes, which are striated, branched, contain many mitochondria, and are under involuntary control.
Cardiomyocytes are joined at their ends by intercalated discs to form long fibers. These interconnections allow the cardiomyocytes to contract together synchronously to enable the heart to work as a pump. Within the intercalated discs are three different types of cell junctions: fascia adherens, desmosomes, and gap junctions. The desmosomes are intercellular structures that anchor cardiac muscle fibers together and are vital in maintaining the structural integrity of the heart. The gap junctions allow for the propagation of coordinated action potentials from one cell to the next in a phenomenon known as electrical coupling.
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Intercalated discs
The individual cardiac muscle cell, or cardiomyocyte, is a tubular structure composed of chains of myofibrils. These myofibrils are rod-like units within the cell, consisting of repeating sections of sarcomeres. Sarcomeres are the fundamental contractile units of the muscle cells, and they are composed of long proteins that organize into thick and thin filaments, called myofilaments.
Cardiac muscle cells branch and form a three-dimensional network. These branch points can sometimes be seen in cross-sections, and the muscle fibres are less parallel than in skeletal muscle. Intercalated discs, which are dark lines crossing the cell transversely, are a distinctive feature of cardiac muscle. They are fairly easy to find in areas where muscle fibres are longitudinally oriented.
Gap junctions form intercellular channels that provide a low-resistance pathway for the direct cell-to-cell passage of electrical charges. Each gap junction channel is composed of two hexameric structures called connexons that dock across the extracellular space and form a permeable pore. The importance of Cx43 in the propagation of the cardiac action potential is well established, and disruptions can lead to lethal arrhythmias.
Understanding the structure and function of intercalated discs is crucial for advancing knowledge about cardiac function in health and disease. Mutations in the intercalated disc gene can lead to various cardiomyopathies that result in heart failure.
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Calcium and contraction
The cardiac muscle, or myocardium, is the thick middle layer of the heart. It is made up of individual cardiac muscle cells, or cardiomyocytes, which are tubular structures composed of chains of myofibrils. These myofibrils are the rod-like units within the cell that consist of repeating sections of sarcomeres—the contractile units of the muscle cells.
The contraction of the cardiac muscle is involuntary and occurs spontaneously or under the control of the autonomic nervous system. This rapid contraction and relaxation are vital for pumping blood throughout the cardiovascular system. The force of contraction is determined by the concentration of calcium in the myocyte.
Calcium is a critical regulator of cardiac function and plays a key role in the process of excitation-contraction coupling. This process links the electric excitation of the surface membrane (action potential) to contraction. Calcium is released from the sarcoplasmic reticulum (SR) when an action potential is delivered to the muscle. The release of calcium activates the sliding of the thin and thick myofilaments past each other, resulting in the formation of "cross-bridges," which causes the heart to contract and generates force. The force of contraction depends on the amount of calcium bound to troponin, which can be influenced by factors such as phosphorylation.
Additionally, the interaction between stretch and calcium also influences cardiac function. Stretch has been shown to alter action potential configuration, generate stretch-activated arrhythmias, and increase the rate of beating of the sino-atrial node. Calcium entering through L-type calcium channels or stretch-activated channels (SACs) influences the secretion of natriuretic peptides and the onset of hypertrophy.
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Myocardium
The myocardium is the middle muscular layer of the heart, composed of specialised muscle cells called cardiomyocytes. It is the thickest of the three layers of the heart, with the endocardium as the innermost layer and the epicardium as the outermost layer. The myocardium is responsible for the contraction of the heart, facilitating the contraction and relaxation of the heart walls to receive and pump blood into the systemic circulation.
The myocardium functions as a syncytium with synchronized contraction due to the presence of intercalated discs, which contain gap junctions that facilitate fast cell-to-cell communication. These intercalated discs are a unique feature of cardiomyocytes. The regular arrangement of intracellular contractile units, sarcomeres, gives cardiac muscle its striated appearance when viewed under a microscope.
In addition to its role in contraction, the myocardium provides a scaffold for heart chambers and conducts electrical stimuli. The extensive network of capillaries in the endomysium ensures a constant supply of oxygen and nutrients to the heart muscle, which is always in demand due to its continuous beating.
The structure of the myocardium reflects its function, with its specialised muscle cells adapted for contractile function. The myocardium's location between the inner and outer layers of the heart allows it to facilitate the contraction and relaxation of the heart walls, contributing to the heart's pumping action.
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Histology
Cardiac muscle cells are long, branched cells, shaped like cylinders joined end-to-end, with one or two nuclei located centrally. The fibres are crossed by linear bands called intercalated discs, which provide attachment points and allow cardiac muscle tissue to function as a syncytium. In other words, the contractile stimuli are propagated from one cell to the next, resulting in a synchronous contraction of the entire tissue section.
The histological features of cardiac muscle tissue include striations, branching fibres, and intercalated discs. In longitudinal sections, the regular branching and anastomosing pattern is the most distinctive feature of cardiac muscle. Higher magnification is required to confirm the presence or absence of myofibrils and cross-striations. Cross sections of A bands (darker) and I bands (lighter) can be seen side by side in the same cell due to the imperfect alignment of myofibrils.
Cardiac muscle tissue has high metabolic, energy, and vascular demands. The tissue is separated by collagenous tissue that supports the capillary network of cardiac tissue. The contractility of cardiac muscle can be altered by the autonomic nervous system and hormones.
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Frequently asked questions
Cardiac muscle, also known as 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.
Cardiac muscle cells branch and form a three-dimensional network. They have their own auto-rhythmicity and are 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.
The rise in calcium causes the cell's myofilaments to slide past each other in a process called excitation-contraction coupling. The myofilaments are oriented along the length of the cell and slide over each other in what is known as the sliding filament theory.
