Maximus Gage
The human body is made up of four basic types of tissues, one of which is muscle tissue. There are three distinct groups of muscle tissues: skeletal, cardiac, and smooth muscle. Cardiac muscle, also known as myocardium, is a structurally and functionally unique subtype of muscle tissue located in the heart. It forms the bulk of the heart, with a thick layer of myocardium sandwiched between the inner endocardium and the outer epicardium (or visceral pericardium). The heart is an involuntary, striated muscle, and visceral muscle is also involuntarily controlled by the unconscious regions of the brain through the autonomic and enteric nervous systems.
| Characteristics | Values |
|---|---|
| Type | One of three types of vertebrate muscle tissues |
| Other Names | Heart muscle, Myocardium |
| Location | Forms the bulk of the heart |
| Layer | Thick middle layer between the outer layer of the heart wall (the pericardium) and the inner layer (the endocardium) |
| Blood Supply | Coronary circulation |
| Composition | Individual cardiac muscle cells joined by intercalated discs, encased by collagen fibers and other substances that form the extracellular matrix |
| Contraction | Similar to skeletal muscle with some differences; strong, continuous, and rhythmic |
| Electrical Stimulation | Release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum |
| T-Tubules | Bigger and wider than those in skeletal muscle, but fewer in number |
| Control | Involuntary |
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What You'll Learn
Cardiac muscle cells
The cardiac syncytium is a network of cardiomyocytes connected by intercalated discs. Each syncytium obeys the all-or-none law. Intercalated discs are complex adhering structures that connect the single cardiomyocytes to an electrochemical syncytium. The discs are responsible mainly for force transmission during muscle contraction. The functional unit of cardiomyocyte contraction is the sarcomere, which consists of thick (myosin) and thin (actin) filaments, the interactions between which form the basis of the sliding filament theory.
The individual cardiac muscle cell 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 myofilaments slide past each other as the muscle contracts and relaxes. This process is activated by the release of calcium from the sarcoplasmic reticulum when delivering an action potential to the muscle, in a process called excitation-contraction coupling. The sliding of actin and myosin past each other produces the formation of “cross-bridges”, which causes contraction of the heart and generation of force.
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Visceral pericardium
The pericardium is a double-walled sac that contains the heart and the roots of the great vessels. The outer layer of the heart wall is the pericardium, and the inner layer is the endocardium. The pericardium has two layers: an outer layer made of strong inelastic connective tissue (fibrous pericardium) and an inner layer made of serous membrane (serous pericardium). The serous membrane, in turn, has two layers: the parietal layer and the visceral layer.
The visceral pericardium, also known as the epicardium, is the innermost layer of the pericardium and adheres to the cardiac tissue. It covers the myocardium of the heart and can be considered its serosa. The visceral pericardium is largely made of a mesothelium overlying some elastin-rich loose connective tissue. The same mesothelium that constitutes the serous pericardium also covers the heart as the epicardium, resulting in a continuous serous membrane. This creates a pouch-like potential space around the heart enclosed between the two opposing serosal surfaces, known as the pericardial space or pericardial cavity. This cavity is filled with a small amount of serous fluid, which lubricates the heart's movements and cushions it from any external jerk or shock. The visceral layer of the serous pericardium aids in the production of this serous fluid, thus permitting friction-free movement of the heart within the pericardial sac.
The visceral pericardium is innervated by sympathetic (aortic and cardiac plexuses) and parasympathetic fibres from the vagus nerve (CN X). As a result of this innervation, the visceral layer is insensitive to pain. The visceral and parietal layers are continuous around the roots of the great vessels and form two recesses, the oblique and transverse sinuses. The transverse sinus is an important landmark in cardiac surgery.
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Electrical stimulation
Cardiac 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 forms the bulk of the heart. The myocardium is a thick middle layer of cardiac muscle cells, or cardiomyocytes, sandwiched between the inner endocardium and the outer epicardium (also known as the visceral pericardium).
In addition, T-tubules within the cardiac muscle cells facilitate the rapid transmission of electrical impulses from the cell surface to the cell's core. They also play a role in regulating calcium concentration within the cell through a process called excitation-contraction coupling. The Purkinje fibres are another type of myocardial conductive fibre that spreads the electrical impulse to the myocardial contractile cells in the ventricles, initiating the contraction that pumps blood out of the ventricles and into the aorta and pulmonary trunk.
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Intercalated discs
The three types of junctions that make up an intercalated disc have distinct functions and characteristics. Fascia adherens junctions are anchoring sites for actin and connect to the closest sarcomere. Desmosomes provide intercellular adhesion and efficient cardiac electrical synchrony. Gap junctions form intercellular channels that allow the direct cell-to-cell passage of electrical charges, enabling the transmission of electrical impulses and the synchronisation of cell contractions.
The intercalated disc is a highly specialised structure that plays a crucial role in maintaining cardiac function. Mutations in the intercalated disc gene can lead to various cardiomyopathies and heart failure. Understanding the structure and function of intercalated discs is essential for advancing our knowledge of cardiac function in health and disease.
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Calcium and contraction
Calcium plays a crucial role in the contraction of cardiac muscle. Cardiac muscle tissue, or myocardium, forms the bulk of the heart. It is an involuntary, striated muscle that constitutes the main tissue of the heart wall. The cardiac muscle contracts in a similar way to skeletal muscle, but with some key differences.
An 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. T-tubules in cardiac muscle are larger and wider than those in skeletal muscle, but fewer in number. They lie close to the sarcoplasmic reticulum and help to regulate the concentration of calcium within the cell in a process known as excitation-contraction coupling.
Calcium triggers contraction by reacting with regulatory proteins that, in the absence of calcium, prevent the interaction of actin and myosin. There are two different regulatory systems found in different muscles: actin-linked regulation and myosin-linked regulation. In actin-linked regulation, troponin and tropomyosin regulate actin by blocking sites on actin required for complex formation with myosin. In myosin-linked regulation, sites on myosin are blocked in the absence of calcium.
Calcium also controls the energetics of the muscle by regulating the provision of ATP when needed. In skeletal muscle, Ca2+ binds to troponin (Tn) C on the thin filament, activating contraction. In addition, Ca2+ may also affect the muscle contraction apparatus through direct interaction with myosin and other motor proteins. The speed of muscle contraction and relaxation, as well as other physiological parameters, are critically dependent on the composition of components belonging to the Ca2+ handling apparatus.
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Frequently asked questions
Cardiac 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.
Visceral muscle is involuntarily controlled by the unconscious regions of the brain through the autonomic and enteric nervous systems. Visceral muscle cells are found in the organs, blood vessels, and bronchioles of the body to move substances.
No, cardiac muscle is not visceral. Visceral muscle is restricted to specific areas like the tongue, upper oesophagus, the pharynx, and the lumbar part of the diaphragm.
Cardiac muscle is structurally and functionally unique, with characteristics from both skeletal and visceral muscle tissues. It is capable of strong, continuous, and rhythmic contractions that are automatically generated. On the other hand, visceral muscle cells are long and thin, and while each cell is very weak, they can work together to produce powerful, long-lasting contractions.
Many visceral muscle cells in the uterus can contract together to push a fetus out of the womb during childbirth.
