Logan Steele
Cardiac muscle, also known as heart muscle or myocardium, is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. Cardiac muscle is striated and found only in the heart. It is composed of individual cardiac muscle cells joined by intercalated discs, which contain gap junctions and desmosomes. These gap junctions allow cardiac muscle cells to be electrically coupled, so they beat in synchrony. Intercalated discs are complex adhering structures that connect single cardiomyocytes to an electrochemical syncytium.
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What You'll Learn
Cardiac muscle cells have intercalated discs
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. Cardiac muscle is composed of individual cardiac muscle cells joined by intercalated discs. These discs are complex adhering structures that connect the single cardiomyocytes to an electrochemical syncytium.
Intercalated discs are found at the ends of cardiac muscle cells, connecting them to form long fibers. They are composed of three different types of cell-to-cell junctions: actin filament anchoring fascia adherens junctions, intermediate filament anchoring desmosomes, and gap junctions. Desmosomes are cell structures that anchor the ends of cardiac muscle fibers together, ensuring the cells do not pull apart during the stress of individual fibers contracting. Gap junctions allow the muscle cells to be electrically coupled, so they beat in synchrony.
The region where the ends of the cells are connected to another cell is called an intercalated disc. These discs allow the cardiac muscle cells to contract in a wave-like pattern, enabling the heart to work as a pump. This joining is called electric coupling, and it allows the quick transmission of action potentials and the coordinated contraction of the entire heart. This network of electrically connected cardiac muscle cells creates a functional unit of contraction called a syncytium.
Intercalated discs are part of the sarcolemma, the cardiomyocyte plasma membrane containing transverse tubules (t-tubules). T-tubules are highly branched invaginations of the cardiomyocyte sarcolemma that function in excitation-contraction coupling (ECC), action potential initiation and regulation, maintaining the resting membrane potential, and signal transduction.
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Intercalated discs contain gap junctions
Intercalated discs are part of the cardiac muscle sarcolemma and play a crucial role in facilitating cardiac muscle contraction. These discs contain gap junctions, which are essential for the transmission of electrical signals between cardiac muscle cells, enabling them to contract in a coordinated manner.
Gap junctions within intercalated discs form channels that allow the flow of depolarizing currents produced by cations from one cardiac muscle cell to the next. This electrical coupling ensures that cardiac muscle cells beat in synchrony, resulting in the coordinated contraction of the entire heart. The depolarization that occurs through gap junctions is vital for the initiation of contraction in cardiac muscles, with a significant portion of the calcium ions required for contraction entering the cells from external sources.
In addition to gap junctions, intercalated discs also contain desmosomes, which are cell structures that act as anchoring points. Desmosomes hold the ends of adjacent cardiac muscle fibres together, providing structural support and ensuring that the cells do not pull apart during the contraction process. This mechanical strength is particularly important as the heart is exposed to increased mechanical load.
The presence of intercalated discs with their gap junctions and desmosomes is a distinctive feature of cardiac muscle cells. These specialised junctions allow for the quick transmission of action potentials and facilitate the wave-like contraction pattern necessary for the heart to function effectively as a pump. The coordinated contraction of the heart, made possible by intercalated discs, is essential for maintaining cardiac function and overall physiological homeostasis.
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Gap junctions allow electrical coupling
Cardiac muscle cells are connected to one another at their ends by intercalated discs. These discs contain gap junctions, which allow the muscle cells to be electrically coupled so that they beat in synchrony. This electrical coupling is essential for the quick transmission of action potentials and the coordinated contraction of the entire heart.
Gap junctions are specialized membrane structures consisting of arrays of intercellular channels that connect adjacent cells in many tissues and organs. In cardiac muscle, gap junctions form channels between adjacent muscle fibers, allowing the depolarizing current produced by cations to flow from one cardiac muscle cell to the next. This results in a wave of contraction that allows the heart to work as a unit, called a functional syncytium.
The specific subcellular distribution of gap junctions, along with the tight packaging of rod-shaped cardiomyocytes, contributes to continuous anisotropic conduction at the macroscopic scale. Gap junctions play a vital role in maintaining the velocity and safety of impulse propagation in cardiac tissue.
Additionally, gap junctions are involved in the orderly and sequential spread of activation from the atrial to ventricular chambers. Connexins, the protein molecules forming gap junction channels, are essential for this process. A reduction or redistribution of connexins in intercalated discs can lead to arrhythmogenic gap junction remodeling, impacting cardiac conduction and heart morphogenesis.
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Intercalated discs contain desmosomes
Intercalated discs, also known as lines of Eberth, are complex structures that connect adjacent cardiac muscle cells. They are only found in cardiac muscles and are responsible for connecting and binding cardiac muscle cells. Intercalated discs contain three components: fascia adherens, desmosomes, and gap junctions.
Desmosomes are cell structures that act as binders during the contraction of cardiac muscles. They are also known as macula adherens (plural: maculae adherentes), which is Latin for "adhering spot". Desmosomes prevent separation during contraction by binding intermediate filaments, anchoring the cell membrane to the intermediate filament network, and joining the cells together.
The remainder of the intercalated disc is composed of fascia adherens and gap junctions. Fascia adherens are ribbon-like protein structures that function as a connector and binder of cardiac muscle cells. They anchor the thin actin filament of one cardiac muscle cell to the actin filaments of the neighbouring cells, allowing the fascia adherens to transmit contractile force between the cardiac muscle cells.
Gap junctions connect the cytoplasms of neighbouring cells electrically, allowing cardiac action potentials to spread between cardiac cells by permitting the passage of ions between cells, producing depolarization of the heart muscle. All of these junctions work together as a single unit called the area composita.
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Desmosomes anchor cardiac muscle fibres
Cardiac muscle cells are connected to one another at their ends by intercalated discs. These discs contain gap junctions, adhering junctions, and desmosomes. Desmosomes are intercellular structures that anchor cardiac muscle fibres together. They are vital in maintaining the structural integrity of the heart.
Desmosomes are cell structures that anchor the ends of cardiac muscle fibres together so that the cells do not pull apart during the stress of individual fibres contracting. This joining is called electric coupling, and in cardiac muscle, it allows the quick transmission of action potentials and the coordinated contraction of the entire heart. This network of electrically connected cardiac muscle cells creates a functional unit of contraction called a syncytium.
The remainder of the intercalated disc is composed of desmosomes. Intercalated discs are part of the sarcolemma and contain two structures important in cardiac muscle contraction: gap junctions and desmosomes. A gap junction forms channels between adjacent cardiac muscle fibres that allow the depolarising current produced by cations to flow from one cardiac muscle cell to the next.
The desmosomal cadherins include four isoforms of desmoglein (DSG1 to DSG-4) and three of desmocollin (DSC1 to DSC-3), all encoded by separate genes that are clustered together on opposite sides of a central region on chromosome 18q12.1. DSC-2 and DSG-2 are expressed in all desmosome-containing tissues, including the colon and cardiac muscle.
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Frequently asked questions
Yes, cardiac muscle cells are joined at their ends by intercalated discs to form long fibers. These discs contain gap junctions for depolarization between cells and desmosomes to hold the fibers together when the heart contracts.
Intercalated discs allow cardiac muscle cells to contract in a wave-like pattern so that the heart can work as a pump. They also allow the quick transmission of action potentials and the coordinated contraction of the entire heart.
Intercalated discs consist of three different types of cell-cell junctions: the actin filament anchoring fascia adherens junctions, the intermediate filament anchoring desmosomes, and gap junctions.
