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Cardiac muscle, also known as myocardium or heart muscle, is an involuntary, striated muscle that constitutes the main tissue of the heart wall. It is composed of individual cardiac muscle cells, or cardiomyocytes, joined by intercalated discs to form long fibers. These muscle fibers are cylindrical in shape, typically measuring 100-150μm by 30-40μm when viewed through a microscope. Each cardiomyocyte contains myofibrils, which are contractile fibers composed of actin and myosin proteins. These myofibrils are arranged in a regular pattern, giving cardiac muscle cells their distinctive striated appearance. The contraction of cardiac muscle fibers is coordinated by electrical impulses, allowing the heart to pump blood efficiently.
| Characteristics | Values |
|---|---|
| Shape | Branched and cylindrical |
| Cell type | Involuntary |
| Cell structure | Striated |
| Cell size | 100–150μm by 30–40μm |
| Cell composition | Individual cardiac muscle cells joined by intercalated discs |
| Cell function | Contracting cells that allow the heart to pump |
| Cell coordination | Functional syncytium |
| Cell communication | Gap junctions |
| Cell contraction | Excitation-contraction coupling |
| Cell components | Myofibrils, T-tubules, sarcomeres |
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What You'll Learn
Cardiac muscle cells are cylindrical and branched
Cardiac muscle cells, also known as myocardium or heart muscle, are found only in the heart. They constitute the main tissue of the heart wall and are responsible for the heart's contractile function. These muscle cells are cylindrical and branched, with a mean diameter of 10-100 μm. They are composed of long, branched fibres that are shaped like cylinders joined end-to-end, with one or two nuclei located centrally. The fibres are separated by collagenous tissue, which supports the capillary network of cardiac tissue.
Cardiac muscle cells exhibit a striped or striated appearance under a microscope due to the regular organisation of myofibrils into sarcomeres. Myofibrils are composed of contractile proteins, actin and myosin, which slide past each other during muscle contraction. The sarcomeres are the fundamental contractile units of muscle cells and are arranged in alternating light and dark bands called striae. This banding pattern is caused by regions of overlap and non-overlap of thick and thin filaments along the length of the myofibrils.
The cardiac muscle cells are joined end-to-end by intercalated discs, which appear as thin, dark-staining lines dividing adjacent muscle cells under light microscopy. These discs provide attachment points, creating a characteristic branched pattern. They also enable the cardiac muscle tissue to function as a syncytium, allowing coordinated contractions. The intercalated discs consist of three types of cell-cell junctions: actin filament anchoring fascia adherens junctions, intermediate filament anchoring desmosomes, and gap junctions.
Cardiac muscle cells are involuntary and excitable, capable of generating electrical impulses and inducing action potentials. The release of calcium ions from the sarcoplasmic reticulum, a network of tubules within the muscle cells, is crucial for initiating contraction. The rise in calcium causes the myofilaments to slide past each other, resulting in muscle contraction. This process is known as excitation-contraction coupling and is similar to that of skeletal muscle, although some differences exist.
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Cardiac muscle cells are joined by intercalated discs
Cardiac muscle cells, or cardiomyocytes, are joined by intercalated discs to form long fibres. Each cardiomyocyte needs to contract in coordination with its neighbouring cells to efficiently pump blood from the heart. This is known as a functional syncytium.
Intercalated discs are complex structures that connect adjacent cardiac muscle cells. They are composed of three types of cell junctions: desmosomes, fascia adherens junctions, and gap junctions. Desmosomes are intermediate filament anchoring sites that prevent separation during contraction by binding intermediate filaments, anchoring the cell membrane to the intermediate filament network, and joining the cells together. Fascia adherens junctions are anchoring sites for actin filaments and connect to the closest sarcomere. 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.
The three types of junctions work together as a single unit called the area composita. Intercalated discs enable the rapid transmission of electrical impulses through the network of cardiomyocytes, enabling the syncytium to act in a coordinated contraction of the myocardium. This allows the heart to work like a pump.
Intercalated discs are also found in the cardiac muscle cells of the black widow spider, Latrodectus mactans. In this species, the intercalated discs support multiple connections to adjacent cells.
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Cardiac muscle cells are striated
Cardiac muscle cells, also known as cardiomyocytes, are striated. They are the individual cells that make up cardiac muscle, which forms a thick middle layer of the heart wall known as the myocardium. This layer sits between the outer layer of the heart wall (the pericardium) or visceral pericardium and the inner layer (the endocardium).
Cardiac muscle cells are roughly rectangular, measuring 100-150μm by 30-40μm. They are joined at their ends by intercalated discs to form long fibres. Each cell contains myofibrils, which are specialised protein contractile fibres of actin and myosin that slide past each other. These are organised into sarcomeres, the fundamental contractile units of muscle cells. The regular organisation of myofibrils into sarcomeres gives cardiac muscle cells their striated appearance when viewed through a microscope.
The striated appearance consists of thick, dark-coloured A-bands composed mainly of myosin, with a relatively bright H-zone in the centre, and lighter-coloured I-bands composed mainly of actin, with a dark central Z-line (also known as a Z-disc) connecting the actin filaments. These lighter and darker bands give cardiac muscle cells their striped or striated look.
The intercalated discs that connect individual cardiac muscle cells are composed of three different types of cell-cell junctions: actin filament anchoring fascia adherens junctions, intermediate filament anchoring desmosomes, and gap junctions. These junctions allow action potentials to spread between cardiac cells by permitting the passage of ions between them, producing depolarisation of the heart muscle. This enables the rapid transmission of electrical impulses through the network of cardiomyocytes, facilitating coordinated contraction of the myocardium.
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Cardiac muscle cells are involuntary
Cardiac muscle cells, also known as cardiomyocytes, are striated, branched, and contain many mitochondria. They are under involuntary control, meaning their movements are automatic and cannot be controlled by a person. This is in contrast to skeletal muscle fibres, which are under voluntary control.
Cardiac muscle cells are located in the walls of the heart and are responsible for keeping the heart pumping and relaxing normally. They receive electrical impulses from the nervous system, which causes them to contract and expand, resulting in the rhythmic, wave-like contractions known as the heartbeat. This process is known as excitation-contraction coupling (ECC). The generation of a cardiac action potential is involuntary and occurs through ECC, where action potentials travel along the sarcolemma and into the t-tubules to depolarize the membrane.
The cardiac syncytium is a network of cardiomyocytes connected by intercalated discs, which enable the rapid transmission of electrical impulses and coordinated contraction of the myocardium. Each cardiomyocyte needs to contract in coordination with its neighbouring cells to efficiently pump blood from the heart. If this coordination breaks down, the heart may not pump at all, as seen in abnormal heart rhythms such as ventricular fibrillation.
Cardiac muscle cells contain branched fibres that are connected via intercalated discs, forming long fibres. These discs contain gap junctions and desmosomes, which allow cardiomyocytes to contract together synchronously. The functional unit of cardiomyocyte contraction is the sarcomere, which consists of thick (myosin) and thin (actin) filaments that slide past each other during contraction.
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Cardiac muscle cells are multinucleated
Cardiac muscle cells are usually mononucleated, meaning there is one nucleus per cell. However, they can sometimes contain more than one nucleus. Cardiac muscle, found only in the heart, has unique properties that distinguish it from skeletal muscle. While skeletal muscle fibres are always multinucleated, cardiac muscle cells typically contain a single nucleus.
The cell membrane of a cardiac muscle cell has several specialized regions, including the intercalated disc and transverse tubules. The intercalated discs are complex adhering structures that connect single cardiomyocytes to an electrochemical syncytium. They are composed of three different types of cell-cell junctions: the actin filament anchoring fascia adherens junctions, the intermediate filament anchoring desmosomes, and gap junctions. Intercalated discs enable the rapid transmission of electrical impulses through the network, allowing the syncytium to act in a coordinated contraction of the myocardium.
The cell membrane is covered by a lamina coat that is approximately 50 nm wide and can be separated into two layers: the lamina densa and lamina lucida. In between these two layers are several different types of ions, including calcium. The release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum, triggers the cell's myofilaments to slide past each other in a process called excitation-contraction coupling.
Cardiac muscle cells are joined at their ends by intercalated discs to form long fibres. Each cell contains myofibrils, specialized protein contractile fibres of actin and myosin that slide past each other during contraction. These are organized into sarcomeres, the fundamental contractile units of muscle cells. The regular organization of myofibrils into sarcomeres gives cardiac muscle cells a striped or striated appearance when viewed through a microscope.
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Frequently asked questions
Yes, cardiac muscle fibers are cylindrical in shape. They are also branched and striated.
Individual cardiac muscle cells are roughly rectangular when viewed through a microscope. They are joined at their ends by intercalated discs to form long fibers.
Cardiac muscle fibers have a mean diameter of 10-100 μm.
Intercalated discs are complex adhering structures that connect individual cardiomyocytes to form a functional syncytium. They enable the rapid transmission of electrical impulses and facilitate the contraction of the myocardium.
Skeletal muscles are voluntary muscles that are attached to bones and tendons to regulate locomotion and maintain body posture. Cardiac muscles, on the other hand, are involuntary and are found only in the heart, constituting the main tissue of the heart wall.
