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The heart is a vital organ in the human body, and its muscular composition is unique. The heart is made up of cardiac muscle, also known as myocardium or heart muscle. This muscle tissue is distinct from other types of muscle in the body, such as skeletal and smooth muscle, due to its specialised structure and function. Cardiac muscle has a striated appearance, with individual muscle cells called cardiomyocytes that are joined by intercalated discs to form long fibres. These striations are caused by the alternating light and dark patterns of contractile proteins actin and myosin. The heart's pacemaker cells respond to signals from the autonomic nervous system, regulating the heart rate and coordinating the contraction of the myocardium to pump blood efficiently throughout the body.
| Characteristics | Values |
|---|---|
| Types of muscle tissue | Cardiac, smooth, and skeletal |
| Location of cardiac muscle | Wall of the heart |
| Appearance of cardiac muscle | Striated |
| Control of cardiac muscle | Involuntary |
| Appearance of smooth muscle | Spindle-shaped, no striations |
| Location of smooth muscle | Walls of hollow visceral organs (liver, pancreas, intestines), arteries and veins, gastrointestinal and respiratory tracts, and genitourinary tracts |
| Control of smooth muscle | Involuntary |
| Appearance of skeletal muscle | Striated |
| Location of skeletal muscle | Attached to the skeleton |
| Control of skeletal muscle | Voluntary |
| Composition of cardiac muscle cells | Individual cardiac muscle cells joined by intercalated discs, encased by collagen fibers and other substances that form the extracellular matrix |
| Composition of skeletal muscle cells | Bundles of long, multinucleated cells called fibers |
| Composition of smooth muscle cells | Short, spindle-shaped, single nucleus in each fiber |
| Function of striated muscle tissue | To create force and contract |
| Function of cardiac muscle tissue | To pump blood throughout the body |
| Function of skeletal muscle tissue | To enable breathing, movement, and posture maintenance |
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What You'll Learn
- Cardiac muscle cells are striated and located in the heart wall
- Skeletal muscle is striated and attached to the skeleton
- Smooth muscle is non-striated and located in visceral organs
- Cardiomyocytes are responsible for the heart's rhythmic contractions
- Cardiac muscle tissue is unable to regenerate in adult humans
Cardiac muscle cells are striated and located in the heart wall
Cardiac muscle cells, also known as cardiomyocytes, are striated and located in the heart wall. They are the contractile myocytes of the cardiac muscle and are responsible for the contractility of the heart, which results in the pumping action. The heart wall is a three-layered structure, with a thick layer of myocardium sandwiched between the inner endocardium and the outer epicardium or visceral pericardium. The cardiac muscle forms the middle layer of the heart wall, with the outer layer being the pericardium and the inner layer being the endocardium.
Cardiac muscle cells are joined at their ends by intercalated discs to form long fibres. These discs are composed of collagen fibres and other substances that form the extracellular matrix. Each cell contains myofibrils, specialised protein contractile fibres of actin and myosin that slide past each other. This sliding movement is caused by the release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum. The regular organisation of myofibrils into sarcomeres gives cardiac muscle cells their striated appearance. These sarcomeres are the fundamental contractile units of muscle cells.
The striated appearance of cardiac muscle cells is similar to that of skeletal muscle cells. However, there are important structural differences between the two types of muscle cells. Unlike skeletal muscle cells, cardiac muscle cells are branched and contain a single, centrally located nucleus surrounded by a cell membrane called the sarcolemma. The sarcolemma contains voltage-gated calcium channels, which are specialised ion channels that skeletal muscle cells do not possess. Additionally, skeletal muscle cells are long and multinucleated, while cardiac muscle cells are cylindrical or roughly rectangular in shape.
Cardiac muscle cells are under involuntary control, meaning they contract without conscious input. They work together in a network that functions as a functional syncytium, where each cell contracts in coordination with its neighbouring cells. This coordinated contraction is essential for efficiently pumping blood from the heart. If this coordination breaks down, the heart may not pump effectively, leading to abnormal heart rhythms such as ventricular fibrillation.
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Skeletal muscle is striated and attached to the skeleton
The human body is made up of three types of muscle tissue: skeletal, cardiac, and smooth muscle. Skeletal muscle is striated and attached to the skeleton via tendons. It is the most common type of muscle in the body, making up between 30% and 40% of total body mass.
Skeletal muscle is composed of long, multinucleated cells called fibers, which are bundled together to form a unique striated pattern. This pattern is caused by the arrangement of actin and myosin filaments, which create light and dark bands. These bands are essential for the contraction of the muscle, as they slide past each other during this process. Skeletal muscle is also known as voluntary muscle, as it can be consciously controlled, unlike cardiac and smooth muscle.
Skeletal muscle is found throughout the body and is attached to bones by tendons. Together, they produce all body movements. They also serve other functions, such as maintaining body posture and position, stabilizing joints, maintaining body temperature, and storing nutrients.
The main functions of skeletal muscle, from a mechanical standpoint, are to convert chemical energy into mechanical energy, thus generating force and power. Skeletal muscle also produces body heat, which is a by-product of muscular activity. This heat is important in homeostatic responses to extreme cold, triggering contractions of shivering to generate heat.
Skeletal muscle disorders can manifest as muscle weakness and can range from mild injuries to serious or even life-threatening myopathies. Maintaining muscle strength is important for overall health, and there are various ways to build and maintain muscle strength.
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Smooth muscle is non-striated and located in visceral organs
The heart is made up of cardiac muscle cells, which are striated and located in the walls of the heart. Cardiac muscle is one of three types of muscle tissue, the other two being skeletal muscle and smooth muscle.
Smooth muscle is non-striated, meaning it has no sarcomeres and therefore no striations or bands. It is one of the three major types of vertebrate muscle tissue, the other two being skeletal and cardiac muscle. Smooth muscle can also be found in invertebrates and is controlled by the autonomic nervous system.
Smooth muscle consists of thick and thin filaments that are not arranged into sarcomeres, giving it a non-striated pattern. On microscopic examination, it appears homogeneous. Smooth muscle cytoplasm contains a large amount of actin and myosin, the main proteins involved in muscle contraction. Actin filaments attach to dense bodies that are spread throughout the cell. The dense bodies can be observed under an electron microscope and appear dark. Another important structure is the calcium-containing sarcoplasmic reticulum, which aids in sustaining contraction.
Smooth muscle is usually arranged in sheets or layers in tubular systems such as arteries and veins, the gastrointestinal and respiratory tracts, and the genitourinary tracts. It is located in the walls of hollow visceral organs, such as the liver, pancreas, and intestines, but not in the heart. Smooth muscle helps regulate blood flow by controlling the diameter of the vessel.
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Cardiomyocytes are responsible for the heart's rhythmic contractions
The heart is a striated muscle, and its rhythmic contractions are made possible by cardiomyocytes. Cardiac muscle or myocardium forms the bulk of the heart. It is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. Striations in cardiac muscle are caused by lighter I bands composed mainly of actin, and darker A bands composed mainly of myosin.
Cardiomyocytes are the contractile myocytes of the cardiac muscle. They are responsible for the rhythmic contractions of the heart, which allow the organ to pump blood. Each cardiomyocyte must contract in coordination with its neighbouring cells, working together to efficiently pump blood from the heart. This coordination is known as a functional syncytium. If this coordination breaks down, the heart may not pump at all, as may occur during abnormal heart rhythms such as ventricular fibrillation.
Cardiomyocytes are connected in a network that functions as a syncytium, which is critical to the coordinated and efficient function of the myocardium during each heartbeat. The intercalated discs that connect the cardiomyocytes consist of three types of cell-cell junctions: the actin filament anchoring fascia adherens junctions, the intermediate filament anchoring desmosomes, and gap junctions. These junctions allow action potentials to spread between cardiac cells by permitting the passage of ions between cells, producing depolarization of the heart muscle.
Cardiomyocytes contain T-tubules, pouches of cell membrane that run from the cell surface to the cell's interior, which help to improve the efficiency of contraction. The majority of these cells contain only one nucleus, unlike skeletal muscle cells which contain many nuclei. The rise in calcium causes the cell's myofilaments to slide past each other in a process called excitation-contraction coupling.
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Cardiac muscle tissue is unable to regenerate in adult humans
The heart is composed of cardiac muscle tissue, also known as myocardium, which is involuntary, striated muscle tissue. Cardiac muscle tissue is located in the walls of the heart and forms a thick middle layer between the outer layer of the heart wall (the pericardium) and the inner layer (the endocardium).
In contrast, lower vertebrates such as newts, salamanders, and zebrafish have an impressive ability to replace lost cardiac tissue and regenerate their hearts. For example, salamanders can take their cardiac myocytes back to an earlier, more primitive state, allowing them to re-enter the cell cycle and create new heart muscle. This process enables them to repair damage and then revert their myocytes back into mature cells once the damage is repaired.
Recent research suggests that mammals may have a brief period during which they can regenerate their hearts, about the first week of life. However, this ability is quickly lost. Researchers are now working on methods to reprogram a patient's own cardiac myocytes within the heart to create new muscle and repair damage. One potential approach involves using nanoparticles to deliver small interfering RNA to the heart to knock out the proteins that keep the myocytes mature. By temporarily blocking these proteins, adult cardiac myocytes may be able to revert to a state where they can proliferate and regenerate damaged heart muscle.
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Frequently asked questions
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. It is an involuntary, striated muscle that constitutes the main tissue of the wall of the heart.
The heart is striated because it is made up of repeating functional units called sarcomeres. Under a microscope, these sarcomeres are visible along muscle fibers, giving a striated appearance to the tissue. The striations are caused by lighter I bands composed mainly of actin, and darker A bands composed mainly of myosin.
The main function of striated muscle tissue is to create force and contract. These contractions in cardiac muscle will pump blood throughout the body. In skeletal muscle, the contractions enable breathing, movement, and posture maintenance.
Cardiac muscle forms the contractile walls of the heart. Skeletal muscle, on the other hand, is attached to the skeleton via tendons and can be consciously controlled. Cardiac myocytes are also single cells with a single centrally located nucleus, while skeletal muscle cells are long and multinucleated.
