Logan Steele
The human body has three types of muscle tissues: skeletal, smooth, and cardiac. Striated muscle tissue is a type of muscle tissue that features repeating functional units called sarcomeres. The two types of striated muscle are skeletal muscle and cardiac muscle. Cardiac muscle, also known as myocardium or heart muscle, is an involuntary, striated muscle that forms the main tissue of the heart wall. This muscle tissue is responsible for the heart's contractions and the subsequent pumping of blood throughout the body.
| Characteristics | Values |
|---|---|
| Type | Striated muscle |
| Other Names | Heart muscle, myocardium |
| Location | Between the outer layer of the heart wall (epicardium/pericardium) and the inner layer (endocardium) |
| Composition | Individual cardiac muscle cells joined by intercalated discs, encased by collagen fibres and other substances that form the extracellular matrix |
| Appearance | Striped or striated due to the regular alternation of contractile proteins actin and myosin |
| Cell Shape | Rectangular |
| Cell Size | 100–150μm by 30–40μm |
| Cell Nuclei | Single |
| Cell Junctions | Anchoring junctions and gap junctions |
| Contractions | Involuntary |
| Contraction Stimulus | 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 |
| Contraction Mechanism | Myofibrils composed of actin and myosin myofilaments slide past each other |
| Contraction Appearance | Longitudinally (becoming shorter from apex to base), radially (becoming narrower from side to side), and with a twisting motion |
| Contraction Uses | Pumps blood throughout the body |
| Regeneration | Adult humans cannot regenerate cardiac muscle tissue after an injury |
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What You'll Learn
Cardiac muscle forms the contractile walls of the heart
Cardiac muscle, also known as myocardium or heart muscle, is one of the three types of vertebrate muscle tissues, the other two being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that forms the contractile walls of the heart. The cardiac muscle constitutes the main tissue of the heart wall and forms a thick middle layer 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, joined by intercalated discs and encased by collagen fibres and other substances that form the extracellular matrix. These intercalated discs contain both anchoring junctions and gap junctions. The anchoring junctions hold adjacent cells together, while the gap junctions allow the cells to communicate and synchronize their actions. The cardiomyocytes are single cells with a single, centrally located nucleus, unlike skeletal muscle cells, which have many nuclei. The cardiomyocytes are joined together to form long, branching cardiac muscle fibres.
The contractile proteins actin and myosin 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 under a microscope, similar to skeletal muscle. These striations are caused by lighter I bands composed mainly of actin and darker A bands composed mainly of myosin. The cardiac muscle cells also contain T-tubules, which enable the release of calcium ions from the sarcoplasmic reticulum, and a large number of mitochondria, which produce ATP for energy and help the heart resist fatigue.
The contractions of the cardiac muscle pump blood throughout the body and are involuntary, like those of smooth muscle. The contractions are due to a myogenic response of the heart's pacemaker cells, which respond to signals from the autonomic nervous system to increase or decrease the heart rate. The pacemaker cells are only weakly contractile and are connected to neighbouring contractile cells via gap junctions. They are located in the sinoatrial node (the primary pacemaker) and the atrioventricular node (secondary pacemaker).
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Cardiac muscle is striated due to the arrangement of actin and myosin filaments
Cardiac muscle, also known as myocardium or heart muscle, is one of the three types of vertebrate muscle tissues, the other two being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that forms the main tissue of the heart wall. The cardiac muscle is a thick middle layer between the outer layer of the heart wall (the pericardium) and the inner layer (the endocardium).
The cardiac muscle is composed of individual cardiac muscle cells, or cardiomyocytes, joined by intercalated discs and encased by collagen fibres and other substances that form the extracellular matrix. These cardiomyocytes are single cells with a single, centrally located nucleus. They are connected in a network that functions as a syncytium, allowing the cells to synchronize their actions and facilitating a coordinated heartbeat.
The striated appearance of cardiac muscle is due to the arrangement of actin and myosin filaments. Each cardiomyocyte contains myofibrils, specialized protein contractile fibres of actin and myosin that slide past each other. 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 under a microscope, similar to skeletal muscle. These striations are caused by lighter I bands composed mainly of actin and darker A bands composed mainly of myosin.
The cardiac muscle's contractile properties are essential for pumping blood throughout the body. The contractions are due to a myogenic response of the heart's pacemaker cells, which respond to signals from the autonomic nervous system to regulate heart rate. The calcium release caused by electrical stimulation triggers the sliding of myofilaments, resulting in muscle contraction.
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Cardiac muscle is involuntary, unlike skeletal muscle
Cardiac muscle, also known as myocardium, is one of the 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 cardiac muscle forms a thick middle layer between the outer layer of the heart wall (the pericardium) and the inner layer (the endocardium).
Cardiac muscle cells (also called cardiomyocytes) are the contractile myocytes of the cardiac muscle. The cells are surrounded by an extracellular matrix produced by supporting fibroblast cells. Under a microscope, cardiac muscle cells appear rectangular and striated with a single nucleus. The striation is due to the regular alternation of the contractile proteins actin and myosin, along with the structural proteins that couple the contractile proteins to connective tissues. The regular organization of myofibrils into sarcomeres gives cardiac muscle cells a striped or striated appearance when viewed through a microscope, similar to skeletal muscle.
Unlike skeletal muscle, cardiac muscle is involuntary. Skeletal muscle is voluntary because it is consciously controlled by the somatic nervous system, which involves motor neurons that connect the brain and spinal cord to skeletal muscle fibers. In contrast, cardiac muscle is regulated by the autonomic nervous system, which controls involuntary body functions like heart rate, digestion, and blood flow. The automaticity of cardiac muscle allows the heart to pump blood constantly and efficiently without us having to think about it.
The cardiac muscle must contract with enough force and blood to supply the metabolic demands of the entire body. When these sheets contract in a coordinated manner, they allow the ventricle to squeeze in several directions simultaneously – longitudinally (becoming shorter from apex to base), radially (becoming narrower from side to side), and with a twisting motion (similar to wringing out a damp cloth) to squeeze the maximum possible amount of blood out of the heart with each heartbeat.
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Cardiac muscle cells are joined by intercalated discs
Cardiac muscle cells, also known as cardiomyocytes, are joined by intercalated discs. Intercalated discs are complex structures that connect adjacent cardiac muscle cells. They are observed under a light microscope as dark-staining lines running in a perpendicular direction to muscle fibres. These discs provide the electrochemical and mechanical connection between neighbouring cardiac muscle cells.
Intercalated discs are composed of three types of cell junctions: desmosomes, fascia adherens junctions, and gap junctions. Desmosomes are junctions that prevent separation during contraction by binding intermediate filaments and anchoring the cell membrane to the intermediate filament network, joining the cells together. Fascia adherens junctions are anchoring sites for actin, connecting to the closest sarcomere. Gap junctions, on the other hand, are responsible for connecting the cytoplasms of neighbouring cells electrically, allowing cardiac action potentials to spread between cardiac cells by permitting the passage of ions between them. This results in the depolarization of the heart muscle.
The presence of intercalated discs allows cardiac muscle cells to work as a single functional syncytium. This means that the individual cardiac muscle cells can function together as a coordinated network, enabling the synchronized contraction of cardiac tissue in a wave-like pattern. This synchronized contraction is essential for the heart to work efficiently as a pump.
Intercalated discs play a crucial role in maintaining the health and function of the heart. Mutations in the intercalated disc gene can lead to various cardiomyopathies, which are diseases of the heart muscle. These mutations can result in heart failure and life-threatening arrhythmias. Understanding the structure and function of intercalated discs is, therefore, important for advancing our knowledge of cardiac function and developing treatments for heart-related conditions.
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Cardiac muscle cells have a single nucleus
Cardiac muscle, or myocardium, is one of the three types of vertebrate muscle tissues, the other two being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that forms the main tissue of the heart wall.
Cardiac muscle cells, or cardiomyocytes, are the contractile myocytes of the cardiac muscle. They are single cells with a single, centrally located nucleus. This is in contrast to skeletal muscle cells, which have many nuclei. The presence of a single nucleus is a feature that cardiac muscle cells share with smooth muscle cells.
Cardiac muscle cells are roughly rectangular in shape and are joined at their ends by intercalated discs to form long fibers. These intercalated discs are specialised regions that contain gap junctions, allowing the regulated transport of molecules and electrical signals from one cardiomyocyte to another. This facilitates a synchronized heartbeat.
The regular organisation of myofibrils into sarcomeres gives cardiac muscle cells their striated appearance when viewed under a microscope. These striations are caused by the lighter I bands, composed mainly of actin, and the darker A bands, composed mainly of myosin. The contractile proteins actin and myosin are arranged in a highly organised lattice, which forms the basis of the force-generating apparatus.
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Frequently asked questions
Yes, the cardiac muscle is striated.
The cardiac muscle, also known as the myocardium or heart muscle, is one of the three types of vertebrate muscle tissues, the other two being skeletal muscle and smooth muscle.
Striated means having a striped appearance. Under a microscope, striated muscles have a striped appearance due to the regular alternation of contractile proteins actin and myosin.
The cardiac muscle forms the contractile walls of the heart and is responsible for pumping blood throughout the body.
Cardiac muscle cells are roughly rectangular and appear striated under a microscope. They have a single, centrally located nucleus, unlike skeletal muscles which have many nuclei.
