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Cardiac muscle, also known as myocardium or heart muscle, is an involuntary striated muscle tissue that forms the heart wall's middle layer. It is composed of cardiomyocytes or cardiac muscle cells, which are joined by intercalated discs. These cells contain myofibrils, which are polymers of sarcomeres, the basic contractile units of cardiac muscle. The sarcomeres are composed of thick and thin filaments of myosin and actin, respectively, that give the muscle its striated appearance. The contraction of cardiac muscle is initiated by electrical stimulation, which triggers the release of calcium from the sarcoplasmic reticulum, leading to muscle contraction through excitation-contraction coupling.
| Characteristics | Values |
|---|---|
| What are myofibrils? | Myofibrils are essentially polymers, or repeating units, of sarcomeres. |
| What are sarcomeres? | Sarcomeres are the functional subunits of myofibrils and the contractile units of cardiac muscle tissue. |
| What are they composed of? | Sarcomeres are composed of thick and thin filaments – myosin and actin, respectively, along with a titin that anchors the myosin to the Z-lines. |
| What is the role of myofibrils? | Myofibrils are the most abundant organelle within myocytes, occupying approximately 50–60% of the cytoplasm. |
| Do cardiac muscles have myofibrils? | Yes, cardiac muscles have myofibrils. |
| What is the role of myosin? | Myosin binds to ATP, pulling actin filaments to the center of the sarcomere, creating the mechanical force of contraction. |
| What is the role of actin? | Actin is a globular protein that combines with other actin globules to form two intertwined strands with positive and negative ends. |
| What is the role of calcium? | Calcium ions are conducted into the cardiomyocyte, leading to the release of more ions into the cytoplasm. Calcium prolongs the period of cardiac muscle cell depolarization before repolarization begins. |
| What is the role of troponin? | Troponin is a component of thin filaments, along with actin and tropomyosin. |
| How do sarcomeres affect contraction? | Changes in sarcomere length are an important mechanism by which the heart regulates its force of contraction. |
| How do myofibrils affect muscle growth? | Growth in the girth of muscle fibers occurs by the splitting of myofibrils, which can be stimulated by development of stress on the sarcomere. |
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What You'll Learn
Cardiac muscle is composed of cardiomyocytes
Cardiac muscle, also called heart muscle or myocardium, is composed of cardiomyocytes. These cardiomyocytes are individual cardiac muscle cells encased by collagen fibres and other substances that form the extracellular matrix. Cardiomyocytes are linked to the basement membrane by specialised glycoproteins called integrins. Humans are born with a set number of cardiomyocytes, which increase in size as the heart grows during childhood development.
Cardiomyocytes are striated, branched, and contain many mitochondria. They are under involuntary control. Each cardiomyocyte contains 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 muscles do not possess. The sarcolemma also contains t-tubules, which are highly branched invaginations that play a role in excitation-contraction coupling, action potential initiation and regulation, maintaining the resting membrane potential, and signal transduction.
Cardiomyocytes are connected by intercalated discs, which 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 depolarisation of the heart muscle. The intercalated discs also allow the cardiomyocytes to contract together synchronously to enable the heart to work as a pump.
Within the myocardium, there are several sheets of cardiomyocytes. The sheets of muscle that wrap around the left ventricle closest to the endocardium are oriented perpendicularly to those closest to the epicardium. When these sheets contract in a coordinated manner, they allow the ventricle to squeeze in several directions simultaneously. This includes becoming shorter from apex to base, becoming narrower from side to side, and a twisting motion similar to wringing out a damp cloth. This enables the heart to squeeze the maximum possible amount of blood out with each heartbeat.
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Sarcomeres are the contractile units of cardiac muscle
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 forms the main tissue of the heart wall. Cardiac muscle contracts in a similar way to skeletal muscle, but with some key differences.
The Z-discs, largely made up of α-actinin, form the boundaries of sarcomeres and provide an attachment site for thin filaments. During the cardiac cycle, the length of the sarcomere changes as the thick and thin filaments slide past one another. The shortening of individual sarcomeres leads to the contraction of the individual muscle fibres, resulting in muscle contractions. The sarcomere length affects force and velocity – longer sarcomeres have more cross-bridges and can generate more force.
The rise in calcium causes the cell's myofilaments to slide past each other in a process called excitation-contraction coupling. Muscle contraction ends when calcium ions are pumped back into the sarcoplasmic reticulum, allowing the contractile apparatus and the muscle cell to relax.
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Sarcomeres are composed of thick and thin filaments
Cardiac muscle, also known as heart muscle or myocardium, is one of three types of vertebrate muscle tissues. It is an involuntary, striated muscle that constitutes the main tissue of the heart wall. Cardiac muscle cells are joined by intercalated discs and encased by collagen fibres and other substances that form the extracellular matrix.
Myofibrils are the most abundant organelle within myocytes, occupying approximately 50-60% of the cytoplasm. They are formed by repeating sarcomeres, the basic contractile units of cardiac muscle. Sarcomeres are composed of thick and thin filaments, which give the muscle its characteristic striated appearance.
The thick filaments are composed of myosin, one of three major classes of molecular motor proteins. Myosin is a bipolar protein that extends throughout the A-band and is cross-linked at the centre by the M-band. The M-band contains the protein myomesin and marks the centre of the sarcomere. The M-line also binds creatine kinase, which facilitates the reaction of ADP and phosphocreatine into ATP and creatine.
The thin filaments are composed of actin, the most abundant protein in most eukaryotic cells. Actin is a pivotal protein in muscle contraction and cell movements. It is the major component of the I-band and extends into the A-band. The thin filaments lie between the two thick filaments and are connected by the Z-disc, which forms the boundary of the sarcomere. The Z-disc is composed largely of α-actinin and provides an attachment site for thin filaments.
During contraction, the actin filaments slide past the myosin filaments, resulting in the shortening of the sarcomere and, consequently, the contraction of the muscle fibre. This process is known as the sliding filament theory and was proposed by scientists using high-resolution microscopes to visualise the actin and myosin filaments within a sarcomere.
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Cardiac muscle is striated due to sarcomeres
Cardiac muscle, also known as myocardium or heart muscle, is one of three types of muscle tissues in the vertebrate body, the other two being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that constitutes the main tissue of the heart wall. The cardiac muscle forms a thick middle layer of the three-layered heart wall, with the outer layer being the pericardium and the inner layer being the endocardium.
The striated appearance of the cardiac muscle is due to the presence of sarcomeres. Sarcomeres are the basic contractile units of the muscle, composed of interdigitating thin (actin) and thick (myosin) filaments. These filaments give the muscle its characteristic striated appearance. The sarcomeres are the functional units of the muscle, and their contraction leads to muscle contraction. The contraction of the cardiac muscle occurs through the excitation-contraction coupling (ECC) mechanism, which involves the release of calcium ions from the sarcoplasmic reticulum. This calcium release causes the cell's myofilaments to slide past each other, resulting in muscle contraction.
The sarcomeres in the cardiac muscle are organised into distinct, repeating units called myofibrils. Myofibrils are polymers or repeating units of sarcomeres, and they make up a significant portion of the muscle volume. The number of myofibrils in a muscle can vary, ranging from 50 per myocyte in fetal muscles to approximately 2000 per myocyte in untrained adult muscles. The growth of myofibrils occurs through the splitting of the myofibrils, which adds to the girth of the muscle fibres. This growth process also results in the addition of new sarcomeres.
The cardiac muscle's contractility is essential for the pumping action of the heart, ensuring that the heart contracts with enough force to supply blood to the entire body. The contractile units, the sarcomeres, play a crucial role in this process, and abnormalities in their structure or function can lead to cardiomyopathies, which are disorders associated with cardiac dysfunction.
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Cardiac muscle is involuntary
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. It is an involuntary, striated muscle that constitutes the main tissue of the heart wall. The cardiac muscle forms 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 discs connect cardiac muscle cells, and gap junctions inside the intercalated discs relay electrical impulses from one cardiac muscle cell to another. Cardiac muscle cells are surrounded by an extracellular matrix produced by supporting fibroblast cells.
Cardiac muscle cells are the contracting cells that allow the heart to pump. Each cardiomyocyte needs to contract in coordination with its neighbouring cells, working together to efficiently pump blood from the heart. If this coordination breaks down, the heart may not pump at all, as can occur during abnormal heart rhythms such as ventricular fibrillation.
The human body contains three kinds of muscle tissue: skeletal, smooth, and cardiac. Only cardiac muscle tissue, comprising cells called cardiomyocytes, produces involuntary movements. This means they are automatic, and a person cannot control them. The heart also contains specialised types of cardiac tissue, including pacemaker cells, which are only weakly contractile and are connected to neighbouring contractile cells via gap junctions. They are responsible for generating and sending out electrical impulses that tell cardiac muscle cells to contract and relax, controlling the heart rate and determining how fast the heart pumps blood.
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Frequently asked questions
Myofibrils are polymers or repeating units of sarcomeres, which are the basic contractile units of muscle fibres. They are made of thin and thick filaments of the proteins actin and myosin, which give muscles their striated appearance.
Yes, cardiac muscle does have myofibrils. Cardiac muscle is composed of individual cardiac muscle cells or cardiomyocytes, which are made up of bundles of myofibrils.
Myofibrils are essential for the contraction of cardiac muscle. The thick and thin filaments of the myofibrils slide past each other, causing the cardiac muscle to contract. This process is known as excitation-contraction coupling (ECC) and is regulated by calcium ions.
