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Cardiac muscle, also known as myocardium, is one of the three types of vertebrate muscle tissues, the other two being skeletal muscle and smooth muscle. Cardiac muscle is striated and involuntary, and constitutes the main tissue of the heart wall. It is composed of cardiomyocytes, or cardiac muscle cells, that are joined by intercalated discs. These cardiomyocytes are responsible for the contractility of the heart, which is the basis for its pumping action. The sarcomeres, or contractile units, of the cardiac muscle are composed of thick and thin filaments, which are attached to bands called the M line and the Z line, respectively.
| Characteristics | Values |
|---|---|
| Type of muscle | Cardiac muscle is one of the three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle |
| Location | Cardiac muscle is found only in the heart |
| Involuntary control | Cardiac muscle is not under voluntary control |
| Rhythmic contractions | Exhibits rhythmic contractions |
| Structure | Cardiac muscle cells form a highly branched cellular network in the heart |
| Contractions | Contractions in cardiac muscle are rapid and involuntary |
| Composition | Cardiac muscle cells are composed of thick and thin filaments called myofilaments |
| Bands | Cardiac muscle contains bands called the M line and the Z line |
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What You'll Learn
Cardiac muscle tissue contains bands called Z lines
Cardiac 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 Z lines are visible as dark lines separating sarcomeres at the light-microscope level. The sarcomeres are arranged in a branched pattern, forming a 3D network in the cytoplasm. They are responsible for the striated appearance of cardiac tissue. The thick and thin filaments slide past each other when a muscle contracts or relaxes, causing the Z lines to come closer together.
The intercalated discs that join the cardiac myocytes together coincide with the Z lines. These discs appear as lines that transverse the muscle fibres perpendicularly when examined with a light microscope. However, they are not linear but have finger-like interdigitations to maximise the contact surface area.
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Sarcomeres are contractile units of cardiac muscle tissue
Cardiac muscle, also known as heart muscle or myocardium, is one of the three types of vertebrate muscle tissues, the other two being skeletal muscle and smooth muscle. Cardiac muscle is an involuntary, striated muscle that forms the thick middle layer of the heart wall, between the outer layer (pericardium) and the inner layer (endocardium).
Sarcomeres are the contractile units of cardiac muscle tissue. They are the functional subunits of myofibrils, composed of thick and thin filaments. The thick filaments are made of polymerised myosin type II protein and are attached to a band called the M line in the middle of the sarcomere. The thin filaments consist of polymers of the protein alpha actin and are attached to the Z lines. The M line contains the protein myomesin and marks the centre of the sarcomere. The Z line, or Z disc, is the area where two actin filaments connect and transverse the I bands. The I band is the zone of thin filaments that are not superimposed by thick filaments. The A band, on the other hand, contains the entire length of a single thick filament and is visible as dark transverse lines across myofibers.
The interaction between actin and myosin filaments in the A band of the sarcomere is responsible for muscle contraction. The sliding filament theory proposes that the active force is generated as actin filaments slide past the myosin filaments, resulting in the contraction of an individual sarcomere. The protein tropomyosin covers the myosin-binding sites of the actin molecules, and for a muscle cell to contract, tropomyosin must be moved to uncover these binding sites. Calcium ions play a crucial role in this process by binding with troponin C molecules, which alters the structure of tropomyosin and forces it to reveal the cross-bridge binding site on the actin.
The sarcomeres give cardiac muscle its striated appearance, with the Z lines appearing as dark lines that anchor the actin myofilaments. The sarcomeres are arranged in a branched pattern, forming a 3D network in the cytoplasm. This structure allows the cardiac muscle to contract with great force, ensuring the heart pumps blood with sufficient force to meet the metabolic demands of the entire body.
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Cardiac muscle cells are connected by intercalated discs
Cardiac muscle cells, also called cardiomyocytes, are the contractile myocytes of the cardiac muscle. These cells are connected by intercalated discs, which are unique to cardiac muscle. Intercalated discs are complex structures that connect adjacent cardiac muscle cells, allowing them to work as a single functional unit called a syncytium. This connection is essential for the synchronized contraction of cardiac tissue in a wave-like pattern, enabling the heart to function as a pump.
Intercalated discs are composed of three types of cell junctions: desmosomes, fascia adherens junctions, and gap junctions. Desmosomes provide mechanical strength and stability to the intercalated discs, ensuring the cells remain firmly attached. Fascia adherens junctions, or adherens junctions, act as anchoring sites for actin filaments and connect to the closest sarcomere, which is the functional subunit of myofibrils. Gap junctions, on the other hand, facilitate electrical communication between neighbouring cells. They allow the passage of ions, enabling the spread of cardiac action potentials and resulting in the depolarization of the heart muscle.
The structure of intercalated discs is quite remarkable. When viewed under a light microscope, they appear as thin, dark-staining lines that run perpendicular to the direction of muscle fibres. However, at the ultrastructural level, intercalated discs exhibit finger-like interdigitations, maximizing the surface contact area between adjacent cells. This intricate design enhances the mechanical and electrical connections between cardiac muscle cells, ensuring synchronized contractions and efficient heart function.
The intercalated discs play a crucial role in maintaining the overall health and functionality of the heart. Mutations in the intercalated disc gene can lead to various cardiomyopathies, which are diseases of the heart muscle. Ruptured intercalated discs, for instance, can be caused by forceful myocardial contractions associated with ventricular fibrillation or electrical injury. Therefore, the integrity and proper functioning of intercalated discs are vital for the normal rhythmic contractions of the heart.
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Electrical impulses cause cardiac muscle contractions
Cardiac 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 thick middle layer of the heart wall. The cardiac muscle is composed of individual cardiac muscle cells, or cardiomyocytes, joined by intercalated discs. These cells form a highly branched cellular network in the heart and are organised into layers of myocardial tissue that wrap around the heart chambers.
The rhythmic contraction of cardiac muscle is regulated by the sinoatrial node of the heart, which serves as the heart's pacemaker. This node generates an electrical stimulus, or impulse, regularly, 60 to 100 times per minute under normal conditions. The electrical impulse then travels through the heart, causing the heart to contract. This process is known as autorhythmicity, which is a unique property of cardiac muscle. The impulse passes through the bundle of His, the right and left bundle branches, and finally through the Purkinje system. The Purkinje fibres have a fast inherent conduction rate, and the electrical impulse reaches all of the ventricular muscle cells in about 75 ms.
The contraction of individual cardiac muscle cells produces force and shortening in these bands of muscle, resulting in a decrease in heart chamber size and the consequent ejection of blood into the pulmonary and systemic vessels. The electrical stimulus triggers the release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum. The rise in calcium causes the cell's myofilaments to slide past each other in a process called excitation-contraction coupling.
The conduction system in the heart contains specialised cells and nodes that control the heartbeat. The myocardial conducting cells initiate and propagate the action potential (the electrical impulse) that travels throughout the heart and triggers the contractions that propel the blood.
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Cardiac muscle is one of three types of vertebrate muscle tissues
Cardiac muscle, also called the myocardium, is one of three types of vertebrate muscle tissues. The other two types are skeletal muscle and smooth muscle. Cardiac muscle is found only in the heart and forms both the atria and the ventricles. 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 endocardium is not cardiac muscle and is made up of simple squamous epithelial cells. It forms the inner lining of the heart chambers and valves. The pericardium, on the other hand, is a fibrous sac surrounding the heart, consisting of the epicardium, pericardial space, parietal pericardium, and fibrous pericardium.
Cardiac muscle cells form a highly branched cellular network in the heart. They are connected end-to-end by intercalated discs and are organised into layers of myocardial tissue that wrap around the chambers of the heart. The contraction of individual cardiac muscle cells produces force and shortening in these bands of muscle, resulting in a decrease in heart chamber size and the ejection of blood into the pulmonary and systemic vessels.
Cardiac muscle cells are the contractile myocytes of the cardiac muscle. They are surrounded by an extracellular matrix produced by supporting fibroblast cells. The extracellular matrix is composed of proteins, including collagen and elastin, along with polysaccharides (sugar chains) known as glycosaminoglycans. The collagenous tissue fibres provide support and a blood supply to the cardiac muscle.
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Frequently asked questions
Yes, cardiac muscle contains bands called sarcomeres. These are the contractile units of cardiac muscle tissue and are composed of thick and thin filaments of myofibrils.
Thick filaments are made of polymerised myosin type II protein and are attached to a band called the M line. Thin filaments are made of polymers of the protein alpha actin and are attached to the Z lines.
Sarcomeres are the contractile units of cardiac muscle and are responsible for the striated appearance of cardiac tissue. The thick and thin filaments slide past each other during contraction, causing the formation of "cross-bridges" and resulting in the heart's pumping action.
Cardiac muscle cells, or cardiomyocytes, are highly branched and contain many mitochondria. They are joined together by intercalated discs, which coincide with Z lines, and are surrounded by a plasma membrane called the sarcolemma.
