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Cardiac muscle, also known as myocardium or heart muscle, is one of three types of vertebrate muscle tissues, the others being skeletal and smooth muscle. It is an involuntary, striated muscle that forms the bulk of the heart. The cardiac muscle is composed of individual cardiac muscle cells or cardiomyocytes, which are joined by intercalated discs. These discs appear as linear bands that provide attachment points, creating a characteristic branched pattern. The intercalated discs also allow the cardiac muscle to function as a syncytium, enabling the synchronous contraction of the entire tissue section. The contractile units of cardiac muscle are called sarcomeres, which are composed of thick and thin filaments, resulting in a striated appearance.
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What You'll Learn
- Cardiac muscle is striated, with alternating dark A bands and light I and H bands
- These bands are due to the arrangement of myofilaments and fibrils in sarcomeres
- Intercalated discs are lines that traverse muscle fibres, connecting cardiac muscle cells
- The discs allow cardiac muscle to function as a functional syncytium, with synchronous contraction
- Calcium is released from the sarcoplasmic reticulum, triggering the myofilaments to slide past each other
Cardiac muscle is striated, with alternating dark A bands and light I and H bands
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 main tissue of the heart wall.
The striated appearance of cardiac muscle is due to the arrangement of contractile proteins. Cardiac muscle tissue is composed of contractile units known as sarcomeres, which are specific portions of myofibrils located between two Z lines. The sarcomeres are composed of thick and thin filaments. Thick filaments are composed of polymerised myosin type II protein and are attached to a band called the M line in the middle of the sarcomere. Thin filaments consist of polymers of the protein alpha-actin and are attached to the Z lines.
The A band corresponds to the length of the myosin filaments and appears darker in colour under a microscope due to its electron-rich composition. The I and H bands appear lighter and represent regions consisting of only thin or thick filaments, respectively. Each sarcomere, the fundamental unit of a muscle's structure, consists of one A band and two I bands. The H-band sits within each A-band and is bisected by the M-line.
The intercalated discs that connect the cardiac muscle cells also coincide with the Z lines, appearing as lines that transverse the muscle fibres perpendicularly when examined with a light microscope.
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These bands are due to the arrangement of myofilaments and fibrils in sarcomeres
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 three-layered heart wall.
The contractile units of cardiac muscle tissue are known as sarcomeres. They are the functional subunits of myofibrils and are composed of thick and thin filaments. The thick filaments are composed of myosin type II protein and are attached to a band called the M line that is situated 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 Z line, or Z disc, is a key component of the sarcomere. It serves to anchor the thin filaments to adjacent sarcomeres. The Z line is also the site of actin and titin cross-linking, and it is where the giant protein titin binds to the thick filament system. The thick and thin filaments are arranged in a hexagonal lattice within each sarcomere, and it is the sliding of these filaments over each other that causes the sarcomere to shorten and the muscle to contract.
The banding pattern of myofibrils is created by the organisation of actin and myosin filaments. The A band is composed of thick filaments containing myosin, and the I band is composed of thin filaments. The H band is the central area of the A band where there is no overlap between thick and thin filaments. The M band is a dense line that runs through the centre of the H band, and it is where the thick filament system is cross-linked.
Therefore, the bands in cardiac muscle tissue are due to the arrangement of myofilaments and fibrils in sarcomeres.
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Intercalated discs are lines that traverse muscle fibres, connecting cardiac muscle cells
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 composed of individual cardiac muscle cells or cardiomyocytes joined by intercalated discs.
Intercalated discs have two important roles. Firstly, they provide attachment points that give the tissue a characteristic branched pattern. Secondly, they allow cardiac muscle tissue to function as a functional syncytium. Essentially, the contractile stimuli are propagated from one cell to the next, resulting in a synchronous contraction of the entire tissue section.
Intercalated discs support the synchronised contraction of cardiac tissue in a wave-like pattern so that the heart can work like a pump. They occur at the Z line of the sarcomere and can be easily visualised when observing a longitudinal section of the tissue. Intercalated discs are complex structures that connect adjacent cardiac muscle cells. The three types of cell junction recognised as making up an intercalated disc are desmosomes, fascia adherens junctions, and gap junctions.
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The discs allow cardiac muscle to function as a functional syncytium, with synchronous contraction
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 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 fibres have a single nucleus, are branched, and are joined to one another by intercalated discs. These discs are linear bands that contain gap junctions and desmosomes. The intercalated discs provide attachment points, giving the tissue a characteristic branched pattern. This allows the cardiac muscle to function as a functional syncytium, with synchronous contraction. Essentially, the contractile stimuli are propagated from one cell to the next, resulting in a synchronous contraction of the entire tissue section.
The contractile units of cardiac muscle tissue are called sarcomeres, which are composed of thick and thin filaments. 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. Thin filaments are made of polymers of the protein alpha-actin and are attached to the Z lines. The sarcomeres are arranged into a branched pattern, forming a 3D network in the cytoplasm.
The rhythmic contractions of cardiac muscle are regulated by the sinoatrial node of the heart and are, therefore, not under voluntary control. The contractions pump blood into the vessels of the circulatory system.
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Calcium is released from the sarcoplasmic reticulum, triggering the myofilaments to slide past each other
Calcium plays a crucial role in muscle contraction. The sarcoplasmic reticulum, a membranous structure within the sarcoplasm, surrounds the myofibrils within a myofibre and occupies about 4% of its volume. It stores calcium ions and releases them to trigger muscle contraction. When calcium is released from the sarcoplasmic reticulum, it binds to troponin, a protein on the thin myofilaments. This movement of calcium ions is detected by troponin, which then moves tropomyosin, another protein on the thin myofilaments. This enables the myosin molecule heads to "grab and swivel" along the thin myofilaments, resulting in the sliding filament model of muscle contraction.
The sliding filament model describes how thin filaments slide past thick filaments within the sarcomeres during muscle contraction. The myosin-binding sites on the actin filaments are exposed when calcium enters the sarcoplasm, allowing the myosin heads to bind to actin and pull the thin filaments. This repeated movement is known as the cross-bridge cycle, requiring energy provided by ATP. The sliding of the myofilaments leads to the shortening of the sarcomeres, with their Z-lines moving closer together.
In cardiac muscle, the myofilaments are arranged in a pattern similar to skeletal muscle, resulting in cross-striations. These cross-striations are formed by the thick and thin protein filaments arranged into contractile units, with the sarcomere extending from Z-line to Z-line. The intercalated discs that connect the cardiac muscle cells coincide with these Z-lines, transmitting the force of contractions from cell to cell.
The contractile stimuli in cardiac muscle are propagated from one cell to the next through these intercalated discs, resulting in a synchronous contraction of the entire tissue section. This coordinated contraction allows the ventricle to squeeze in several directions simultaneously, maximising the amount of blood ejected from the heart with each heartbeat. Thus, the release of calcium from the sarcoplasmic reticulum triggers a series of events that ultimately lead to the sliding of the myofilaments, driving muscle contraction and the pumping action of the heart.
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Frequently asked questions
Cardiac muscles, also known as heart muscles or myocardium, are one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. They are found only in the heart.
The bands in cardiac muscles are called intercalated discs. They are lines that transverse the muscle fibres perpendicularly and appear linear when examined with a light microscope.
Intercalated discs have two main roles. They provide attachment points, giving the tissue a branched pattern, and they allow cardiac muscle tissue to function as a functional syncytium.
