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Cardiac muscle, also called heart muscle or myocardium, is a type of vertebrate muscle tissue that constitutes the main tissue of the heart wall. Cardiac muscle cells, or cardiomyocytes, are the contractile myocytes of the cardiac muscle. These cells are surrounded by an extracellular matrix and are connected by intercalated discs. The myocyte is composed of bundles of myofibrils that contain myofilaments. The myofibrils have distinct, repeating microanatomical units, termed sarcomeres, which represent the basic contractile units of the myocyte. The sarcomere is the smallest functional unit of striated muscle tissue and is directly responsible for most of its mechanical properties.
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What You'll Learn
Sarcomeres are composed of thick and thin filaments
Sarcomeres are the fundamental units of contraction in striated muscles, such as skeletal and cardiac muscle. They are composed of thick and thin filaments, which are made of proteins and generate force and movement during muscle contraction.
The thick filaments are composed mainly of the protein myosin, while the thin filaments are made up of the proteins actin, tropomyosin, and troponin. The actin and myosin filaments are arranged in a repeating pattern, with the myosin filaments at the centre of the sarcomere and the actin filaments attached to the Z-discs at either end of the sarcomere. During muscle contraction, the myosin filaments slide past the actin filaments, causing the sarcomere to shorten and the muscle to contract. This sliding is mediated by the interaction between the myosin heads and the actin filaments, which are regulated by the presence of calcium ions.
The thin filament is 7-8nm in diameter and extends the entire length of the I-band. It is composed of three proteins: actin, tropomyosin, and troponin. The actin molecules are arranged in chains that twist around each other like a double strand of pearls. Tropomyosin is another protein that twists around actin, and troponin is a third major protein in the thin filament. Troponin is a 3 polypeptide complex, with one binding to actin, another binding to tropomyosin, and the third binding calcium ions (Ca^2+) to start muscle contraction.
The thick filaments are bipolar and extend throughout the A-band, where they are cross-linked at the centre by the M-band. The giant protein titin (also known as connectin) extends from the Z-line of the sarcomere, where it binds to the thick filament (myosin) system, to the M-band. Titin is the biggest single highly elasticated protein found in nature and provides binding sites for numerous proteins. It is thought to play an important role as a sarcomeric ruler and as a blueprint for the assembly of the sarcomere. Another giant protein, nebulin, is hypothesised to extend along the thin filaments and the entire I-Band.
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Cardiac muscle cells are also called cardiomyocytes
Cardiac muscle cells, also called cardiomyocytes, are a type of muscle tissue found only in the heart. They are the individual cells that make up the myocardium, the thick middle layer of the heart. Cardiomyocytes are tubular structures composed of chains of myofibrils, which are rod-like units within the cell. The myofibrils consist of repeating sections of sarcomeres, which are the fundamental contractile units of the muscle cells.
Sarcomeres are highly ordered multiprotein complexes that give the characteristic striated look of cardiomyocytes and skeletal muscle cells. They are composed of long proteins that organize into thick and thin filaments, called myofilaments. The thick filaments mainly consist of myosin molecules, while the thin myofilaments contain actin. The sliding of these filaments past each other produces the formation of "cross-bridges", which causes contraction of the heart and the generation of force.
The sarcomere length in cardiac myocytes rarely exceeds 2.2 micrometres. Small changes in cardiac sarcomere length can produce large changes in tension development. Stretching the sarcomere increases TN-C affinity for calcium, leading to increased tension development, referred to as length-dependent activation.
Cardiomyocytes are connected to each other by intercalated discs, which contain gap junctions that facilitate intercellular communication. When one cardiac muscle cell is stimulated to contract, a gap junction transfers the stimulation to the next cardiac cell, allowing the muscle to contract in a coordinated manner. Cardiomyocytes typically only have one nucleus, in contrast to skeletal muscle cells, which can have multiple nuclei.
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The sarcomere is the smallest functional unit of striated muscle tissue
Sarcomeres are composed of two main protein filaments: thin actin and thick myosin filaments. These filaments slide past each other, resulting in the contraction of an individual sarcomere. The sliding filament theory, which explains the mechanism of muscular contraction, describes the active force generated by the sliding of actin and myosin filaments. The interaction between these filaments in the A-band of the sarcomere is responsible for muscle contraction. The thick filament mainly consists of myosin molecules, which are ubiquitous in eukaryotes and participate in several cellular motile processes.
The thin actin filaments are the major component of the I-band and extend into the A-band. 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. It includes both thick and thin filaments, with a paler region called the H-zone, which contains only the myosin. The giant protein titin, also known as connectin, extends from the Z-line to the M-band and stabilizes contraction. It also acts as a bridge between the thin and thick filaments, contributing to the passive force that helps to stabilize sarcomeres.
Changes in sarcomere length are an important mechanism for regulating the force of contraction. Stretching the sarcomere increases TN-C affinity for Ca++, leading to increased tension development, known as length-dependent activation. Small changes in cardiac sarcomere length can produce large changes in tension development. This length-dependent activation is also influenced by titin strain, which affects both myosin and troponin positioning.
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Sarcomeres are connected in series at the Z-disk
Sarcomeres are highly ordered multiprotein complexes that are longitudinally aligned to give the characteristic striated look of cardiomyocytes and skeletal muscle cells. They are the contractile units of the cardiac myocyte, with their myofilaments arranged in parallel, cross-striated bundles of thin and thick fibres. The thin fibres contain actin, tropomyosin, and the troponin complex, while the thick fibres are primarily composed of myosin and its supporting proteins.
The sarcomere is defined as the segment between two neighbouring Z-lines (or Z-discs). The Z-disk anchors the thin filaments of actin that extend into each adjacent sarcomere and are located in the I band. The sarcomere length, or the distance between Z-lines, ranges from about 1.6 to 2.2 μ in human hearts.
The actin filaments from adjacent sarcomeres are connected by a single zigzag link. In transverse sections, interdigitating square lattices are seen with links between the opposing actin filaments. This observation led to the model in which four links from the end of one actin filament connect to the four neighbouring opposite polarity actin filaments from the adjoining sarcomere.
The sliding of thick and thin filaments relative to each other results in muscle contraction. Changes in sarcomere length are an important mechanism by which the heart regulates its force of contraction. With increased sarcomere length, there is an increase in the force of contraction or tension development by the muscle fibre.
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The sarcomere is composed of the proteins actin, myosin, and titin
The sarcomere is the smallest functional unit of striated muscle tissue. It is the repeating unit between two Z-lines. Cardiac muscles contain sarcomeres, which are responsible for the striated appearance of the muscle. The sarcomere is composed of the proteins actin, myosin, and titin. Actin is a thin filament, myosin is a thick filament, and titin is an elastic filament. Together, they form the thin, thick, and elastic filament system.
Actin and myosin are the active structures responsible for muscular contraction. The widely accepted theory describing muscular contraction is the sliding filament theory, which proposes that active force is generated as actin filaments slide past the myosin filaments, resulting in the contraction of an individual sarcomere. The sliding filament theory was proposed by scientists who, through the use of high-resolution microscopes, visualised the actin and myosin filaments within a sarcomere. They were able to observe the length of the sarcomere when relaxed and its shortening as it contracted.
The thick filament mainly consists of myosin molecules. The myosin superfamily encodes 18 classes of myosin motors, which are ubiquitous in eukaryotes and participate in several cellular motile processes. The type of myosin present in muscle is myosin II, a very large protein consisting of two identical heavy chains and two pairs of light chains. Each heavy chain consists of a globular head region and a long α-helical tail. The α-helical tails of the two heavy chains twist around each other in a coiled-coil structure to form a dimer, and two light chains associate with the neck of each head region to form the complete myosin II molecule.
Actin is the most abundant protein in most eukaryotic cells and has a pivotal role in muscle contraction as well as in cell movements. Actin filaments and titin molecules are cross-linked in the Z-disc via the Z-line protein alpha-actinin. The interaction between actin and myosin filaments in the A-band of the sarcomere is responsible for muscle contraction. The protein tropomyosin covers the myosin-binding sites of the actin molecules in the muscle cell. For a muscle cell to contract, tropomyosin must be moved to uncover the binding sites on the actin.
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Frequently asked questions
A sarcomere is the smallest functional unit of striated muscle tissue. It is the repeating unit between two Z-lines.
Yes, cardiac muscle does have sarcomeres. Cardiac muscle cells, also called cardiomyocytes, are the contractile myocytes of the cardiac muscle. They are composed of bundles of myofibrils that contain myofilaments. The myofibrils have distinct, repeating microanatomical units, termed sarcomeres, which represent the basic contractile units of the myocyte.
Sarcomeres are responsible for the generation of active and passive forces in the cardiac muscle. Changes in sarcomere length are an important mechanism by which the heart regulates its force of contraction.
Sarcomeres are composed of long, fibrous proteins as filaments that slide past each other when a muscle contracts or relaxes. Two of the important proteins are myosin, which forms the thick filament, and actin, which forms the thin filament.
Titin is a giant elastic protein that spans the half-sarcomere from the Z-disk to the M-band and stabilizes contraction. It is the biggest single highly elasticated protein found in nature and provides binding sites for numerous proteins. Titin contributes to passive force, which helps to stabilize sarcomeres and is driven through the structural components of the protein during muscle stretch.
