Dominic Stone
A sarcomere is the basic contractile unit of muscle fibre. It is a highly organised structure composed of thick and thin protein filaments, mainly actin and myosin, which are responsible for muscle contraction and performance. The thick myosin filaments contain numerous heads, which when attached to the thinner actin filaments, create actin-myosin cross-bridges. The force of a muscular contraction is determined by the number of these cross-bridges. The sarcomere also contains other proteins such as troponin and tropomyosin, which regulate the interaction of thick and thin filaments and prevent the binding of actin and myosin filaments until a signal for muscle contraction is received.
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Sarcomere structure
A sarcomere is the basic contractile unit of muscle fibre. It is a highly organised structure composed of thick and thin protein filaments. The thick filaments are composed of myosin proteins, while the thin filaments are made up of actin proteins. The coordinated performance of actin and myosin proteins is responsible for the function of the sarcomere.
The thick and thin filaments are arranged in a specific pattern within the sarcomere. The thick filaments are positioned in the centre, with the thin filaments extending from either side. The thick filaments are arranged in a hexagonal lattice, with several myosin heads projected outwards. These myosin heads bind to the thin filaments during muscle contraction. The thin filaments, also known as actin filaments, are composed of actin protein, along with troponin and tropomyosin proteins. Troponin and tropomyosin regulate the interaction between the thick and thin filaments, thereby regulating the muscle contraction process.
The sarcomere structure also includes distinct bands, namely the A-band and the I-band, formed due to the arrangement of thick and thin filaments. The A-band is the region of the sarcomere containing both thick and thin filaments, while the I-band is the region of thin filaments that are not superimposed with the thick filaments. As a result, the A-band appears darker under a microscope, while the I-band appears lighter, earning it the name "light band". Within the A-band, there is a region called the H-zone, which contains only thick filaments. The H-zone is the area between the M-line and the Z-disc. The M-line, composed of myomesin protein, marks the centre of the sarcomere and interconnects the thick filaments.
The Z-disc, or Z-line, is another crucial component of the sarcomere structure. It is the area where two actin filaments connect and transverse the I-bands. The Z-disc defines the boundaries of the sarcomere, with each sarcomere being the segment between two neighbouring Z-discs. Actin molecules are bound to the Z-disc, and it plays a key role in anchoring the thin filaments. The Z-disc also contains other proteins, such as alpha-actinin, which cross-links the actin filaments and titin molecules.
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Actin and myosin interaction
The sarcomere is the basic unit of striated muscles, a complex multicomponent biological system responsible for converting chemical energy released during ATP hydrolysis into mechanical work. It is the basic contractile unit of muscle fibre, composed of two main protein filaments—actin and myosin. The actin molecule is called F-actin, formed by the polymerization of G-actin. Actin is the most abundant protein in most eukaryotic cells and has a pivotal role in muscle contraction and cell movements.
Actin and myosin interact to generate muscle tension and contraction. The sliding filament theory, first proposed by Huxley and Hanson in 1954, explains that active force is generated as actin filaments slide past the myosin filaments, resulting in contraction of an individual sarcomere. The thick myosin filament contains numerous heads, which, when attached to the thinner actin filaments, create actin-myosin cross-bridges. The myosin head is similar to a cocked spring, which, on binding with an actin filament, flexes and produces a power stroke. The power stroke slides the actin filament past the myosin, resulting in force generation and the shortening of an individual sarcomere.
The force of a muscular contraction is determined by the number of actin-myosin cross-bridges that are formed. The myosin-actin cycling refers to the process by which the myosin S1 segment binds and releases actin, forming cross-bridges that extend from the thick myosin filaments to the thin actin filaments. The contraction of the myosin's S1 region is called the power stroke, which requires the hydrolysis of ATP (Adenosine triphosphate), breaking a high-energy phosphate bond to release energy, resulting in force generation and the shortening of an individual sarcomere.
The myosin head also binds to ATP, which is the source of energy for muscle movement. Myosin can only bind to actin when the binding sites on actin are exposed by calcium ions. Calcium is required by two proteins, troponin and tropomyosin, which regulate muscle contraction by blocking the binding of myosin to filamentous actin. In a resting sarcomere, tropomyosin blocks the binding of myosin to actin. Tropomyosin must rotate around the actin filaments to expose the myosin-binding sites.
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Z-discs
The sarcomeric Z-disc, also known as the Z-line or Z-band, defines the lateral borders of the sarcomere. It is composed of connectin protein that anchors actin filaments. Actin molecules are bound to the Z-line, which forms the borders of the sarcomere. The Z-line is visible as dark lines separating sarcomeres at the light-microscope level. It is the area where two actin filaments connect and transverse the I bands.
The core of a Z-disc consists of actin filaments coming from adjacent sarcomeres, which are cross-linked by α actinin molecules. The giant protein titin (connectin) extends from the Z-line of the sarcomere, where it binds to the thick filament (myosin) system, to the M-band, where it is thought to interact with the thick filaments. Titin (and its splice isoforms) is the biggest single highly elasticated protein found in nature. It provides binding sites for numerous proteins and is thought to play an important role as a sarcomeric ruler and as a blueprint for the assembly of the sarcomere.
Mature Z-discs are probably composed of hundreds of different proteins, and they are regarded as one of the most complex macromolecular structures in biology. Major Z-disc proteins include cardiac actin, which is cross-linked by α actinin and “capped” by CapZ, titin, which spans a complete half sarcomere, and nebulin/nebulette, which runs along actin filaments. Z-discs are difficult to detect in conventional light microscopy. They appear in the longitudinal view of electron microscopy as dense zigzag bands with varying but myofibre-specific sizes, ranging between 30 to 50 nm in fast muscle and between 100 to 140 nm in slow muscle and cardiac myocytes.
The Z-disc has been found to serve as a nodal point for signalling in general and mechanosensation and mechanotransduction in particular. The Z-disc also links to the t-tubular system, the sarcoplasmic reticulum, and several E3 ubiquitin ligases localized to the Z-disc and link this structure to protein turnover and probably autophagy.
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M-line
The M-line, derived from the German word "Mittelscheibe" meaning "middle disc", is a crucial component of the sarcomere, which is the basic contractile unit of muscle fibres. The M-line is a thin vertical line that runs through the centre of the sarcomere, marking its midpoint. It is formed by the cross-connection of elements of the cytoskeleton.
The M-line is primarily composed of the protein myomesin, which acts as an anchor for the thick myosin filaments, arranging them in a lattice-like structure. The M-line is essential for the structural integrity of the sarcomere, as it provides a binding site for the thick filaments to attach. This cross-linking of filaments by the M-line is vital for the overall function of the sarcomere.
In addition to myomesin, the M-line also contains other proteins such as C-protein and titin. Titin, a giant protein, plays a significant role in the sarcomere's structure and function. It spans from the Z-line to the M-line, providing elastic properties and passive resistance to muscle fibre stretching. This elasticity is crucial for the contraction and relaxation of muscles.
During muscle contraction, the myosin heads, which are part of the thick filaments, swing towards the M-line. This movement results in the sliding of the attached thin actin filaments towards the M-line, ultimately shortening the length of the sarcomere. This dynamic process is known as the sliding filament theory, which describes how active force is generated during muscle contraction.
The M-line also has a role in energy production. It binds creatine kinase, which facilitates the conversion of ADP and phosphocreatine into ATP and creatine. This energy production is essential for muscle function, as ATP provides the energy required for muscle movement and contraction.
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Sarcomere contraction
The sliding filament theory is the most popular model that explains muscular contraction. This theory states that active force is generated as actin filaments slide past the myosin filaments, resulting in the contraction of an individual sarcomere.
Actin and myosin are the two main protein filaments that make up a sarcomere, and they are the active structures responsible for muscular contraction. The thick myosin filament contains numerous heads, which attach to the thinner actin filaments to create actin-myosin cross-bridges. The myosin head is similar to a cocked spring, which, on binding with an actin filament, flexes and produces a power stroke. The power stroke slides the actin filament past the myosin, resulting in force generation and shortening of an individual sarcomere.
The force of a muscular contraction is determined by the number of actin-myosin cross-bridges that are formed. The contraction of myosin's S1 region is called the power stroke, which requires the hydrolysis of ATP (Adenosine triphosphate), breaking a high-energy phosphate bond to release energy, resulting in force generation and shortening of an individual sarcomere.
The cross-bridge cycle refers to the mechanism by which the thick and thin filaments slide past one another to generate a muscle contraction. At the beginning of the cycle, when myosin is tightly bound to actin, no ATP is bound to myosin, a state known as rigor. Then, ATP binds to the myosin head, inducing a conformational change in myosin that decreases its affinity for actin. Consequently, myosin dissociates from actin and the myosin head becomes cocked toward the end of the sarcomere. The ATP bound to myosin then becomes hydrolyzed to ADP and P, which causes the myosin heads to change conformation and move toward the positive end of the actin, cocking the myosin head. The phosphate is then released, and the ADP-bound myosin binds to a new location on the actin filament. ADP is then released, which causes the myosin to return to its original position, pulling on the actin filament and causing the sarcomere (and, therefore, the muscle fibre) to contract.
The calcium-induced calcium release (CICR) mechanism is used for cardiac muscle contraction. CICR involves the conduction of Ca ions into the cardiomyocyte, leading to the further release of ions into the cytoplasm. Calcium is required by two proteins, troponin and tropomyosin, that regulate muscle contraction by blocking the binding of myosin to filamentous actin. In a resting sarcomere, tropomyosin blocks the binding of myosin to actin. For myosin to bind to actin, calcium ions are needed to expose the binding sites on actin.
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Frequently asked questions
A sarcomere is the basic contractile unit of a striated muscle cell. It is a highly organised structure made up of thick and thin protein filaments, mainly of actin and myosin proteins.
The thick filaments are composed of myosin proteins and the thin filaments are made up of actin proteins. Other proteins like troponin and tropomyosin are also present in a sarcomere. The thick filaments are arranged in a hexagonal lattice in the centre of the sarcomere, with the thin filaments extending from either side.
The interaction of myosin and actin proteins is at the core of our understanding of sarcomere shortening. The sliding filament theory states that the sliding of actin past myosin generates muscle tension and results in contraction of an individual sarcomere. This is called the power stroke, which requires the hydrolysis of ATP (Adenosine triphosphate) to release energy.
