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The sarcomere is the basic unit of striated muscles, which are responsible for converting chemical energy released during ATP hydrolysis into mechanical work. It is composed of two main protein filaments, actin and myosin, which are the active structures responsible for muscular contraction. The sliding filament theory describes muscular contraction as the sliding of actin past myosin, resulting in the shortening of an individual sarcomere. The force of a muscular contraction is determined by the number of actin-myosin cross-bridges formed. The sarcomere gives skeletal and cardiac muscle their striated appearance, with dark and light bands visible under a microscope. The structure of the sarcomere affects its function, with the length of the actin and myosin filaments impacting force and velocity.
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What You'll Learn
Sarcomere structure
The sarcomere is the basic unit of striated muscles, which are a complex multicomponent biological system responsible for converting chemical energy released during ATP hydrolysis into mechanical work. Each sarcomere is composed of two main protein filaments: actin and myosin. These are the active structures responsible for muscular contraction.
The structure of the sarcomere is traditionally described with dark and light bands visible under a microscope. This banding pattern is due to the arrangement of thick and thin myofilaments in each unit. The thick filaments are called myosin, and the thin filaments are called actin. The bundles of myofilaments are called myofibrils. The myofibrils' main function is to produce a muscular contraction in which the filaments slide over each other.
The A-band is a dark band that contains whole thick filaments (myosin). The I-band is a light band that contains only the thin filaments (actin) and is located between the two thick filaments. The Z disc is the area where two actin filaments connect and transverse the I bands. The sarcomere can also be described as the structure between the two Z discs.
The M line marks the middle of the sarcomere and contains the protein called myomesin. The H zone is the area between the M line and Z disc and contains only myosin. The M line also binds creatine kinase, which facilitates the reaction of ADP and phosphocreatine into ATP and creatine.
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Actin and myosin filaments
Actin and myosin are both proteins that are found in every type of muscle tissue. Thick myosin filaments and thin actin filaments work together to generate muscle contractions and movement.
The thick filaments of muscle consist of several hundred myosin molecules, associated in a parallel staggered array by interactions between their tails. The globular heads of myosin bind actin, forming cross-bridges between the thick and thin filaments. The myosin heads bind to the exposed binding sites on the actin filaments, allowing muscle contraction to begin. This process is known as myosin-actin cycling.
The sliding filament theory explains that the sliding of actin past myosin generates muscle tension and contraction. The myosin pulls the actin filaments towards the centre of the sarcomere, causing the sarcomere to shorten and contract. The actin filaments are attached at their plus ends to the Z disc, which includes the crosslinking protein α-actinin. The myosin filaments are anchored at the M line in the middle of the sarcomere.
The force of a muscular contraction is determined by the number of actin-myosin cross-bridges that are formed. The contraction of the sarcomere causes the muscle fibre to contract and generates muscle movement.
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Sliding filament theory
The sliding filament theory is a widely accepted explanation of the mechanism underlying muscle contraction. It was introduced in 1954 by two separate research teams: Andrew Huxley and Rolf Niedergerke from the University of Cambridge, and Hugh Huxley and Jean Hanson from the Massachusetts Institute of Technology. Hugh Huxley first conceived of the theory in 1953.
The sliding filament theory explains that muscle contraction occurs when myosin (thick filaments) slide past actin (thin filaments). The myosin filaments use energy from ATP to "walk" along the actin filaments, pulling them closer together. This movement results in the shortening of the sarcomere, which is the basic unit of striated muscles.
The sarcomere consists of contractile actin and myosin filaments, which are integrated in a paracrystalline order with accessory cytoskeleton proteins, forming the sarcomeric cytoskeleton. The myosin filaments have numerous heads, and the actin filaments have multiple binding sites. When the myosin heads attach to the actin filaments, they form cross-bridges, which result in a power stroke that slides the actin filament past the myosin. This process is known as the cross-bridge cycle or cross-bridge theory, which was introduced alongside the sliding filament theory.
The force generated during muscle contraction is determined by the number of actin-myosin cross-bridges formed. The simultaneous contraction of all sarcomeres in a muscle fiber leads to the shortening of the entire muscle fiber. This process of muscle contraction is also known as the walk-along theory or ratchet theory.
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Sarcomere contraction
The sarcomere is the basic unit of striated muscles, which are responsible for converting chemical energy into mechanical work. It is composed of two main protein filaments—actin and myosin—which are the active structures responsible for muscular contraction.
The sliding filament theory describes the process of 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. The myosin filament contains numerous heads, which attach to the thinner actin filaments to create actin-myosin cross-bridges. The power stroke, which occurs when a myosin head binds with an actin filament, results in the sliding of the actin filament past the myosin, leading to force generation and the shortening of the sarcomere.
The force of muscular contraction is influenced by the number of actin-myosin cross-bridges formed. During contraction, the sarcomere shortens as the actin filaments at each end of a central myosin filament slide towards the myosin's centre. This process, known as myosin-actin cycling, involves the repetitive binding and releasing of actin by myosin.
In striated muscle fibres, actin and myosin filaments are organised into repeating arrays called sarcomeres, which contribute to their striated microscopic appearance. The striated appearance results from the alternation of thick-filament-containing (A-Band) and thin-filament-containing (I-band) regions. The A-band, located at the centre of each sarcomere, contains the thick filaments, which may overlap with thin filaments. The I-band, also known as the isotropic band, contains only the thin filament (actin).
The contraction of cardiac muscle, in particular, involves excitation-contraction coupling (ECC) and calcium-induced calcium release (CICR). Electrical stimuli induce an influx of Ca ions into the cardiomyocyte, leading to further release into the cytoplasm. Calcium binds to cardiac troponin C, moving the troponin complex away from the actin-binding site. This frees actin, allowing it to bind to myosin and initiate contraction.
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Sarcomere assembly
A sarcomere is the basic contractile unit of muscle fibre. It is composed of two main protein filaments—actin and myosin—which are the active structures responsible for muscular contraction. The sarcomere is a repeating unit of interdigitating actin and myosin filaments that serve as the building blocks of the myofibrils in striated muscle cells.
The assembly of sarcomeric proteins into the highly organised structure of the sarcomere is an ordered and complex process involving an array of structural and associated proteins. It is considered one of the most complex macromolecular assemblies in biology. The sarcomere is being redefined as a dynamic network of proteins capable of generating force and signalling with other cellular compartments and metabolic enzymes capable of controlling many facets of striated myocyte biology.
The sarcomere consists of a bundle of myosin-containing thick filaments flanked and interdigitated with bundles of actin-containing thin filaments. The striated appearance of muscle results from the alternation of thick-filament-containing (A-Band) and thin-filament-containing (I-band) regions. The centre of each A-band consists of a specialised region (M-line). The thin filament lies between the two thick filaments. The Z disc is the area where two actin filaments connect and transverse the I bands. The M line contains the protein called myomesin and it marks the centre of the sarcomere.
The most popular model that describes muscular contraction is the sliding filament theory. In this theory, active force is generated as actin filaments slide past the myosin filaments, resulting in contraction of an individual sarcomere. Each myosin filament has numerous heads, and each actin filament has numerous binding sites. This is important because, for a sarcomere to maximally contract, numerous power strokes must occur. The force of a muscular contraction is determined largely by the number of actin-myosin cross-bridges that are formed.
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Frequently asked questions
A sarcomere is the basic unit of striated muscles, which are responsible for converting chemical energy released during ATP hydrolysis into mechanical work.
A sarcomere is made up of two main protein filaments: actin and myosin. These are the active structures responsible for muscular contraction.
The sliding filament theory is a model that describes muscular contraction. In this theory, active force is generated as actin filaments slide past the myosin filaments, resulting in contraction of an individual sarcomere.
The function of a sarcomere is to facilitate muscle contraction and performance.
