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Muscle filaments, or myofilaments, are the three protein filaments of myofibrils in muscle cells. The main proteins involved are myosin, actin, and titin. Myosin and actin are contractile proteins, while titin is an elastic protein. Myosin forms the thick filaments, and actin forms the thin filaments. These filaments interact to generate muscle contraction, with the sliding of actin past myosin generating muscle tension. This process is known as the sliding filament model of muscle contraction. The structure of the thick and thin filaments, along with their interactions, allow for the generation of force by the sarcomeres.
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What You'll Learn
Thick and thin filaments
Myofilaments are the three protein filaments of myofibrils in muscle cells. The main proteins involved are myosin, actin, and titin. Myosin and actin are the contractile proteins, while titin is an elastic protein. The myofilaments act together in muscle contraction.
There are three types of myofilaments: thick, thin, and elastic filaments. Thick filaments are approximately 15 nm in diameter and are made of several hundred molecules of myosin, a motor protein. Myosin molecules are shaped like golf clubs, with a tail formed of two intertwined chains and a double globular head projecting from it at an angle. Half of the myosin heads angle to the left, and half angle to the right, creating an area in the middle of the filament known as the M-region or bare zone.
Thin filaments, on the other hand, are 7 nm in diameter and consist primarily of the protein actin, specifically filamentous F-actin. Each F-actin strand is composed of a string of subunits called globular G-actin. Each G-actin has an active site that can bind to the head of a myosin molecule. Each thin filament also has approximately 40 to 60 molecules of tropomyosin, a protein that blocks the active sites of the thin filaments when the muscle is relaxed.
Elastic filaments, 1 μm in diameter, are made of titin, a large springy protein. They run through the core of each thick filament and anchor it to the Z-line, the endpoint of a sarcomere. Titin also stabilizes the thick filament, centering it between the thin filaments and preventing overstretching.
The arrangement and interactions between thin and thick filaments allow sarcomeres to generate force. When a motor neuron signals a skeletal muscle fiber, cross-bridges form between the thick and thin filaments, and the thin filaments are pulled, sliding past the thick filaments within the fiber's sarcomeres. This process is known as the sliding filament model of muscle contraction. While the sarcomere shortens, the individual proteins and filaments do not change length but simply slide next to each other.
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Myosin and actin
Myosin is a molecular motor, a protein that converts chemical energy in the form of ATP to mechanical energy, thus generating force and movement. It is the prototype of a molecular motor. Myosin has a double-headed structure, with the heads positioned at opposite ends of the molecule. The globular end of each myosin protein nearest the actin, called the S1 region, has multiple hinged segments that can bend and facilitate contraction. The slimmer and typically longer "tail" region of myosin (S2) also exhibits flexibility and rotates in concert with the S1 contraction.
Actin, usually in association with myosin, is responsible for many types of cell movements. Each actin filament is composed of a string of subunits called globular G-actin. Each G-actin has an active site that can bind to the head of a myosin molecule. Each thin filament also has approximately 40 to 60 molecules of tropomyosin, a protein that blocks the active sites of the thin filaments when the muscle is relaxed.
The interaction of myosin and actin proteins is at the core of our current understanding of sarcomere shortening and muscle contraction. The sliding filament theory, proposed in 1954, states that the sliding of actin past myosin generates muscle tension. The sliding filament process of contraction can only occur when myosin-binding sites on the actin filaments are exposed by a series of steps that begin with Ca++ entry into the sarcoplasm. The bending of the myosin S1 region helps explain how myosin moves or "walks" along actin. This process is known as myosin-actin cycling.
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Sarcomere shortening
A sarcomere is the region of a myofibril contained between two cytoskeletal structures called Z-discs or Z-lines. A muscle fibre is composed of many myofibrils, which contain sarcomeres. The striated appearance of skeletal muscle fibres is due to the arrangement of thick and thin myofilaments within each sarcomere.
The thick and thin myofilaments are composed of the proteins myosin and actin, respectively. Myosin and actin are the contractile proteins, while titin is an elastic protein. The thick myosin filaments have a double-headed structure, with the heads positioned at opposite ends of the molecule. During muscle contraction, the heads of the myosin filaments attach to oppositely oriented thin actin filaments and pull them past one another. This action of myosin attachment and actin movement results in sarcomere shortening.
The sliding filament theory of muscle contraction states that the sliding of actin past myosin generates muscle tension. Because actin is tethered to structures located at the lateral ends of each sarcomere (the Z-discs), any shortening of the actin filament length would result in a shortening of the sarcomere and thus the muscle. This process is known as the sliding filament model of muscle contraction. While the sarcomere shortens, the individual proteins and filaments do not change length but simply slide past each other.
The process of sarcomere shortening begins with the release of calcium ions from the sarcoplasmic reticulum into the sarcoplasm. This causes troponin and tropomyosin to move away from the myosin-binding sites on the actin filaments, exposing them. Myosin can then bind to actin, and the sarcomere shortens as the actin filaments slide towards the centre of the myosin filament.
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Sliding filament theory
Myofilaments are the three protein filaments of myofibrils in muscle cells. The main proteins involved are myosin, actin, and titin. Myosin and actin are contractile proteins, while titin is an elastic protein. Myosin is a type of motor protein, and myosin II is the type found in muscle cells.
The sliding filament theory explains the mechanism of muscle contraction based on muscle proteins that slide past each other to generate movement. According to the theory, the myosin (thick filaments) of muscle fibres slide past the actin (thin filaments) during muscle contraction, while the two groups of filaments remain at a relatively constant length. The sliding filament theory was introduced in 1954 by two independent research teams. One team consisted of Andrew Huxley and Rolf Niedergerke from the University of Cambridge, and the other team consisted of Hugh Huxley and Jean Hanson from the Massachusetts Institute of Technology. The theory was originally conceived by Hugh Huxley in 1953.
The sliding filament theory directly introduced a new concept called cross-bridge theory, which explains the molecular mechanism of sliding filaments. Cross-bridge theory states that actin and myosin form a protein complex by the attachment of myosin heads to actin filaments, forming a cross-bridge between the two filaments. This cross-bridge cycle is facilitated by the presence of calcium ions and ATP, which are cofactors required for the contraction of muscle cells. Calcium ions bind to troponin and tropomyosin, which are proteins that regulate muscle contraction by blocking or exposing the binding sites on the actin filament for myosin.
The sliding filament theory is based on the structure of the thick and thin filaments within sarcomeres, which are the functional units of skeletal muscle fibres. The thick filament, myosin, has a double-headed structure, and during muscle contraction, the myosin heads attach to oppositely oriented thin filaments, actin, and pull them past one another, resulting in sarcomere shortening. The arrangement and interactions between the thin and thick filaments allow the sarcomeres to generate force. When a skeletal muscle fibre is activated by a signal from a motor neuron, cross-bridges form between the thick and thin filaments, and the thin filaments are pulled and slide past the thick filaments within the sarcomeres.
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Muscle contraction
Sarcomeres are highly stereotyped and repeated throughout muscle cells, and they are responsible for the striated appearance of skeletal and cardiac muscles. Each sarcomere contains many parallel actin (thin) and myosin (thick) filaments, which are bound together by troponin and tropomyosin. These filaments are the three protein filaments of myofibrils in muscle cells, with myosin and actin being contractile proteins, and titin being an elastic protein. The thick filaments are composed of several hundred myosin molecules, while the thin filaments are primarily made up of the protein actin. The thick filaments have a double-headed structure, with the heads positioned at opposite ends of the molecule.
During muscle contraction, the heads of the myosin filaments attach to oppositely oriented thin filaments, actin, and pull them past one another. This action results in sarcomere shortening, which is the fundamental mechanism of muscle contraction. The shortening of sarcomeres leads to the contraction of individual skeletal muscle fibres and ultimately the whole muscle. The actin filaments slide past the myosin filaments towards the middle of the sarcomere, causing the sarcomere to shorten without any change in the length of the individual filaments.
The process of muscle contraction involves several key steps. Firstly, calcium ions are released from the sarcoplasmic reticulum into the sarcoplasm, which is the cytoplasm of muscle cells. This release of calcium ions triggers the binding of myosin to actin filaments, allowing myosin to function as a motor that drives filament sliding. The troponin-tropomyosin complex uses calcium ion binding to regulate when the myosin heads form cross-bridges to the actin filaments. Cross-bridge formation and filament sliding occur in the presence of calcium, and this signalling process leading to muscle contraction is known as Excitation-Contraction Coupling.
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Frequently asked questions
Filaments are the three protein filaments of myofibrils in muscle cells. They are responsible for muscle contraction.
There are thick filaments, made of myosin, and thin filaments, made of actin. There are also very thin filaments made of titin, which is an elastic protein.
The thick and thin filaments interact to generate force and shorten the sarcomeres, which are the functional units of a skeletal muscle fibre. This process is known as the sliding filament model of muscle contraction.
Athletes can use the knowledge of how filaments respond to exercise to inform their training techniques. For example, research has shown that resistance training leads to changes in myofilament proteins in humans.
