Alec Titan
A sarcomere is the smallest functional unit of a muscle fibre, or myofibril. It is the repeating unit between two Z-lines or Z-discs. The Z-disc anchors the thin filaments of actin that extend into each adjacent sarcomere and are located in the I-band. The thick filaments, composed of myosin, are located in the A-band. The interaction between actin and myosin filaments in the A-band of the sarcomere is responsible for muscle contraction. The sliding filament theory proposes that the active force is generated as actin filaments slide past the myosin filaments, resulting in the contraction of an individual sarcomere. The sliding theory was proposed by scientists who visualised the actin and myosin filaments within a sarcomere using high-resolution microscopes.
| Characteristics | Values |
|---|---|
| Definition | The smallest functional unit of striated muscle tissue |
| Appearance | Alternating dark and light bands under a microscope |
| Composition | Two main protein filaments (thin actin and thick myosin filaments) |
| Contraction | Occurs when actin and myosin filaments slide past each other |
| Calcium | Required for contraction; binds with troponin C molecules to expose myosin-binding sites on actin |
| ATP | Supplies energy for contraction |
| Role | Basic contractile unit of a myocyte (muscle fiber) |
| Z-line | Area where two actin filaments connect and transverse the I bands |
| M-line | Contains the protein myomesin and marks the center of the sarcomere |
| H-zone | Area between the M-line and Z-disc containing only myosin |
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What You'll Learn
Sarcomere structure
A sarcomere is the smallest functional unit of striated muscle tissue. It is the repeating unit between two Z-lines, with the Z-line anchoring the thin filaments of actin that extend into each adjacent sarcomere. The sarcomere is the main contractile unit of muscle fibre in the skeletal muscle. Each sarcomere is composed of protein filaments (myofilaments) that include mainly the thick filaments called myosin and thin filaments called actin.
The structure of the sarcomere is traditionally described with dark and light bands visible under the microscope. This banding pattern in sarcomeres is due mainly to the arrangement of thick and thin myofilaments in each unit. The A-band (or anisotropic band) is a dark band that contains whole thick filaments (myosin). The I-band (or isotropic band) is a light band that contains only the thin filaments (actin) and is located between the two thick filaments. The Z-line is an area that traverses the I-bands and marks the point of connection between the two neighbouring actin filaments. The M-line bisects the sarcomere and divides the A-band, which is formed by an array of thick filaments composed of myosin. The M-line contains the protein myomesin and marks the centre of the sarcomere. The H-zone is the area between the M-line and Z-line and contains only myosin.
The sliding filament model explains how sarcomeres and muscles can be shortened by sliding thick and thin filaments past one another. This process is ATP-dependent and requires direct interaction between the myosin head and actin. It also requires a change in the conformation of the myosin head to transform chemical energy from ATP hydrolysis into mechanical energy for myosin head movement along an actin filament. The sliding filament theory proposes that the active force is generated as actin filaments slide past the myosin filaments, resulting in the contraction of an individual sarcomere.
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Sarcomere contraction
A sarcomere is the smallest functional unit of striated muscle tissue. It is composed of two main protein filaments: thin actin and thick myosin filaments. These filaments are the active structures responsible for muscular contraction.
The widely accepted theory describing muscular contraction is the sliding filament theory, which proposes that the active force is generated as actin filaments slide past the myosin filaments, resulting in the contraction of an individual sarcomere. The sliding theory was proposed by scientists who, through the use of high-resolution microscopes, visualised the actin and myosin filaments within a sarcomere. They could see the length of the sarcomere when relaxed and its shortening as it contracted, and were able to give names to particular zones.
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. Calcium ions bind with troponin C molecules (which are dispersed throughout the tropomyosin protein) and alter the structure of the tropomyosin, forcing it to reveal the cross-bridge binding site on the actin.
The concentration of calcium within muscle cells is controlled by the sarcoplasmic reticulum, a unique form of endoplasmic reticulum in the sarcoplasm. Muscle cells are stimulated when a motor neuron releases the neurotransmitter acetylcholine, which travels across the neuromuscular junction (the synapse between the terminal button of the neuron and the muscle cell).
The binding of myosin to actin is also facilitated by the presence of ATP. A portion of the energy released in this reaction changes the shape of the myosin head and promotes it to a high-energy configuration. Through the process of binding to the actin, the myosin head releases ADP and an inorganic phosphate ion, changing its configuration back to one of low energy. The myosin remains attached to actin in a state known as rigor, until a new ATP binds the myosin head. This binding of ATP to myosin releases the actin by cross-bridge dissociation. The ATP-associated myosin is ready for another cycle, beginning with the hydrolysis of the ATP.
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Actin and myosin filaments
Myosin, on the other hand, has a long fibrous tail and a globular head that binds to actin. The myosin head binds to actin and releases ADP and an inorganic phosphate ion, changing its configuration back to one of low energy. The myosin remains attached to actin in a state known as rigor until a new ATP binds to the myosin head. This binding releases the actin by cross-bridge dissociation.
The sliding filament theory, first proposed in 1954, explains the interaction between actin and myosin filaments in muscle contraction. According to this theory, the sliding of actin past myosin generates muscle tension and results in the contraction of an individual sarcomere. The actin filaments slide past the myosin filaments in a cyclic rowing action, producing the macroscopic muscular movements we observe.
The sliding filament theory also explains the formation of the actomyosin complex responsible for muscular contraction. The actin-myosin interaction is driven by ATP-driven motors, with myosin molecules acting as motors and actin filaments as molecular tracks. The myosin molecules move along the actin filaments in a cyclic process known as myosin-actin cycling.
In summary, actin and myosin filaments are the key components of muscle contraction, with actin providing the essential building blocks and myosin facilitating contraction through their interaction and movement along the actin filaments.
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Calcium and ATP
ATP, or adenosine triphosphate, is the molecule that muscle cells use to store and transfer energy. During muscle contraction, ATP is hydrolyzed into ADP (adenosine diphosphate) and inorganic phosphate, releasing energy needed for myosin heads to perform power strokes and ultimately for muscles to contract. This process is known as ATP hydrolysis, a chemical process that breaks down ATP. The energy released changes the shape of the myosin head, promoting it to a high-energy configuration. The myosin head then binds to the actin filament, and the muscle contracts.
Calcium ions play a crucial role in this process by binding to a protein called troponin, which is attached to the actin filament. This binding causes a shift in the position of another protein, tropomyosin, exposing the myosin-binding sites on actin. The myosin heads then attach to these exposed binding sites, forming cross-bridges. The power stroke occurs when myosin releases stored energy by hydrolyzing ATP into ADP and inorganic phosphate, causing the myosin head to move and pull the actin filament, resulting in muscle contraction.
ATP is also vital for muscle relaxation. After contraction, ATP binds to myosin, leading to dissociation from the actin filament, breaking the cross-bridge. Additionally, ATP powers the calcium pumps that restore low Ca2+ levels post-contraction, helping the muscle return to its resting state.
The concentration of calcium within muscle cells is controlled by the sarcoplasmic reticulum, a unique form of endoplasmic reticulum. Muscle cells are stimulated when a motor neuron releases the neurotransmitter acetylcholine, which travels across the neuromuscular junction. This triggers an increase in calcium ions within the muscle cell, initiating the contraction process.
The relationship between calcium and ATP is not limited to muscle contraction and relaxation. Calcium signalling, which relies on the transmembrane concentration gradient for Ca2+, is fundamental for various cellular functions, including heartbeat, neurotransmission, and cell energetics. The maintenance of this gradient depends on ATP-dependent Ca2+ transport, emphasizing the inseparable links between calcium and ATP.
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Sarcomere identification
Sarcomeres are the smallest functional units of striated muscle tissue. They are composed of long, fibrous proteins that slide past each other when a muscle contracts or relaxes.
The identification of sarcomeres can be done by examining the structure and composition of muscle tissue, particularly the presence of alternating dark and light bands under a microscope. These bands, known as Z-lines, I-bands, and A-bands, are characteristic of sarcomeres.
Z-lines, or Z-discs, are dark lines that appear between the I-bands and anchor the actin myofilaments. They are formed by the connection of two actin filaments. The I-band, or isotropic band, is the zone of thin filaments (actin) that do not overlap with the thick filaments (myosin). The A-band, or anisotropic band, on the other hand, contains the entire length of a single thick filament (myosin).
Additionally, the identification of sarcomeres can be aided by understanding the role of key proteins such as myosin and actin. Myosin forms the thick filaments, while actin forms the thin filaments. The interaction and sliding of these filaments past each other during muscle contraction or relaxation is a fundamental characteristic of sarcomeres.
Recent advances in muscle biology, including the identification of key genes and the discovery of signalling molecules, have also contributed to our ability to identify and understand sarcomeres and their role in muscle function and disease.
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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. Skeletal muscles are composed of tubular muscle cells (called muscle fibers or myofibers) which are formed during embryonic myogenesis.
Sarcomeres are composed of long, fibrous proteins as filaments that slide past each other when a muscle contracts or relaxes. The two main protein filaments are thin actin and thick myosin filaments.
The sliding filament theory proposes that the active force is generated as actin filaments slide past the myosin filaments, resulting in the contraction of an individual sarcomere. The presence of calcium is essential for the contraction mechanism.
