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A sarcomere is the smallest functional unit of striated muscle tissue. It is a repeating unit between two Z-lines, composed of long, fibrous proteins that slide past each other when a muscle contracts or relaxes. The interaction between actin and myosin filaments in the A-band of the sarcomere is responsible for muscle contraction. Skeletal muscles are composed of tubular muscle cells (called muscle fibres or myofibers) which are formed during embryonic myogenesis. Muscle fibres contain numerous tubular myofibrils, which are composed of repeating sections of sarcomeres. These sarcomeres give skeletal and cardiac muscle their striated appearance. However, the myofibrils of smooth muscle cells are not arranged into sarcomeres, and thus, they do not contain the troponin complex required for skeletal muscle contraction.
| Characteristics | Values |
|---|---|
| Definition | The smallest functional unit of striated muscle tissue |
| Structure | Composed of long, fibrous proteins as filaments that slide past each other when a muscle contracts or relaxes |
| Appearance | Alternating dark and light bands under a microscope |
| Components | Myosin (thick filament) and actin (thin filament) |
| Function | Controls changes in muscle length |
| Location | Skeletal and cardiac muscles |
| Absence | Smooth muscle |
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What You'll Learn
What is a sarcomere?
A 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. In other words, it is the fundamental unit within a muscle that is responsible for contraction. Each sarcomere is a multinucleated cell surrounded by a 50-nm thick basement membrane called the sarcolemma.
The sarcomere consists of a bundle of myosin-containing thick filaments flanked and interdigitated with bundles of actin-containing thin filaments. The thick myosin filament contains 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 that are formed. The myosin head binds ATP and hydrolyzes it into ADP, inorganic phosphate, and free energy. Some of the energy released by ATP hydrolysis is transferred to the myosin head itself, which undergoes an allosteric change in conformation to a high-energy configuration. This energized myosin binds to a specific site on actin, forming a cross-bridge. The stored energy is then released, and the myosin head relaxes to a low-energy configuration. This relaxation event alters the angle of attachment between the myosin head and tail. Thus, as myosin turns inward on itself, it exerts tension on the thin filament to which it is bound, pulling the thin filament toward the centre of the sarcomere. This process is called myosin-actin cycling, and the contraction of myosin's S1 region is called the power stroke, which requires the hydrolysis of ATP.
The structure of a sarcomere can be described by the following zones:
- A bands (or anisotropic bands): It is also called the dark band and contains the whole thick filament (myosin) as well as the end of actin filaments.
- I bands (or isotropic bands): It is called the light band that contains only the thin filament (actin). The thin filament lies between the two thick filaments.
- H band: The area within the A band in which the thin and thick filaments do not overlap.
- Z disc: The area where two actin filaments connect and transverse the I bands.
- M line: The M line contains the protein called myomesin and it marks the centre of the sarcomere. It bisects the sarcomere and divides the A band.
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What is the role of calcium in muscle contraction?
All muscles, whether they are skeletal, cardiac, or smooth muscles, contain sarcomeres. These sarcomeres are the fundamental units of muscle contraction and are responsible for generating the force that leads to muscle shortening or lengthening. While the presence of sarcomeres is universal in muscles, their structure and arrangement vary between muscle types.
Now, moving on to the role of calcium in muscle contraction:
Calcium plays a crucial role in muscle contraction, acting as a trigger for the initiation of the process. The role of calcium differs slightly between skeletal and cardiac muscles and smooth muscles due to their distinct structures and functions. In skeletal and cardiac muscles, which are striated muscles, calcium binds to troponin, a regulatory protein, causing a conformational change. This change exposes binding sites on the thin filament, allowing myosin (the motor protein) heads to attach and initiate the cross-bridge cycle. The cross-bridge cycle is the sequence of events where myosin heads attach to actin (the thin filament) and pull, generating tension and causing the muscle to contract.
In smooth muscles, which lack the organized sarcomere structure of striated muscles, calcium interacts with calmodulin, a calcium-binding protein. The calcium-calmodulin complex then activates myosin light-chain kinase, which phosphorylates the myosin heads, making them active and able to interact with actin filaments. This process is similar to the activation of myosin in striated muscles, but the structural arrangement is different, leading to a more gradual and sustained contraction in smooth muscles.
Additionally, calcium is essential in muscle relaxation as well. When calcium levels in the cytosol drop, either due to pumping out of the cell or sequestration into the sarcoplasmic reticulum, the troponin-calcium complex dissociates, and the tropomyosin (a filamentous protein) shifts back to its original position, blocking the binding sites on the actin filament. This prevents further cross-bridge formation, leading to muscle relaxation.
In summary, calcium acts as a critical signaling molecule in muscle contraction, facilitating the interaction between myosin and actin filaments. Its dynamic interaction with regulatory proteins and its concentration changes in the cytosol help control the initiation, strength, and duration of muscle contractions, making it a key player in the complex process of muscle function.
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How does muscle contraction work?
Muscle contraction is an increase in muscle tension or a decrease in muscle length. Mammals have three types of muscles: skeletal, cardiac, and smooth. Skeletal muscles, which are attached to bones and give the body structure and strength, are the only type of muscle that can be contracted voluntarily.
Skeletal muscle is composed of bundles of muscle fibres called myofibres, which contain several myofibrils. Each myofibril represents a muscle cell with its basic cellular unit, the sarcomere. A single sarcomere is a multinucleated cell surrounded by a 50-nm thick basement membrane called the sarcolemma. The sarcomere is composed of thin and thick filaments of proteins. The thick filaments are composed of myosin, and the thin filaments are composed of actin.
When a motor neuron produces an action potential, it stimulates a skeletal muscle contraction. This occurs when the neuron releases acetylcholine at the neuromuscular junction, which depolarizes the sarcolemma. This change in membrane potential is converted to excitation–contraction (E–C) coupling. The depolarization spreads down the T-tubule and causes a conformational change in the dihydropyridine receptor (DHPR), which in turn triggers the opening of ryanodine receptor 1 (RyR1) in the adjacent SR, allowing Ca2+ to be released from the SR to the cytosol.
The released Ca2+ binds to a calmodulin protein, which then activates myosin light chain kinase (MLCK). The MLCK phosphorylates the myosin light chain, which hydrolyzes ATP, increasing its affinity to actin. The myosin can then readily bind to actin, forming a cross-bridge. The ATP is then hydrolyzed into 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 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 contraction cycle continues until calcium levels in the myocyte fall, causing tropomyosin to cover the actin filaments' myosin-binding sites, which inhibits actin-myosin interaction and leads to muscle relaxation.
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What is the 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 independent research teams: one consisting of Andrew Huxley and Rolf Niedergerke from the University of Cambridge, and the other consisting 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 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 theory introduced a new concept called cross-bridge theory, which explains the molecular mechanism of sliding filaments. The cross-bridge theory states that actin and myosin form a protein complex (called actomyosin) by the attachment of the myosin head to the actin filament, forming a cross-bridge between the two filaments.
In the sliding filament theory, myosin filaments use energy from ATP to "walk" along the actin filaments with their cross-bridges, pulling the actin filaments closer together. This movement of the actin filaments also pulls the Z lines closer together, thus shortening the sarcomere. When all the sarcomeres in a muscle fibre shorten, the fibre contracts. The number of fibres that contract determines the strength of the muscular force.
The sliding filament theory was developed through the study of muscle fibres using interference and electron microscopes.
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What are the different types of muscle?
All muscles are composed of thousands of individual fibers or sarcomeres. A single sarcomere is a multinucleated cell surrounded by a 50-nm thick basement membrane.
There are three types of muscle tissue in the body: skeletal, smooth, and cardiac. Skeletal muscles are part of the musculoskeletal system and work with bones, tendons, and ligaments to support the body's weight and enable movement. They are under voluntary control and receive neural inputs, allowing conscious control of muscles. Skeletal muscles are composed of bundles of muscle fibers called myofibers, which contain several myofibrils. Each myofiber represents a muscle cell with its basic cellular unit, the sarcomere. Skeletal muscles can contract quickly, using short bursts of energy, or slowly, such as those that help with posture. They are responsible for producing movement, sustaining body posture and position, maintaining body temperature, storing nutrients, and stabilizing joints.
Smooth muscle tissue lines some organs, including the liver, pancreas, and intestines, and is also found in the walls of hollow visceral organs. It is under involuntary control.
Cardiac muscle, also known as myocardium, makes up the middle layers of the heart and is under involuntary control by the autonomic nervous system. Cardiac muscle cells have one central nucleus, are striated, and appear striped. The contraction of cardiac muscle is involuntary, strong, and rhythmic.
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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.
No, only striated muscles have sarcomeres. Striated muscles include skeletal and cardiac muscles. Smooth muscles do not have sarcomeres.
The key components of a sarcomere are actin and myosin. Actin forms the thin filament, while myosin forms the thick filament.
Sarcomeres are responsible for the shortening of muscles during contraction. The interaction between actin and myosin filaments in the sarcomere allows for muscle contraction, which provides animals with flexibility and movement.
