Jonah Patel
Muscle contraction is a complex process that involves the interaction of various proteins and enzymes. The power stroke is a critical step in this process, and it occurs when the myosin head moves toward the M line, pulling the actin along with it. This movement results in a shortening of the sarcomere and the muscle, generating force and causing the muscle to contract. The power stroke requires energy, which is provided by the hydrolysis of ATP, and it is a key part of the cross-bridge cycle that drives muscle contraction.
| Characteristics | Values |
|---|---|
| Definition | The power stroke is the step at which force is produced during muscle contraction. |
| Occurrence | The power stroke occurs when ATP is hydrolyzed to ADP and phosphate, and when ADP and phosphate dissociate from the myosin head and the actin active site. |
| Movement | The myosin head moves toward the M line, pulling the actin along with it, causing the sarcomere to shorten and the muscle to contract. |
| Energy | The myosin head contains energy in a "cocked" position before the power stroke, and expends this energy during the power stroke, ending in a low-energy position. |
| Result | After the power stroke, ADP is released, but the cross-bridge formed is still in place, and actin and myosin remain bound together. |
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What You'll Learn
The role of actin and myosin filaments
The sliding filament theory of muscle contraction was developed following the publication of two groundbreaking papers in 1954 that described the molecular basis of muscle contraction. The papers detailed the positions of actin and myosin filaments at various stages of contraction in muscle fibres and proposed how their interaction produced contractile force.
Actin and myosin are both proteins found in every type of muscle tissue. They work together to generate muscle contractions and facilitate movement. Thick myosin filaments (15 nm in diameter) and thin actin filaments (7 nm in diameter) work together to generate force, producing the muscle cell contractions that facilitate the movement of the muscles and, therefore, of body structures.
The sliding filament theory states that the sliding of actin past myosin generates muscle tension. Actin is tethered to structures located at the lateral ends of each sarcomere called Z discs or Z bands. When a muscle contracts, the Z discs move closer together, shortening the sarcomere and thus the muscle. The thick filaments of myosin remain central and constant in length while the thinner actin filaments change in length along with the sarcomere.
The mechanism of contraction is the binding of myosin to actin, forming cross-bridges that generate filament movement. Myosin binds to actin at a binding site on the globular actin protein. Myosin has another binding site for ATP at which enzymatic activity hydrolyzes ATP to ADP, releasing an inorganic phosphate molecule and energy. The energy released during ATP hydrolysis changes the angle of the myosin head into a "cocked" position. The myosin head is then in a position for further movement, possessing potential energy, but ADP and Pi are still attached. If the actin binding sites are uncovered, a cross-bridge will form; that is, the myosin head spans the distance between the actin and myosin molecules. Pi is then released, allowing myosin to expend the stored energy as a conformational change. The myosin head moves toward the M line, pulling the actin along with it. As the actin is pulled, the filaments move approximately 10 nm toward the M line. This movement is called the power stroke, as it is the step at which force is produced.
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The sliding filament theory
The mechanism of contraction involves the binding of myosin to actin, forming cross-bridges that generate filament movement. The myosin reaches forward, binds to actin, contracts, releases actin, and then reaches forward again to bind to actin in a new cycle. This process is known as myosin-actin cycling. The contraction of the myosin's S1 region is called the power stroke, which requires the hydrolysis of ATP to release energy.
In summary, the sliding filament theory explains muscle contraction by the sliding of actin and myosin filaments past each other, with the formation of cross-bridges that generate filament movement and muscle tension. The power stroke occurs during the contraction of the myosin's S1 region, releasing energy and causing the muscle to shorten.
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ATP and the cross-bridge cycle
The cross-bridge cycle is a four-step process that involves the interaction of actin, myosin, and calcium ions to generate muscle contractions. This cycle is identified in all muscle types, including cardiac, smooth, and skeletal muscles.
The first step in the process of contraction is for calcium ions (Ca2+) to bind to troponin, a protein found in thin actin filaments, causing a conformational change that leads to the movement of tropomyosin. This movement exposes the myosin-binding sites on the actin filaments, allowing cross-bridge formation between the actin and myosin microfilaments.
In the second step, the myosin head binds to the exposed actin-binding site, forming the cross-bridge. This step is driven by the energy from ATP hydrolysis, which powers the myosin head's movement and prepares it for the power stroke.
The third step is the power stroke, during which the myosin head attached to the actin filament rotates, generating a relative sliding movement between the actin and myosin filaments. This movement results in the production of force required for muscle contraction, with the sarcomere shortening and the muscle contracting.
In the final step, ADP (adenosine diphosphate) and phosphate, the products of ATP hydrolysis, are released from the myosin head, preparing it for a new ATP molecule to bind and initiating the next cycle.
ATP plays a crucial role in the cross-bridge cycle by providing the energy required for muscle contraction. It also ensures that muscles return to their relaxed state by breaking the cross-bridge formed between actin and myosin. Without ATP, muscles would remain in a contracted state.
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The role of calcium ions
Muscle contraction is regulated by calcium ions. An action potential generated by a motor neuron activates voltage-gated calcium channels, allowing calcium ions to flow into the muscle cell. This influx of calcium ions triggers the release of more calcium ions stored inside the sarcoplasmic reticulum, which then diffuse between the myosin and actin filaments of the muscle fibrils.
Calcium ions play a crucial role in facilitating the interaction between actin and myosin. They do this by binding with troponin, which is found on the actin filaments. This binding causes a conformational change in troponin, which then changes the position of tropomyosin. This movement exposes the actin binding sites, allowing myosin to bind to actin and initiate the cross-bridge cycle.
The cross-bridge cycle is a key step in muscle contraction. During this cycle, myosin and actin filaments slide past each other, generating muscle tension and causing the sarcomere to shorten. The sarcomere is the basic unit of muscle tissue, consisting of thick and thin filaments that slide by each other during muscle contraction. The sliding of actin past myosin, driven by the calcium-induced exposure of actin binding sites, is what generates the force necessary for muscle contraction.
In summary, calcium ions are essential for muscle contraction as they activate the interaction between actin and myosin. This interaction leads to the formation of cross-bridges and the subsequent sliding of filaments, resulting in muscle tension and contraction. Without the presence of calcium ions, regulatory proteins would block the molecular binding sites on actin, preventing contraction.
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Types of muscle cells
The power stroke of muscle contraction occurs when ATP is hydrolyzed to ADP and phosphate, resulting in the release of energy. This energy allows the myosin head to move towards the M line, pulling the actin along with it, and producing force. This movement causes the sarcomere to shorten, leading to muscle contraction.
Now, let's discuss the various types of muscle cells:
There are three main types of muscle cells or tissue: cardiac, smooth, and skeletal. Each type has distinct characteristics, locations, and functions in the body.
Cardiac Muscle Cells
Cardiac muscle cells are found in the walls of the heart and are responsible for its contraction. They exhibit an involuntary control, meaning individuals do not need to consciously think about making the heart contract. Cardiac muscle cells have a striped or striated appearance due to the presence of sarcomeres, which are basic units that change in length during contraction, causing the overall length of the muscle to change. The cell membrane of cardiac muscle cells has specialized regions, including intercalated discs and transverse tubules, and is covered by a lamina coat.
Smooth Muscle Cells
Smooth muscle fibers are located in the walls of hollow visceral organs, such as the liver, pancreas, and intestines, but are absent in the heart. They have an involuntary control similar to cardiac muscle cells. Smooth muscle cells do not possess sarcomeres and myofibrils but contain large amounts of the contractile proteins actin and myosin. They play a crucial role in organ function and movement.
Skeletal Muscle Cells
Skeletal muscle cells, also known as muscle fibers, are the individual contractile cells within a muscle. They are attached to the skeleton and work with bones, tendons, and ligaments to support body weight and facilitate movement. Skeletal muscles are under voluntary control, meaning individuals can consciously decide to move them. These muscles contain myofibrils, which are composed of long protein chains of myofilaments. There are three types of myofilaments: thin (actin), thick (myosin), and elastic (titin). These myofilaments work together during muscle contraction, with the thin and thick filaments sliding over each other to shorten the fiber length.
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Frequently asked questions
The power stroke of muscle contraction occurs when the myosin head moves toward the M line, pulling the actin along with it.
ATP provides the energy required for the power stroke. It attaches to myosin, allowing the release of actin and the formation of a cross-bridge.
The power stroke is the step at which force is produced during muscle contraction. It results in the movement of actin filaments, causing the muscle to shorten and contract.
