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The sliding filament theory explains how the muscles contract to generate force.
Skeletal muscle cells are long and cylindrical. Due to their appearance, they are referred to as muscle fibres or myofibers. Skeletal muscle fibres are multinucleated cells, meaning that they consist of multiple nuclei (singular nucleus) because of the fusion of hundreds of precursor muscle cells (embryonic myoblasts) during early development.
Moreover, these muscles can be pretty large in humans.
Muscle fibres are highly differentiated. They have acquired particular adaptations, making them efficient for contraction. Muscle fibres consist of the plasma membrane in muscle fibres is called the sarcolemma, and the cytoplasm is called the sarcoplasm. As well as, myofibers which possess a specialised smooth endoplasmic reticulum called the sarcoplasmic reticulum (SR), adapted for storing, releasing, and reabsorbing calcium ions.
Myofibers contain many contractile protein bundles called myofibrils which extend along with the skeletal muscle fibre. These myofibrils are composed of thick myosin and thin actin myofilaments, which are the critical proteins for muscle contraction, and their arrangement gives the muscle fibre its striped appearance. It is important not to confuse myofibers with myofibrils.
Figure 1 The ultrastructure of a microfibre.
Another specialised structure seen in skeletal muscle fibre is T tubules (transverse tubules), protruding of the sarcoplasm into the centre of the myofibers (Figure 1). T tubules play a crucial role in coupling muscle excitation with contraction. We will elaborate further on their roles further on in this article.
Skeletal muscle fibres contain many mitochondria to supply a large amount of ATP needed for muscle contraction. Furthermore, having multiple nuclei allows muscle fibres to produce large amounts of proteins and enzymes required for muscle contraction.
Skeletal myofibers have a striated appearance due to the sequential arrangement of thick and thin myofilaments in myofibrils. Each group of these myofilaments is called sarcomere, and it is the contractile unit of a myofiber.
The sarcomere is approximately 2 μm (micrometres) in length and has a 3D cylindrical arrangement. Z-lines (also called Z-discs) to which the thin actin and myofilaments are attached border each sarcomere. In addition to actin and myosin, there are two other proteins found in sarcomeres that play a critical role in regulating the function of actin filaments in muscle contraction. These proteins are tropomyosin and troponin. During muscle relaxation, tropomyosin binds along actin filaments blocking the actin-myosin interactions.
Troponin is composed of three subunits:
Since actin and its associated proteins form filaments thinner in size than the myosin, it is referred to as the thin filament.
On the other hand, the myosin strands are thicker due to their larger size and multiple heads that protrude outwards. For this reason, myosin strands are called thick filaments.
The organisation of thick and thin filaments in sarcomeres gives rise to bands, lines, and zones within sarcomeres.
Figure 2. Arrangement of filaments in sarcomeres. Source: OpenStax College, CC-BY-4.0.
The sarcomere is split into the A and I bands, H zones, M lines, and Z discs.
For a skeletal muscle to contract, its sarcomeres must shorten in length. The thick and thin filaments do not change; instead, they slide past one another, causing the sarcomere to shorten. This process is known as the sliding filament theory.
As the sarcomere shortens, some zones and bands contract while others stay the same. Here are some of the main observations during contraction (Figure 3):
Figure 3. Changes in the length of sarcomere bands and zones during muscle contraction.
Muscle events can be broken down into three steps: muscle stimulation, muscle contraction, and muscle relaxation.
Figure 4. Actin-myosin cross-bridge formation cycle.
Energy in the form of ATP is needed for the movement of myosin heads and the active transportation of Ca ions into the sarcoplasmic reticulum. This energy is generated in three ways:
Z disc: Disc where the thin actin filaments are anchored. The Z-disc marks the border of the adjacent sarcomeres.
According to the sliding filament theory, a myofiber contracts when myosin filaments pull actin filaments closer towards the M line and shorten sarcomeres within a fibre. When all the sarcomeres in a myofiber shorten, the myofiber contracts.
Yes, the sliding filament theory applies to striated muscles.
The sliding filament theory explains the mechanism of muscle contraction based on actin and myosin filaments that slide past each other and cause sarcomere shortening. This translates to muscle contraction and muscle fibre shortening.
Step 1: Calcium ions are released from the sarcoplasmic reticulum into the sarcoplasm. Myosin head does not move.
Step 2: Calcium ions cause tropomyosin to unblock actin-binding sites and permit cross bridges to form between actin filament and myosin head.
Step 3: Myosin head utilises ATP to pull on actin filament toward the line.
Step 4: Sliding of actin filaments past myosin strands results in shortening of sarcomeres. This translates to contraction of the muscle.
Step 5: When calcium ions are removed from the sarcoplasm, tropomyosin moves back to block calcium-binding sites.
Step 6: Cross bridges between actin and myosin are broken. Hence, the thin and thick filaments slide away from each other and the sarcomere returns to its original length.
According to the sliding filament theory, myosin binds to actin. The myosin then alters its configuration using ATP, resulting in a power stroke that pulls on the actin filament and causes it to slide across the myosin filament towards the M line. This causes the sarcomeres to shorten.
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