Architectonics of the sarcomere and filaments
The cross-striation of the muscle fiber is due to the strict arrangement of myofilaments and supporting structures in the sarcomere.
Thin (actin) myofilaments They form the light I-bands and partially extend into the dark zones. The basis of the filament is a double helix of globular actin (about 350 molecules). Regulatory proteins are woven into this helix: fibrillar tropomyosin and globular troponin (in a proportion of 50 molecules per 350 actin molecules). It is the regular presence of this complex that distinguishes muscle filaments from the conventional cytoskeleton structures of non-muscle cells. Thin filaments attach to the telophragma (Z-line), a net-like plate whose base is formed by the protein $\alpha$-actinin. Directed towards each other from neighboring Z-lines, these filaments do not meet in the center of the sarcomere, forming a clear gap—the H-zone.
Thick (myosin) myofilaments Consist of the protein myosin (about 300 molecules per filament). The molecule has a long tail and a double head. The tails are densely packed inside the filament, while the heads protrude outward to interact with actin. Thick filaments determine the width of the dark A band. In the center of the sarcomere, they are fixed by the M-protein on the M-line (mesophragma). To prevent overstretching, the ends of the myosin filaments are connected to the Z-line by a giant protein — titin.
To maintain this entire structure, there is a supporting apparatus. Intermediate filaments made of desmin cross-link the Z- and M-lines of adjacent myofibrils, while costameres made of vinculin anchor the I bands to the sarcolemma.
Resting state
In an inactive muscle, protein interaction is securely blocked. The concentration of $Ca^{2+}$ ions in the sarcoplasm is maintained at an extremely low level.
Active binding sites on actin molecules are physically (sterically) blocked by a complex of regulatory proteins — troponin and tropomyosin. At this moment, myosin heads are in a "strained", energized conformation. They cannot bind to actin yet, but they already contain ATP hydrolysis products (ADP and phosphate) and are fully ready to perform the working stroke as soon as a signal arrives.
Cross-bridge cycle
Excitation of the muscle fiber leads to an increase in $Ca^{2+}$ concentration in the sarcoplasm, which triggers the molecular mechanism of contraction:
- Actin activation. Calcium ions bind to troponin. This changes the spatial configuration of the entire troponin-tropomyosin complex and opens active sites on the actin filaments.
- Cross-bridge formation. Energized myosin heads instantly bind to the exposed actin sites. At this moment, ADP and phosphate are released.
- Working stroke. The myosin head changes conformation, performing a pivoting movement. It bends, pushing the thin filament toward the center of the sarcomere.
- Bridge detachment. For the bond formed between actin and myosin to break, a new ATP molecule must attach to the myosin head. Crucial fact: protein detachment is impossible without ATP.
- Reactivation. Hydrolysis of the ATP molecule occurs. The released energy returns the myosin head to its original "cocked" state, preparing it for the next cycle.
Sliding filament theory and rigor mortis phenomenon
As the bridges work, thin filaments penetrate deeper between the thick ones. This leads to strict metric changes in the sarcomere:
- Light I bands and the central H zone gradually narrow.
- Dark regions of the overlap zone in the A band widen.
- The total width of the A band remains strictly unchanged since the length of the thick filaments does not change.
In a cross-section within the overlap region (A band), filaments are hexagonally packed: one thick filament is surrounded by six thin ones, and each thin one by three thick ones. Contraction reaches its limit when the I bands completely disappear and the thick filaments abut against the Z-lines.
Rigor mortis This physiological phenomenon is directly explained by the described mechanism. After death, metabolism stops, and ATP levels drop critically. Deprived of energy, the calcium pump stops working, causing the calcium concentration in the sarcoplasm to rise. Calcium exposes actin sites, and cross-bridges form. However, due to the complete absence of new ATP molecules, myosin heads cannot detach from actin. Muscles become tightly fixed in a contracted state until tissue decomposition processes begin.
Briefly: answers to topical questions
What subunits does the troponin protein consist of, and what is the function of each?
Troponin is a globular heterotrimeric protein consisting of three specialized subunits.
- Troponin T — responsible for binding to tropomyosin.
- Troponin C — binds calcium ions.
- Troponin I — binds actin and inhibits muscle contraction.
At rest, the complex performs an inhibitory function. When calcium ions bind to the subunit, a conformational change occurs in troponin, which eliminates the blocking effect of tropomyosin and enables contact between actin and myosin.
How is excitation transmitted from the nerve terminal to the muscle fiber?
Excitation is transmitted from the nerve terminal to the muscle fiber at a chemical neuromuscular synapse using a neurotransmitter.
The neurotransmitter acts as a chemical messenger, ensuring the transfer of the action potential from the nerve to the postsynaptic membrane of the muscle. After excitation occurs in the synapse, the electrical impulse rapidly propagates along the plasma membrane and deep invaginations (T-tubules) into the interior of the fiber to the myofibrils to initiate muscle contraction.
What structures form the muscle triad and what is their role?
A muscle triad is a structural and functional unit consisting of one T-tubule (a sarcolemmal invagination) and two adjacent terminal cisterns of the sarcoplasmic reticulum.
The main role of the triad is to provide electromechanical coupling — converting the incoming electrical signal into a mechanical response. Excitation passing along the T-tubules causes depolarization and interaction of dihydropyridine receptors with ryanodine receptors. This leads to the opening of calcium channels in the terminal cistern membrane and a massive efflux of calcium ions into the cytoplasm to initiate muscle contraction.
For quick memorization of sarcomere zone changes: I bands Isolate (shorten/disappear), A band Absolutely unchanged.
Class and exam questions
Does the length of the protein filaments themselves change during muscle contraction?
No, neither actin nor myosin myofilaments shorten. Only their sliding relative to each other occurs, which changes the width of the functional zones of the sarcomere.
What is the fundamental difference between muscle thin filaments and non-muscle cell microfilaments?
In muscle tissues, thin filaments are distinguished by the regular presence of a regulatory complex consisting of two proteins — troponin and tropomyosin.
Why does a muscle fail to relax when there is a deficiency of ATP?
The attachment of a new ATP molecule to the myosin head is absolutely essential to break the physical bond with actin. Without it, the cross-bridges remain locked.
What else to study on this topic
- Spatial hexagonal packing of myofilaments
- Molecular structure of the troponin-tropomyosin complex
- Functional role of cytoskeleton proteins: desmin and vinculin
- Operation of the calcium pump and sarcoplasmic reticulum
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