Sarcomere Architecture and Filaments
The cross-striated appearance of a muscle fiber is due to the precise arrangement of myofilaments and supporting structures within the sarcomere.
Thin (Actin) Myofilaments They form light I bands and partially extend into the dark zones. The backbone of the filament is a double helix of globular actin (about 350 molecules). Intertwined in this helix are regulatory proteins: fibrillar tropomyosin and globular troponin (in a ratio of 50 molecules to 350 actin molecules). The regular presence of this complex distinguishes muscle filaments from ordinary non-muscle cytoskeletal structures. Thin filaments anchor to the Z disc (Z line), a mesh-like plate whose backbone is composed of $\alpha$-actinin. Extending toward each other from neighboring Z lines, these filaments do not meet in the center of the sarcomere, forming a gap known as the H zone.
Thick (Myosin) Myofilaments Composed of the protein myosin (about 300 molecules per filament). Each molecule has a long tail and a double head. The tails are tightly packed within the filament core, while the heads project outward to interact with actin. Thick filaments define the width of the dark A band. In the center of the sarcomere, they are anchored by M-protein at the M line (mesophragma). To prevent overstretching, the ends of the myosin filaments are connected to the Z line by the giant protein titin.
To maintain this entire structure, a supporting apparatus exists. Intermediate filaments made of desmin cross-link the Z and M lines of adjacent myofibrils, while costamere complexes containing vinculin anchor the I bands to the sarcolemma.
Resting State
In an inactive muscle, protein interaction is reliably blocked. The concentration of $Ca^{2+}$ ions in the sarcoplasm is kept extremely low.
Active binding sites on actin molecules are physically (sterically) blocked by the regulatory protein complex—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 prepared to execute a power stroke once a signal arrives.
Cross-Bridge Cycle
Excitation of the muscle fiber leads to an increase in sarcoplasmic $Ca^{2+}$ concentration, triggering the molecular mechanism of contraction:
- Actin Activation. Calcium ions bind to troponin, altering the spatial conformation of the entire troponin-tropomyosin complex and exposing active binding sites on the actin filaments.
- Cross-Bridge Formation. Energized myosin heads rapidly bind to the exposed actin sites, accompanied by the release of ADP and phosphate.
- Power Stroke. The myosin head changes conformation, executing a pivoting movement (bending) that slides the thin filament toward the center of the sarcomere.
- Bridge Detachment. To break the bond formed between actin and myosin, a new ATP molecule must bind to the myosin head. Crucial fact: without ATP, protein detachment is impossible.
- Reactivation. Hydrolysis of the ATP molecule occurs. The released energy resets the myosin head to its original 'cocked' state, preparing it for the next cycle.
Sliding Theory and Rigor Mortis
As the cross-bridges cycle, thin filaments penetrate deeper among the thick filaments. This results in specific metric changes within the sarcomere:
- Light I bands and the central H zone progressively narrow.
- The dark regions of overlap within the A band widen.
- The total width of the A band remains strictly unchanged because the length of the thick filaments does not alter.
In cross-section within the overlap region (A band), filaments are arranged hexagonally: each thick filament is surrounded by six thin filaments, and each thin filament by three thick ones. Contraction reaches its limit when the I bands completely disappear and the thick filaments abut the Z lines.
Rigor Mortis This physiological phenomenon is directly explained by the described mechanism. After death, metabolism ceases and ATP levels critically drop. Deprived of energy, the calcium pump stops functioning, causing sarcoplasmic calcium concentration to rise. Calcium exposes actin-binding sites, and cross-bridges form. However, due to the total absence of new ATP molecules, myosin heads cannot detach from actin. Muscles become rigidly locked in a contracted state until tissue decomposition processes begin.