Biochemical Foundations of Human Vital Activity - Volkov N.I., Nesen E.N. 2000

Biochemistry of Sports
Biochemistry of Muscle and Muscular Contraction
Structural and Biochemical Changes in Muscles during Contraction and Relaxation

In a relaxed Muscle, Actin filaments extend into the space between Myosin filaments at the edges of the A-discs, but do not make contact with them (see Fig. 115). The ATPase activity centers located on the myosin heads bind ATP without hydrolyzing it. Activation of myosin ATPase requires the presence of Ca²⁺ ions. In the sarcoplasm of a resting muscle, the concentration of free Ca²⁺ ions is extremely low (10^-7 mol ⋅ l^-1) because they are sequestered within the vesicles of the sarcoplasmic reticulum. ATP prevents myosin filaments from contacting actin filaments. In this state, ATP acts as a plasticizing agent that hinders The formation of cross-bridges between Actin and myosin. Furthermore, in the absence of Ca²⁺ ions, troponin molecules—situated in the "groove" between two twisted polypeptide chains of actin within the thin filaments—also block the active sites where actin interacts with myosin. This dual inhibitory effect prevents cross-bridge formation between thick and thin filaments in myofibrils, protects the resting muscle from wasteful ATP consumption, and imparts elasticity to it in this state.

The main biochemical processes occurring During Muscle contraction are illustrated in Fig. 119. Contraction is triggered by a Nerve Impulse. At the synapse (1)—the site of contact between the nerve terminal and the sarcolemma—the neurotransmitter acetylcholine (2) is released. Acetylcholine (ACh) excites the sarcolemma, accompanied by membrane depolarization and the generation of an Action Potential on its surface (3). The action potential propagates deep into the fiber via T-system tubules, which contact the membranes of the sarcoplasmic reticulum. Excitation reaches the membrane structures of the sarcoplasmic reticulum, prompting Ca²⁺ ions to exit the reticulum vesicles into the sarcoplasm (5). An elevated concentration of free Ca²⁺ ions near the myosin filaments activates the ATPase centers in the myosin heads. ATP Hydrolysis takes place, but the reaction products—ADP and P_i—remain attached to the myosin molecule (6). In this state, the myosin heads are already capable of interacting with actin, yet their interaction site is blocked by troponin. Ca²⁺ ions also participate in removing this block and exposing the actin active sites On the surface of the actin filaments by binding to troponin and lifting the block (7). Cross-bridges in the form of an Actomyosin complex are formed between the myosin heads and the active sites of actin. The formation of actomyosin complexes stimulates the dissociation of ADP and P_i from the myosin molecule heads, and The energy released in the process is utilized for Conformational Changes in the contractile Proteins (8).

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Fig. 119 Main biochemical processes providing muscle contraction

Myosin molecule heads bend, adopting a arrowhead-shaped position relative to the axis of the myosin filament, while tension develops between the thick and thin filaments, sliding the thin filament toward the center of the sarcomere. Each cross-bridge between the actin and myosin filaments acts independently of other bridge formations during contraction.

The overall contraction process, manifested as the shortening of the muscle fiber and The Development of tension, is the result of the simultaneous summation of numerous cross-bridges formed along the entire length of the myofibrils involved in the contraction of an excited muscle. The magnitude of tension in a contracting muscle is proportional to the number of cross-bridges or their overlap area within each sarcomere, as dictated by The Nervous system (Fig. 120). Upon significant muscle stretch (sarcomere length exceeding 3.65 µm), the thin filaments extend completely beyond the A-discs, and muscle tension is absent. As the thin filaments penetrate between the thick ones and their overlap area increases, muscle tension gradually rises, reaching a maximum at a sarcomere length of 2.25 to 2.00 µm. With more pronounced muscle shortening, the thin filaments overlap at the center of the A-discs and crowd into the H-zone, forming a contraction band. Muscle tension at this stage of contraction rapidly decreases.

The reviewed processes of muscle contraction are consistent with the sliding filament theory proposed by the English biophysicist H. Huxley (1968).

Fig. 120 Changes in sarcomere length and tension development resulting from the summation of formed cross-bridges during contraction