Protein Chemistry: Structure, Properties, and Research Methods - Shendryk, A. N. 2022

Proteins of blood and muscle tissue
Proteins of the contractile system
Mechanism of muscle contraction

When examining the Molecular Mechanism of Muscle contraction, two fundamental questions arise:

> how the contraction mechanism is "switched on" or triggered;

> how the relaxation of contracted sarcomeres occurs.

According to current understanding, the signal that triggers muscle contraction is an electrical impulse arriving from a motor nerve (motor neuron) via the motor end-plate, or Neuromuscular Junction. This impulse is transmitted to the muscle Cell and rapidly propagates along the sarcolemma. At rest, a potential difference of approximately 60mV is maintained across the sarcolemma. As the electrical impulse spreads along the sarcolemma, this potential difference rapidly collapses, a phenomenon known as depolarization. It is hypothesized that the depolarization effect is driven by a sharp increase in the permeability of inorganic cations (K⁺, Na⁺, Ca²⁺).

Observations show that the propagation velocity of the electrical impulse in muscle is significantly greater than what could be achieved by simple diffusion of a chemical mediator from the sarcolemma to the internal myofilaments. A more complex mechanism is at play here. According to Electron Microscopy studies, its essence is as follows. It has been established that the sarcolemma forms numerous tubular invaginations, which in some cases run perpendicular to the muscle in the region of the Z-line of myofibrils, and in others at the junction of the A- and I-bands. With this arrangement, the sarcolemma comes into contact with nearly all myofibrils. This branching system of tubules is referred to as the T-system. Upon excitation of the sarcoplasm by an electrical impulse, the entire T-system is depolarized almost instantaneously, thereby transmitting the impulse signal simultaneously to virtually all sarcomeres (see Fig. 3.7).

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Fig. 3.7 Sequence of muscle contraction.

Stage (a) - the motor neuron releases acetylcholine, which binds to receptors located on the sarcolemma. Once a sufficient amount of acetylcholine has bound, an Action Potential is generated in the muscle fiber.

Stage (b) - the resulting action potential triggers the release of Ca²⁺ ions into the sarcoplasm.

Stage (c) - Ca²⁺ ions bind to troponin on the Actin filament (thin filament), causing troponin to "detach" Tropomyosin from the active sites. This enables Myosin heads to attach to the actin filament.

Following the passage of the electrical impulse through the sarcoplasmic reticulum and the release of Ca²⁺ ions into the sarcoplasm, the sarcolemma and sarcoplasmic reticulum return to their initial polarized state. This recovery process proceeds at approximately the same rate as the contraction process.