Muscle contraction mechanism (MCM) is the collective set of processes that lead to the shortening or tensioning of muscles in response to stimulation triggered by the discharge of motor neurons (motor neurons).
A model of MCM is widely accepted, according to which the process of muscle contraction can be roughly divided into three periods:
- Latent Period: During this phase, when the membrane surface of the muscle fiber is excited, the action potential spreads initially through the system of T-tubules (transverse tubules) and then to the membranes of the cisterns of the sarcoplasmic reticulum. Subsequently, calcium ions (Ca2+) are released from the cisterns (known as the “calcium spike”), initiating the excitation-contraction coupling process. Calcium binds to the protein troponin, causing a change in the spatial structure of tropomyosin and the release of active sites on actin. Due to the high affinity of actin for myosin, the release of active sites on actin leads to the formation of cross-bridges.
- Contraction Period: This stage is described by the theory of “sliding filaments.” Actin and myosin myofilaments, oriented in parallel to each other and the axis of the muscle fiber, begin to shift relative to each other, thanks to the cross-bridge system. As a result, the length of the sarcomere (the basic structural unit of a muscle) decreases due to the sliding of actin filaments along myosin, with the filaments themselves not shortening. Each sarcomere can contract by approximately 20% of its initial length.
- Relaxation Period: Following the excitation of the muscle fiber, a repolarization wave stimulates the process of pumping calcium ions back into the sarcoplasmic reticulum. As a result, troponin and tropomyosin block the active sites on actin, leading to the dissociation (disruption) of cross-bridges, and the muscle fiber relaxes.
This intricate mechanism of muscle contraction enables muscles to perform a wide range of movements and functions within the body. Understanding this process serves as the foundation for studying muscle function and can be valuable for training and rehabilitation purposes.
