Muscle silk glide theory
志联
2024-11-23
Muscle silk glide theory
The intermediary process between the excitation process, characterized by electrical changes in the muscle cell membrane, and the contraction process based on myofilament sliding is called excitation-contraction coupling. Let's learn about the sliding filament theory! The muscle filament excitation-coupling process includes three main steps: the transmission of electrical excitation to the deep part of the muscle cell through the transverse tubule system, the transmission of information from the triad structure, the release of Ca2+ from the sarcoplasmic reticulum to the cytoplasm, and the re-uptake of Ca2+ from the cytoplasm back into the sarcoplasmic reticulum.
Reaggregation of myofilament sliding caused by cross-bridge movement
Muscle filaments Sliding theory
Muscle filaments Shortening is due to the sliding of thin filaments between thick filaments in the sarcomere. In other words, when the muscle contracts, the thin filaments emanating from the Z-line slide towards the middle of the A-band by some force. As a result, the adjacent Z-lines approach each other, the length of the sarcomere shortens, and the myofibril, muscle fiber, and the entire muscle shorten.
Muscle filaments Molecular mechanism of fiber contraction
After the increased Ca2+ in actin binds to actin, actin moves from the edge of the groove of the actin double helix to the bottom of the groove, exposing the actin-myosin binding site. The cross-bridge containing ATP binds to this site, forming an actin-myosin-ATP complex. At the same time, the myosin ATPase in the cross-bridge is activated by actin, rapidly hydrolyzing the ATP on the cross-bridge to release energy, causing the cross-bridge to swing towards the A-band, pulling the thin filament towards the middle of the A-band. Then it automatically separates from the active site on actin, binds to the next site on actin, and swings the cross-bridge again, pulling the thin filament towards the middle of the A-band. Before the Ca2+ concentration in the sarcoplasm decreases, the cross-bridge repeatedly binds, swings, and separates from actin, slowly pulling the thin filaments towards the middle of the A-band, shortening the muscle fiber.
Muscle filaments Expansion after fiber contraction
After the excitation from the motor neuron stops, the release of Ca2+ also stops immediately, and the calcium pump is activated. Under the action of the calcium pump, the calcium-binding protein pumps Ca2+ back into the longitudinal tubules of the sarcoplasmic reticulum, diffusing throughout the sarcoplasmic reticulum. The Ca2+ concentration in the sarcoplasm decreases, Ca2+ and troponin separate, troponin recovers its conformation, tropomyosin covers the myosin-binding site on actin again, separating the cross-bridge and actin, restoring the original state of the thick and thin filaments, and expanding the muscle fiber.
Muscle filaments During fiber contraction, the energy driving contraction mainly comes from a substance called adenosine triphosphate (ATP) in the cell. If the brain and spinal cord instruct motor neurons to release the neurotransmitter acetylcholine at the neuromuscular junction, muscle contraction will occur. Once acetylcholine is detected, calcium is released into the area surrounding the fiber. Calcium exposes the binding sites on actin and myosin. If sufficient ATP is present, myosin will bind to the receptor sites on actin, forming cross-bridges. Myosin filaments pull actin filaments towards the center, shortening the sarcomere. Because all sarcomeres shorten simultaneously, the entire length of the muscle fiber shortens. Simultaneously stimulating multiple muscle fiber filaments to contract causes the entire muscle to contract.
The above is an introduction to the sliding filament theory. If you need to learn more, please feel free to contact us!
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