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#muscle relaxation

3 public questions tagged with this topic.

Cytoskeleton

Which event leads to muscle relaxation?

Muscle relaxation requires rapid removal of activating calcium signal and restoration of inhibitory thin filament state to prevent ATP waste. During excitation depolarization opens dihydropyridine receptors coupled to ryanodine receptors of sarcoplasmic reticulum flooding sarcoplasm with calcium that binds troponin C or calmodulin initiating cross bridge cycling. For relaxation three mechanisms lower cytosolic calcium efficiently: sarcoplasmic reticulum Ca ATPase SERCA a P type pump hydrolyzing one ATP to transport two calcium ions into lumen against concentration gradient, plasma membrane Ca ATPase PMCA and sodium calcium exchanger NCX extruding calcium to extracellular space, and cytosolic calcium buffers like parvalbumin and calsequestrin transiently binding calcium. As free calcium falls below about 300 nM calcium dissociates from troponin C tropomyosin returns to blocked B position and myosin light chain phosphatase dephosphorylates smooth muscle myosin heads. Myosin heads can bind ATP and detach but cannot reattach strongly tension decays sarcomeres return to resting length passively by elastic titin and connective tissue forces.

Ref: Alberts et al., Molecular Biology of the Cell 7th ed., Chapter 16: SERCA calcium reuptake leading to muscle relaxation mechanism.

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Cytoskeleton

Which protein prevents actin-myosin interaction in relaxed muscle?

In relaxed skeletal and cardiac muscle actomyosin interaction is prevented not by direct myosin inhibition but by steric blocking of thin filament through tropomyosin troponin complex evolutionarily conserved. Tropomyosin is alpha helical coiled coil dimer about 40 nm long that polymerizes head to tail along major groove of filamentous actin spanning seven actin monomers and forming continuous cable. In low calcium near 100 nM troponin I holds tropomyosin in blocked B state covering myosin binding sites on outer domain of actin. Elevation of calcium to micromolar via ryanodine receptor release allows calcium binding to troponin C N lobe causing structural shift of troponin complex moving tropomyosin azimuthally toward closed C state partially exposing sites then to open M state upon myosin strong binding. Only then can myosin heads attach release Pi and produce force. CapZ caps barbed ends gelsolin and cofilin sever or depolymerize actin but they do not regulate relaxation steric block essential for preventing wasteful ATP consumption at rest.

Ref: Alberts et al., Molecular Biology of the Cell 7th ed., Chapter 16: Tropomyosin blocks myosin binding in relaxed muscle.

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