Practice question

Coordinated muscle contraction depends on interplay between chemical fuel and regulatory ion signals ensuring contraction only when needed. ATP provides both detach signal and energy currency: ATP binding to myosin head after power stroke dissociates rigor cross bridge, while its hydrolysis to ADP Pi stores elastic energy in lever arm for next stroke, and Pi plus ADP release execute mechanical work. Without ATP muscle enters rigor state as observed in cadaveric stiffness. Calcium provides temporal switch: at rest sarcoplasmic reticulum SERCA maintains cytosolic calcium about 100 nM insufficient for troponin C binding keeping tropomyosin blocked and myosin light chain kinase inactive. In striated excitation opens ryanodine receptors raising calcium tenfold unlocking thin filament. In smooth calcium calmodulin activates myosin light chain kinase phosphorylating regulatory light chains increasing actin activated ATPase. Dynein is microtubule minus end motor unrelated to actin myosin regulation. Hence availability of ATP determines whether cycles can turn while calcium determines whether they are permitted to start linking excitation to contraction.