Practice question

Triggering of all muscle types converges on transient elevation of calcium as intracellular messenger linking excitation to contraction coupling electrical and mechanical events effectively. Resting cytosolic calcium held near 100 nanomolar by SERCA pumps actively sequestering calcium in sarcoplasmic endoplasmic reticulum lumen. Action potential propagating along sarcolemma and T tubules activates voltage gated L type channels physically coupled to ryanodine receptors in striated muscle causing explosive calcium release raising concentration to 10 micromolar in microdomains near thin filaments. Calcium binds to EF hand motifs: troponin C in skeletal cardiac exposing myosin binding sites on actin via tropomyosin movement and calmodulin in smooth muscle activating myosin light chain kinase phosphorylating regulatory light chain. Sodium influx initiates excitation but does not directly activate contractile proteins potassium and magnesium maintain electrochemical gradients and serve as ATP Mg cofactor. Rapid pump mediated calcium reuptake ends signal making calcium ideally suited for rapid reversible control of force and speed of contraction.