Expansion
Filaments slide past each other without changing length, powered by ATP
Expansion
- Calcium binds troponin C, moving tropomyosin off the actin binding sites
- Cross-bridge formation: the energised myosin head binds actin
- Power stroke: the head pivots, pulling actin past myosin; ADP and phosphate are released
- Detachment: a new ATP binds myosin, releasing it from actin
- Re-cocking: ATP hydrolysis re-energises the head
Filaments slide; neither actin nor myosin changes length. The A band stays constant while the I band and H zone shorten.
Two consequences of step 4 are worth holding onto. Rigor mortis occurs because ATP is required for detachment, so without it the cross-bridges remain locked. And the length-tension relationship follows from filament overlap: force is maximal at the sarcomere length giving optimal overlap, and falls at both shorter and longer lengths.
Malignant hyperthermia involves uncontrolled calcium release through a defective ryanodine receptor, causing sustained contraction, massive ATP consumption, heat and rhabdomyolysis, treated with dantrolene.