The cross-bridge cycle
Ca²⁺
Ca²⁺
Ca²⁺
M line →
Thin filament (actin; dashed line marks one point on it)
Thick filament (myosin)
ADP
P
ATP
Hinge
P
ADP
1
2
3
4
5
6
Repeats while calcium
is present
The cycle
1
Active sites exposed
Calcium binds troponin; tropomyosin rolls aside
2
Cross-bridge forms
Head binds actin (cross-bridge); phosphate released
3
Power stroke
Head swings towards the M line, dragging the thin filament; ADP released
4
ATP binds: head lets go
A new ATP binds the head itself; the head lets go
5
ATP split: head re-cocked
ATP split to ADP + P: the head is re-cocked, opposite a new active site
6
Ready to bind again
Lined up with the next active site
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