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Figure 9.12
The cross bridge cycle is the series of events
during which myosin heads pull thin filaments toward the
center of the sarcomere.
Actin
Ca
2+
1
Cross bridge formation.
Energized
myosin head attaches to an actin
myofilament, forming a cross bridge.
2
The power (working) stroke.
ADP and P
i
are released and the myosin head pivots
and bends, changing to its bent low-energy
state. As a result it pulls the actin filament
toward the M line.
3
Cross bridge detachment.
After ATP
attaches to myosin, the link between myosin
and actin weakens, and the myosin head
detaches (the cross bridge “breaks”).
4
Cocking of the myosin head.
As ATP is
hydrolyzed to ADP and P
i
, the myosin
head returns to its prestroke high-energy,
or “cocked,” position.
*
Myosin
cross bridge
Thick filament
Thin filament
ADP
Myosin
P
i
ADP
P
i
ATP
hydrolysis
ADP
P
i
ATP
ATP
*
This cycle will continue as long as ATP is
available and Ca
2+
is bound to troponin.
In the absence
of ATP, myosin
heads will not
detach, causing
rigor mortis.
FOCUS
Cross Bridge Cycle
 
292
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