The co n tractio n (cross-bridge) cycle • Figure 6.4
Key:
O = Ca2+
,2+
Ca:
ADP
ADP
Myosin
ATP
ATP
Actin
Tropomyosin
Troponin
The contraction cycle
continues as long as ATP is
available and Ca2+ levels in
the cytoplasm are high.
ADP
ADP
3
ADP
ADP
Power stroke
Myosin head picks up
fresh ATP, drops actin,
and resets to again
form cross-bridges.
ATP
ATP
Myosin head bends
toward H zone, pulling
actin and Z disk
inward.
The myosin heads grab onto the binding sites of
the actin filaments to form cross-bridges and then
pull the actin filaments along in a repeating process
called the cross-bridge cycle or contraction cycle (
Fig-
ure 6.4
). This contraction cycle requires calcium ions
(Ca2+) and ATP. The contraction cycle repeats as long
as ATP and Ca2+ are available in the sarcoplasm.
At
any one instant, some of the myosin heads are attached
to actin, forming cross-bridges, and generating force,
and other myosin heads are detached from actin and
getting ready to bind again. How is muscle contraction
initiated? How is it stopped? This process, called
exci-
tation-contraction coupling,
begins with a nerve impulse.
Let’s take a closer look.
Muscles Are Built to Move 159
p r o c e ss
diagram
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