The co n tractio n (cross-bridge) cycle • Figure 6.4
O = Ca2+
The contraction cycle
continues as long as ATP is
available and Ca2+ levels in
the cytoplasm are high.
Myosin head picks up
fresh ATP, drops actin,
and resets to again
Myosin head bends
toward H zone, pulling
actin and Z disk
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 (
). This contraction cycle requires calcium ions
(Ca2+) and ATP. The contraction cycle repeats as long
as ATP and Ca2+ are available in the sarcoplasm.
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
begins with a nerve impulse.
Let’s take a closer look.
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