Page 343 - Fundamentals of anatomy physiology

Spotlight TFhigeureC1o0–n1t1raction Cycle and Cross-Bridge Formation

1 Contraction Cycle Begins 2 Active-Site Exposure 3 Cross-Bridge Formation

The contraction cycle involves a Calcium ions bind to troponin, Once the active sites are
series of interrelated steps. It begins weakening the bond between exposed, the energized myosin
with the arrival of calcium ions actin and the troponin–tropomyosin heads bind to them, forming
(Ca2+) within the zone of overlap in complex. The troponin molecule then cross-bridges.
a sarcomere. changes position, rolling the
tropomyosin molecule away from the
active sites on actin and allowing
interaction with the energized myosin
heads.

ADP Ca2+ Myosin head ADP Cytosol ADP
+P Troponin +P Ca2+ +P
Ca2+
Ca2+ Ca2+
ADP
Tropomyosin Actin ADP Ca2+ Active ADP P+
P+ site P+

RESTING SARCOMERE M line Zone of Overlap
(shown in
In the resting sarcomere, each
myosin head is already sequence above)
“energized”—charged with the
energy that will be used to
power a contraction. Each
myosin head points away from
the M line. In this position, the
myosin head is “cocked” like
the spring in a mousetrap.
Cocking the myosin head
requires energy, which is
obtained by breaking down
ATP; in doing so, the myosin
head functions as ATPase, an
enzyme that breaks down ATP.
At the start of the contraction
cycle, the breakdown prod-
ucts, ADP and phosphate
(represented as P), remain
bound to the myosin head.

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