Module 9.7
Watch
A muscle fiber contraction uses
ATP in a cycle that repeats
during the contraction
Mitosis
The contraction cycle is a series of interrelated
steps that begins with the arrival of calcium ions
within the zone of overlap in a sarcomere.
1
ADP
+
P
In the resting sarcomere, each myosin head is
already “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 this process, the myosin head functions as
ATPase, an enzyme that breaks down ATP. At the
start of the contraction cycle, the breakdown
products, ADP and phosphate (represented as P),
remain bound to the myosin head.
Ca2+
Once the active sites are exposed, the
energized myosin heads bind to them,
forming cross-bridges.
ADP
+ P
Tropomyosin
ADP
+
P
Ca2+
Ca2+
Troponin
Actin
3
Cytosol
ADP
+
P
Ca2+
Myosin reactivates when the free myosin head
splits ATP into ADP and P. The energy released
is used to “recock” the myosin head.
Cross-Bridges Detach
Myosin Heads Pivot
When another ATP binds to the myosin
head, the link between the myosin head
and the active site on the actin molecule is
broken. The active site is now exposed and able
to form another cross-bridge.
After the cross-bridges form, the energy that
was stored in the resting state is released as the
myosin heads pivot toward the M line. This action is
called the power stroke; when it occurs, the bound
ADP and phosphate group are released.
Ca2+
Ca2+
Ca2+
Ca2+
ADP
P +
4
ADP + P
ATP
Ca2+
ADP
+
P
ADP
P +
5
6
Ca2+
Ca2+
Active
site
ADP
+
P
The entire cycle repeats several times each second, as
long as Ca2+ concentrations remain elevated and ATP
reserves are sufficient. Calcium ion levels will remain
elevated only as long as action potentials continue to
pass along the T tubules and stimulate the terminal
cisternae. Once that stimulus is removed, the calcium
channels in the SR close and calcium ion pumps pull
Ca2+ from the cytosol and store it within the terminal
cisternae. Troponin molecules then shift position,
swinging the tropomyosin strands over the active
sites and preventing further cross-bridge formation.
Calcium ions bind to troponin, weakening the
bond between actin and the troponin–
tropomyosin complex. The troponin molecule then
changes position, rolling the tropomyosin molecule
away from the active sites on actin and allowing
interaction with the energized myosin heads.
Myosin head
Myosin Reactivates
Contracted Sarcomere
Cross-Bridges Form
2
Contraction Cycle Begins
Resting Sarcomere
Active Sites Exposed
Ca2+
ADP + P
ATP
Module 9.7 Review
a. What molecule supplies the energy for
a muscle contraction?
b. List the interrelated steps that occur
once the contraction cycle begins.
c. What triggers myosin reactivation?
318
• Chapter 9: Skeletal Muscle Tissue
M09_MART0000_00_SE_CH09.indd 318-319
9.7 Describe the role of ATP in a muscle contraction, and explain
the steps involved in the contraction of a skeletal muscle fiber.
Section 1: Functional Anatomy of Skeletal Muscle Tissue •
319
7/19/13 3:28 PM