292
UNIT TWO
f laments, shortens the sarcomere and thus shortens the mus-
cle (f g. 9.11). When another ATP binds, the cross-bridge is
f rst released From the actin binding site (see f
g. 9.10
5
), then
breaks down the ATP to return to the cocked position (see
f
g. 9.10
6
). This cross-bridge cycle may repeat as long as
ATP is present and nerve impulses release ACh at that neu-
romuscular junction.
reaction releases energy (see chapter 4, p. 119) that provides
the Force For muscle contraction. Breakdown oF ATP puts the
myosin cross-bridge in a “cocked” position (see f
g. 9.10
6
).
When a muscle is stimulated to contract, a cocked cross-
bridge attaches to actin (see f
g. 9.10
3
) and pulls the actin
f
lament toward the center oF the sarcomere (see f
g. 9.10
4
). This causes a greater overlap oF the actin and myosin
Thin flament
Actin monomers
Troponin
Tropomyosin
Thick flament
Release oF Ca
+
2
From sarcoplasmic
reticulum exposes binding sites on
actin:
Muscle contraction
Muscle relaxation
Active transport oF Ca
+
2
into sarcoplasmic
reticulum, which requires ATP, makes
myosin binding sites unavailable.
Ca
+
2
binds to troponin
Tropomyosin pulled aside
Binding sites on
actin exposed
3
Cross-bridges
bind actin to
myosin
5
New ATP binds to myosin, releasing linkages
6
ATP splits, which
provides power to
“cock” the myosin
cross-bridges
ATP
Contraction cycle
4
Cross-bridges pull thin flament (power stroke),
ADP and
released From myosin
P
P
ADP +
P
ADP +
P
ADP +
Ca
+
2
Ca
+
2
2
Exposed binding sites on actin molecules
allow the muscle contraction cycle to occur
1
Relaxed muscle
Ca
+
2
Ca
+
2
Ca
+
2
P
ADP +
P
ADP +
P
ADP +
P
ADP +
P
ADP
P
ADP
ATP
ATP
ATP
P
ADP +
P
ADP +
ATP
FIGURE 9.10
According to the sliding f
lament theory (
1–3
) when calcium ion concentration rises, binding sites on actin f
laments open, and
cross-bridges attach. (
4
) Upon binding to actin, cross-bridges spring From the cocked position and pull on actin f
laments. (
5
) ATP binds to the
cross-bridge (but is not yet broken down), releasing it From the actin f
lament. (
6
) ATP breakdown provides energy to “cock” the unattached myosin
cross-bridge. As long as ATP and calcium ions are present, the cycle continues. When calcium ion concentration is low in the cytosol, the muscle cell
remains relaxed. Not all cross-bridges Form and release simultaneously.
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