368 Chapter 12
myosin head will then split ATP to ADP and P i , and—if noth-
ing prevents the binding of the myosin head to the actin—a
new cross-bridge cycle will occur ( fig. 12.12 ).
Note that the splitting of ATP is required before a cross
bridge can attach to actin and undergo a power stroke, and that
the attachment of a new ATP is needed for the cross bridge to
release from actin at the end of a power stroke ( fig. 12.12 ).
A single cross-bridge power stroke pulls the actin filament
a distance of 6 nanometers (6 nm), and all of the cross bridges
acting together in a single cycle will shorten the muscle by
less than 1% of its resting length. Muscles can shorten up to
60% of their resting lengths, so the contraction cycles must
be repeated many times. For this to occur the cross bridges
must detach from the actin at the end of a power stroke, reas-
sume their resting orientation, and then reattach to the actin
and repeat the cycle.
During normal contraction, however, only a portion of
the cross bridges are attached at any given time. The power
strokes are thus not in synchrony, as the strokes of a com-
petitive rowing team would be. Rather, they are like the
actions of a team engaged in tug-of-war, where the pullingextend from the axis of the thick filaments to form “arms”
that terminate in globular “heads” ( fig. 12.10 ). A myosin pro-
tein has two globular heads that serve as cross bridges. The
orientation of the myosin heads on one side of a sarcomere
is opposite to that of the other side, so that, when the myosin
heads form cross bridges by attaching to actin on each side of
the sarcomere, they can pull the actin from each side toward
the center.
Isolated muscles are easily stretched (although this is
opposed in the body by the stretch reflex, described in a later
section), demonstrating that the myosin heads are not attached
to actin when the muscle is at rest. Each globular myosin head
of a cross bridge contains an ATP-binding site closely associ-
ated with an actin-binding site ( fig. 12.10 , left ). The globular
heads function as myosin ATPase enzymes, splitting ATP into
ADP and P i.
This reaction must occur before the myosin heads can bind
to actin. When ATP is hydrolyzed to ADP and P i , the phos-
phate binds to the myosin head, phosphorylating it and causing
it to change its conformation so that it becomes “cocked” (by
analogy to the hammer of a gun). The position of the myosin
head has changed and it now has the potential energy required
for contraction. Perhaps a more apt analogy is with a bow and
arrow: The energized myosin head is like a pulled bowstring; it
is now in position to bind to actin ( fig. 12.10 , right ) so that its
stored energy can be released in the next step.
Once the myosin head binds to actin, forming a cross
bridge, the bound P i is released (the myosin head becomes
dephosphorylated). This results in a conformational change
in the myosin, causing the cross bridge to produce a power
stroke ( fig. 12.11 ). This is the force that pulls the thin fila-
ments toward the center of the A band.
After the power stroke, with the myosin head now in
its flexed position, the bound ADP is released as a new ATP
molecule binds to the myosin head. This release of ADP and
binding to a new ATP is required for the myosin head to break
its bond with actin after the power stroke is completed. The
Figure 12.9 The sliding filament model of muscle contraction. (a) The upper image is an electron micrograph of
sarcomeres in a relaxed muscle. The lower two images are photo illustrations of the changes that occur during contraction. (b) An
illustration of the sliding filament model of striated muscle contraction as a relaxed muscle fiber (1) partially contracts (2) and then
contracts fully (3). Although the sarcomeres shorten, the filaments slide rather than shorten.
I
H
ZA
ZRelaxedPartially ContractedFully ContractedZ(a) (b)HAI
Z132CLINICAL APPLICATION
Rigor mortis is the stiffening of the body that begins a
couple of hours after death and lasts for about two days,
depending on the temperature. In rigor mortis, the myosin
heads form rigor complexes with actin that do not detach.
This is because ATP is lacking in dead cells, and without
ATP the active transport pumps needed to move Ca^2 1 from
the cytoplasm into the sarcoplasmic reticulum (SERCA
pumps, discussed shortly under muscle relaxation) can-
not function. As a result, Ca^2 1 remains attached to troponin
so that tropomyosin cannot inhibit cross bridge formation.
After a while, the myosin heads cannot release from actin
because they cannot bind to a new ATP.