258
Chapter 9
The space between overlapping thick and thin fi la-
ments is bridged by projections known as
cross-bridges.
These are portions of myosin molecules that extend from
the surface of the thick fi laments toward the thin fi laments
(see Figure 9–2). During muscle contraction, the cross-
bridges make contact with the thin fi laments and exert force
on them.
Molecular Mechanisms
of Skeletal Muscle Contraction
The term
contraction,
as used in muscle physiology, does not
necessarily mean “shortening.” It simply refers to activation of the
force-generating sites within muscle fi bers—the cross-bridges.
For example, holding a dumbbell at a constant position requires
muscle contraction, but not muscle shortening. Following con-
traction, the mechanisms that generate force are turned off, and
tension declines, allowing
relaxation
of the muscle fi ber.
Sliding-Filament Mechanism
When force generation produces shortening of a skeletal mus-
cle fi ber, the overlapping thick and thin fi laments in each sar-
comere move past each other, propelled by movements of the
cross-bridges. During this shortening of the sarcomeres, there
is no change in the lengths of either the thick or thin fi
laments
(
Figure 9–5
). This is known as the
sliding-fi
lament mecha-
nism
of muscle contraction.
During shortening, each myosin cross-bridge attached
to a thin fi
lament actin molecule moves in an arc much like
an oar on a boat. This swiveling motion of many cross-bridges
forces the thin fi laments attached to successive Z lines to
move toward the center of the sarcomere, thereby shorten-
ing the sarcomere (
Figure 9–6
). One stroke of a cross-bridge
produces only a very small movement of a thin fi lament rela-
tive to a thick fi lament. As long as a muscle fi ber remains acti-
vated, however, each cross-bridge repeats its swiveling motion
many times, resulting in large displacements of the fi laments.
It is worth noting that a common pattern of muscle shorten-
ing involves one end of the muscle remaining at a fi xed posi-
tion while the other end shortens toward it. In this case, as
fi laments slide and each sarcomere shortens internally, the
center of each sarcomere also slides toward the fi
xed end of
Myofibril
Thick
filament
Thin
filament
(a)
(b)
Figure 9–4
(a) Electron micrograph of a cross section through three
myofi brils in a single skeletal muscle fi ber. (b) Hexagonal
arrangements of the thick and thin fi laments in the overlap
region in a single myofi bril. Six thin fi laments surround each
thick fi lament, and three thick fi laments surround each thin
fi lament. Titin fi laments and cross-bridges are not shown.
From H. E. Huxley,
J. Mol. Biol.,
37:507–520 (1968).
Figure 9–4
physiological
inquiry
Draw a cross-section diagram like the one in part b for a
slice taken: (1) in the H zone, (2) in the I band, (3) at the
M line, and (4) at the Z line (ignore titin).
Answer can be found at end of chapter.
Figure 9–5
The sliding of thick fi laments past overlapping thin fi laments shortens the
sarcomere with no change in thick or thin fi lament length. The I band
and H zone are reduced.
Relaxed
(a)
Shortened
(b)
A band
H zone
I
band
H zone
reduced
I
band
reduced
Z line
Z line
Z line
A band
unchanged
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