Muscle
257
For example, some of the muscles that move the fi ngers are in
the forearm (wiggle your fi ngers and feel the movement of the
muscles in your lower arm). These muscles are connected to
the fi ngers by long tendons.
The striated pattern in skeletal (and cardiac) muscle results
from the arrangement of numerous thick and thin fi laments in
the cytoplasm into
myofi
brils,
approximately cylindrical bun-
dles 1 to 2 μm in diameter (see Figure 9–2). Most of the cyto-
plasm of a fi
ber is fi
lled with myofi brils, each extending from
one end of the fi ber to the other and linked to the tendons at
the ends of the fi ber.
The thick and thin fi laments in each myofi bril (see
Figure 9–2 and
Figure
9–3
) are arranged in a repeating pat-
tern along the length of the myofi bril. One unit of this repeat-
ing pattern is known as a
sarcomere
(Greek,
sarco,
muscle;
mer,
part). The
thick fi
laments
are composed almost entirely
of the protein
myosin.
The
thin fi
laments
(which are about
half the diameter of the thick fi laments) are principally com-
posed of the protein
actin,
as well as two other proteins—
troponin
and
tropomyosin
—that play important roles in
regulating contraction.
The thick fi laments are located in the middle of each sar-
comere, where their orderly, parallel arrangement produces a
wide, dark band known as the
A band
(see Figures 9–2 and
9–3). Each sarcomere contains two sets of thin fi laments, one
at each end. One end of each thin fi lament is anchored to a net-
work of interconnecting proteins known as the
Z line,
whereas
the other end overlaps a portion of the thick fi laments. Two
successive Z lines defi ne the limits of one sarcomere. Thus, thin
fi laments from two adjacent sarcomeres are anchored to the two
sides of each Z line. (The term “line” refers to the appearance
of these structures in two dimensions. Because myofi brils are
cylindrical, it is more realistic to think of them as “Z disks.”)
A light band known as the
I band
(see Figures 9–2 and
9–3) lies between the ends of the A bands of two adjacent sar-
comeres and contains those portions of the thin fi laments that
do not overlap the thick fi laments. The I band is bisected by
the Z line.
Two additional bands are present in the A-band region
of each sarcomere (see Figure 9–3). The
H zone
is a narrow,
light band in the center of the A band. It corresponds to the
space between the opposing ends of the two sets of thin fi la-
ments in each sarcomere. A narrow, dark band in the center
of the H zone, known as the
M line
(also technically a disk),
corresponds to proteins that link together the central region
of adjacent thick fi laments. In addition, fi laments composed
of the elastic protein
titin
extend from the Z line to the M
line and are linked to both the M-line proteins and the thick
fi laments. Both the M-line linkage between thick fi laments
and the titin fi laments act to maintain the alignment of thick
fi laments in the middle of each sarcomere.
A cross section through the A bands (
Figure 9–4
),
shows the regular arrangement of overlapping thick and thin
laments. Each thick fi
lament is surrounded by a hexago-
nal array of six thin fi laments, and each thin fi lament is sur-
rounded by a triangular arrangement of three thick fi laments.
Altogether there are twice as many thin as thick fi laments in
the region of fi lament overlap.
Figure 9–3
(a) High magnifi cation of a sarcomere within myofi brils. (b) Arrangement of the thick and thin fi laments in the sarcomere shown in (a).
Sarcomere
M line
Z line
Z line
I
band
H zone
A band
Titin
Thin filament
Thick filament
(a)
(b)
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