288
UNIT TWO
Thick (myosin)
filaments
Thin (actin)
filaments
Myofibril
Sarcomere
Sarcoplasmic
reticulum
(a)
Skeletal muscle fiber
Z line
H zone
M line
I band
A band
I band
A band
(b)
Z line
Sarcomere
(a)
I band
Z line
I band
Z line
A band
Thin filaments
Titin
(b)
Thick filaments
Myosin
molecule
Cross-bridges
Actin molecule
Troponin
Tropomyosin
Thin filament
Thick
filament
FIGURE 9.5
A sarcomere. (
a
) Micrograph (16,000×). (
b
) The
relationship of thin and thick F
laments in a sarcomere. The size of the
H zone may change depending on the degree of F
lament overlap.
Compare with the size of the H zone and F
lament overlap in
F
gure 9.4
a
and
b
.
FIGURE 9.4
Skeletal muscle F
ber. (
a
) A skeletal muscle F
ber contains numerous myoF
brils, each consisting of (
b
) repeating units called
sarcomeres. The characteristic striations of a sarcomere re±
ect the organization of actin and myosin F
laments.
FIGURE 9.6
Thick F
laments are composed of the protein myosin,
and thin F
laments are primarily composed of the protein actin. Myosin
molecules have cross-bridges that extend toward nearby actin F
laments.
Within the sarcoplasm of a muscle F ber is a network of
membranous channels that surrounds each myoF bril and runs
parallel to it. These membranes form the
sarcoplasmic reticu-
lum,
which corresponds to the endoplasmic reticulum of other
cells (see F gs. 9.2 and 9.4). A set of membranous channels, the
transverse tubules
(T-tubules), extends into the sarcoplasm
as invaginations continuous with the sarcolemma and contains
extracellular fl uid. Each transverse tubule lies between two
enlarged portions of the sarcoplasmic reticulum called
cister-
nae.
These three structures form a
triad
near the region where
the actin and myosin F laments overlap
(f g. 9.7)
.
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