104
UNIT 2
Basic Tissues
Figure 6-4A.
Skeletal muscle, striations.
H&E,
3
1,480; inset
3
1,800
A pattern of light and dark stripes is readily apparent in higher
magniF
cations of
skeletal muscle
. This pattern gives skeletal mus-
cle its alternate name,
striated
(
“striped”
)
muscle
. The names of
the striations are based on their behavior under polarized light.
The dark bands are named
A bands
because they are
anisotropic
(rotate polarized light strongly), whereas the
I bands
are
isotropic
(rotate polarized light only slightly). The
A bands
correspond to
regions in which myosin molecules and actin molecules overlap
to a large extent;
I bands
correspond to regions in which actin
molecules predominate. In the center of each I band is a thin dark
line, the
Z line
, which corresponds to a membrane-like structure
to which the ends of actin molecules are attached. This striated
pattern was recognized from the early history of light microscopy,
but its structural signiF
cance was not understood until the advent
of practical electron microscope techniques in the 1950s.
A
I
Z
I
A
J.Lynch
Z line
M line
H band
Sarcomere
Sarcomere
I band
A band
Actin
myofilaments
Actin
Myosin
myofilaments
Myosin
B
C
Actin
1
2
34
Myosin
Myosin
Actin
Actin
Myosin
Myosin
Actin
Figure 6-4B.
Skeletal muscle—sarcomeres, myof
laments.
EM,
3
17,600
A
sarcomere
is deF ned as the portion of a myoF
bril between two
adjacent
Z lines
. The electron micrograph at left illustrates two sar-
comeres. The basic correspondence between the features on the elec-
tron micrograph and the constituent molecules is illustrated in the
diagram at left below.
Actin myof laments
(each consisting of many
actin molecules and other accessory molecules) are anchored at the
Z lines.
Myosin myof laments
(each consisting of hundreds of myo-
sin molecules) partially overlap the actin F laments. In cross section,
both actin and myosin F laments are arrayed hexagonally.
Figure 6-4C.
Muscle contraction.
Myosin
and
actin f laments
(1) are not in contact with each other in
the resting muscle. (2) When a contraction is initiated, myosin mol-
ecules undergo a conformational change and contact adjacent actin
F laments. (3) An energy (adenosine triphosphate [ATP]) consum-
ing reaction causes a further conformational change in the “head”
of the myosin molecule, which produces a translational movement
between the myosin and actin F laments. (4) The myosin molecule
is released from the actin F lament and the conformational changes
are reversed. The process is repeated millions of times in a fraction
of a second to produce contraction of the whole muscle.
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