C h a p t e r 1
Cell Structure and Function
25
represents the cell membrane. Embedded throughout
the sarcoplasm are the contractile elements actin and
myosin, which are arranged in parallel bundles (i.e.,
myofibrils
). Each myofibril consists of regularly repeat-
ing units called
sarcomeres
along its length.
Sarcomeres, which are the structural and functional
units of striated muscle, extend from one Z line to
another Z line (Fig. 1-17B). The central portion of the
sarcomere contains the dark band (A band) consisting
mainly of myosin filaments, with some overlap with
actin filaments. Straddling the Z line, the lighter I band
contains only actin filaments; therefore, it takes two sar-
comeres to complete an I band. An H zone is found in
the middle of the A band and represents the region where
only myosin filaments are found. In the center of the H
zone is a thin, dark band, the M band or M line, pro-
duced by linkages between the myosin filaments. Z lines
consist of short elements that interconnect and provide
the thin actin filaments from two adjoining sarcomeres
with an anchoring point. Muscle contraction involves
the actin filaments sliding inward among the myosin
filaments (Fig. 1-17C). In the relaxed state, the ends of
the actin filaments extend from two successive Z lines,
barely overlapping one another. In the contracted state,
these actin filaments have been pulled inward among the
myosin filaments so their ends overlap one another.
The
sarcoplasmic reticulum
, which is comparable to
the smooth ER, is composed of longitudinal tubules that
run parallel to the muscle fiber and surround each myo-
fibril (Fig. 1-17D). This network ends in enlarged, saclike
regions called the
lateral sacs
or
terminal cisternae
. These
sacs store calcium that is released during muscle contrac-
tion. A second system of tubules consists of the
transverse
or
T tubules
, which are extensions of the cell membrane
and run perpendicular to the muscle fiber. The hollow por-
tion or lumen of the T tubule is continuous with the extra-
cellular fluid compartment. Action potentials, which are
rapidly conducted over the surface of the muscle fiber, are
in turn propagated by the T tubules into the sarcoplasmic
reticulum. As the action potential moves through the lat-
eral sacs, the sacs release calcium, initiating muscle con-
traction. The membrane of the sarcoplasmic reticulum also
has an active transport mechanism for pumping calcium
ions back into the reticulum. This prevents interactions
between calcium ions and the actin and myosin myofila-
ments after cessation of a muscle contraction.
Skeletal Muscle Contraction.
Muscle contraction
involves the sliding of the thick myosin and thin actin
filaments over each other to produce shortening of the
muscle fiber, while the actual length of the individual
thick and thin filaments remains unchanged. The thick
myosin filaments consist of a thin tail, which provides
the structural backbone for the filament, and a globu-
lar head that forms cross-bridges with the thin actin
filaments (Fig. 1-18A). Myosin molecules are bundled
together side by side in the thick filaments such that one
half have their heads toward one end of the filament and
their tails toward the other end, and the other half are
arranged in the opposite manner. Each globular myosin
head contains a site able to bind to a complementary
site on the actin molecule. Besides the binding site for
actin, each myosin head has a separate active site that
catalyzes the breakdown of ATP to provide the energy
needed to activate the myosin head so it can form a
cross-bridge with actin. After contraction, myosin also
Tail
Head
Actin binding site
ATP binding site
Myosin
Cross-
bridge
A
Troponin complex
TnI
TnT
TnC
Tropomyosin Actin
Actin
B
ADP
Cocking of myosin head
Cross-bridge attachment
P
i
Power stroke
Cross-bridge detachment
ATP
C
1
2
3
4
FIGURE 1-18.
Molecular structure of the thicker myosin filament
(A)
and the thinner actin
filament
(B)
of striated muscle.The thin filament is a double-stranded helix of actin molecules with
tropomyosin and troponin molecules lying along the grooves of the actin strands.
(C)
Sequence of
events involved in sliding of adjacent actin and myosin filaments: (1) cocking of the myosin head,
which occurs as adenosine triphosphate (ATP) is split to adenosine diphosphate (ADP); (2) cross-
bridge attachment; (3) power stroke during which the myosin head bends as it moves the actin
forward; and (4) cross-bridge detachment, which occurs as a new ATP attaches to the myosin head.
