SECTION 21.1
Muscle Systems
457
A b a n d
M y o fib rils
S a r c o p la s m ic
re tic u lu m (S R )
T r a n s v e r s e
,
tu b u le s (T -s y ste m )
P la s m a
m e m b r a n e '
Mitochondrion
F IG U R E 2 1 -2
Schematic representation of the internal organization of a muscle
fiber. Note the parallel arrangement of myofibrils and the location
of the sarcoplasmic reticulum and T-tubules with respect to them.
This arrangement is typical of a mammalian fast-twitch fiber,
with the T-tubules and terminal cisternae located at or near the
Z-lines.
the sarcolemma action potential to activation of contrac-
tion. SR membranes bear large numbers of Ca2+ pump
proteins whose role is to pump Ca2+ into the lumen of
the SR where it is sequestered until the fiber is stimu-
lated. Crossing the SR perpendicular to the cell axis are
transverse tubules (T-tubules), which are invaginations
of the sarcolemma. They conduct the action potential into
the interior of the cell which triggers the release of Ca2+
from SR.
Muscle cells contain many mitochondria which are
often present as reticulum-like structures extending lon-
gitudinally in the fiber near the sarcolemma, rather than
as discrete ellipsoidal organelles found in many other
cells. These provide much of the high-energy phosphate
needed to power contraction and to operate the Ca2+
pumps that control the cytosolic calcium concentration.
Different types of skeletal muscle fibers differ consider-
ably in the extent and organization of both their SR and
mitochondria.
Muscle fibers contain long cylindrical myofibrils (typ-
ically
1 - 2
[ju
m in diameter) aligned longitudinally and
consisting of interpenetrating arrays of thin myofilaments
(6-7 nm in diameter) and thick filaments (15-16 nm in
diameter). These structures are the contractile apparatus
of the fiber (Figure 21-3).
Myofibrils
Skeletal and cardiac muscle exhibit alternating light and
dark bands (striae) visible by light microscopy and are
called striated muscle. This banded pattern is due to re-
gions of overlap alternating with nonoverlap of the thick
and thin filaments (Figure 21-3). In cross-sections, these
filaments are arranged in interpenetrating hexagonal ar-
rays. Between the thick and thin filaments, cross-bridges
may form.
The sarcolemma has more or less regularly spaced
electron-dense patches or rings of large membrane-
spanning multiunit proteins. These are analogous to
desmosomes, and the proteins therein are structural pro-
teins linking the meshwork of cytoskeletal filaments that
tie the myofibrils together to the extracellular matrix. The
complex of cytoskeletal actin and IFs and the membrane
proteins is called a
costamere.
One end of each thin fil-
ament is anchored to a dense fibrous structure (
Z-band
)
whose central region contains actin, a-actinin, and other
proteins. The Z-disk periphery contains predominantly
actin and desmin and lesser amounts of filamin, vimentin,
and synemin.
In vitro,
a-actinin and filamin bind actin, the major pro-
tein of thin filaments. One a-actinin molecule binds to
two actin filaments, one from each side of the Z-disk.
Four a-actinin molecules bind to each actin filament at
90° angles, so that the actin filaments are bound into
the Z-disks in a square array, although in most of the
length of the sarcomere their arrangement is hexagonal.
Desmin,
an intermediate filament protein, forms a net-
work from one Z-disk to the next across the myofibril.
Such links, aided by attachment of desmin and dystrophin
to the sarcolemma, help hold the sarcomere structure in
M lin e
A band
b a n d
I b a n d
T h in fila m e n t
T h ick
0 - 0 M-m lo n g )
fila m e n t
(
1.6
(j.m lo n g )
F IG U R E 2 1 -3
Portion of a myofibril. The Z-lines mark the ends of sarcomeres. An
A-band consists of thick filaments. An I-band consists of thin filaments
only. The thin filaments extend part way into the A-band leaving a central
region, the H-zone, free of thin filaments. A faintly staining M-line is
present in the center of the H-zone.
