Covering, Support, and Movement of the Body
mitochondria and glycogen granules, both involved in produc-
ing the energy used during contraction.
Te SR regulates intracellular levels of ionic calcium. It stores
calcium and releases it on demand when the muscle fiber is
stimulated to contract. As you will see, calcium provides the
final “go” signal for contraction.
T Tubules
At each A band–I band junction, the sarcolemma
of the muscle cell protrudes deep into the cell interior, form-
ing an elongated tube called the
T tubule
(± for “transverse”).
Te ± tubules, shown in gray in Figure 9.5, tremendously
increase the muscle fiber’s surface area. Possibly the result of
fusing tubelike caveolae (inpocketings of the sarcolemma),
(cavity) of the ± tubule is continuous with the
extracellular space.
Along its length, each ± tubule runs between the paired ter-
minal cisterns of the SR, forming
, successive groupings
of the three membranous structures (terminal cistern, ± tubule,
and terminal cistern). As they pass from one myofibril to the
next, the ± tubules also encircle each sarcomere.
Muscle contraction is ultimately controlled by nerve-
initiated electrical impulses that travel along the sarcolemma.
Because ± tubules are continuations of the sarcolemma, they
conduct impulses to the deepest regions of the muscle cell and
every sarcomere. Tese impulses signal for the release of cal-
cium from the adjacent terminal cisterns. Tink of the ± tubules
as a rapid telegraph system that ensures that every myofibril in
the muscle fiber contracts at virtually the same time.
Sarcoplasmic Reticulum
Shown in blue in
Figure 9.5
, the
sarcoplasmic reticulum (SR)
is an elaborate smooth endo-
plasmic reticulum. Its interconnecting tubules surround each
myofibril the way the sleeve of a loosely crocheted sweater
surrounds your arm.
Most SR tubules run longitudinally along the myofibril,
communicating with each other at the H zone. Others called
terminal cisterns
(“end sacs”) form larger, perpendicular cross
channels at the A band–I band junctions and they always occur
in pairs. Closely associated with the SR are large numbers of
Thin filament (actin)
Thick filament (myosin)
Myosin heads
Figure 9.4
Myosin heads forming cross bridges that gener-
ate muscular contractile force.
Part of a sarcomere is seen in a
transmission electron micrograph (277,000
Tubules of
the SR
I band
I band
A band
H zone
Z disc
Z disc
Part of a skeletal
muscle fiber (cell)
T tubule
of the SR (2)
Figure 9.5
Relationship of the
sarcoplasmic reticulum and T tubules
to myofibrils of skeletal muscle.
tubules of the SR (blue) fuse to form a net of
communicating channels at the level
of the H zone and the saclike terminal
cisterns abutting the A-I junctions. The
T tubules (gray) are inward invaginations of
the sarcolemma that run deep into the cell
between the terminal cisterns. (See detailed
view in Figure 9.11, pp. 290–291.) Sites
of close contact of these three elements
(terminal cistern, T tubule, and terminal
cistern) are called triads.
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