CHAPTER 6
Muscle
105
B
J.Lynch
Triad
Triad
Mitochondrion
Mitochondrion
Openings into
transverse tubules
Sarcoplasmic reticulum
Transverse tubules
A band
I band
I band
Z line
Sarcolemma
Myofibril
Mitochondrion
Nucleus
A
A band
T-tubule
Triad
Sarcoplasmic reticulum
(terminal cistern)
Voltage-gated
calcium channel
Mitochondrion
Resting state
Muscle action potential opens calcium channels
Calcium ions
C
D
Figure 6-5.
Muscle contraction: Transverse tubule system
(T tubules) and the sarcoplasmic reticulum.
EM,
3
40,000
Skeletal muscle contracts very quickly after a nerve action
potential
releases
acetylcholine
(
Ach
) at the
neuromuscular junction
(see
Fig. 6-6A,B). The Ach causes Na
+
channels in the
sarcolemma
to
open and a wave of electrical excitation (
depolarization
) sweeps
down the length of the muscle ±
ber. The depolarization is carried
into the interior of the muscle ±
ber by a system of tubules, the
transverse tubules (T tubules)
that are themselves extensions of
the cell membrane. The
T tubules
branch within the muscle ±
ber
and encircle each
myof
bril
. Immediately adjacent to each T tubule
are two enlargements of the
sarcoplasmic reticulum
called
ter-
minal cisterns
. The three structures together form a
muscle triad
(Fig. 6-5A,B). In mammalian skeletal muscle, these triads lie at
the junction of the A and I bands. The sarcoplasmic reticulum, a
specialized form of
endoplasmic reticulum
, is a plexus of membra-
nous channels that ±
lls much of the space between the myo± brils.
It serves as a reservoir for
Ca
++
ions
, which are essential to the
process of muscle contraction (Fig. 6-5C). When a muscle action
potential is initiated, the depolarization spreads through the entire
T-tubule system almost instantaneously and causes the
voltage-
gated calcium channel
proteins to change con± guration, permit-
ting large amounts of Ca
++
to move from the terminal cisterns into
the surrounding cytosol (Fig. 6-5D). Here, the Ca
++
initiates the
reaction between the actin and myosin ±
laments that produces
muscle contraction (see Fig. 6-4C). At the end of the contraction,
the Ca
++
is quickly returned to the sarcoplasmic reticulum by an
ATP-dependent pump
in its membrane.
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