Essential Cell Biology 5th edition

604 CHAPTER 17 Cytoskeleton
Figure 17–44 T tubules and the
sarcoplasmic reticulum surround
each myofibril. (A) Drawing of the two
membrane systems that relay the signal
to contract from the muscle cell plasma
membrane to all of the myofibrils in the
muscle cell (see Figure 17–40). (B) Electron
micrograph showing a cross section of a
T tubule and the adjacent sarcoplasmic
reticulum compartments. (B, courtesy of
Clara Franzini-Armstrong.)
plasma membrane
myofibril
transverse (T)
tubules formed
from invaginations
of plasma membrane
(A)
sarcoplasmic reticulum
(B)
0.2 µm
QUESTION 17–9
Compare the structure of
intermediate filaments with that of
the myosin-II filaments in skeletal
muscle cells. What are the major
similarities? What are the major
differences? How do the differences
in structure relate to their function?
Muscle Contraction Is Triggered by a Sudden Rise in
Cytosolic Ca 2+
The force-generating molecular interaction between myosin and actin
filaments takes place only when the skeletal muscle receives a signal
to contract from a motor neuron. The neurotransmitter released from
the nerve terminal triggers an action potential in the muscle cell plasma
membrane (as discussed in Chapter 12). This electrical excitation spreads
in a matter of milliseconds into a series of membranous tubes, called
transverse (or T) tubules, that extend inward from the plasma membrane
around each myofibril. The electrical signal is then relayed to the sarcoplasmic
reticulum, an adjacent sheath of interconnected flattened vesicles
that surrounds each myofibril ECB5 e17.44-17.44
like a net stocking (Figure 17–44).
The sarcoplasmic reticulum in muscle cells is a specialized region of the
endoplasmic reticulum. It contains a very high concentration of Ca 2+ . In
response to electrical excitation, which passes along the plasma membrane
and to the T tubules, much of this Ca 2+ is released into the cytosol
through a specialized set of ion channels that open in the sarcoplasmic
reticulum membrane (Figure 17–45). As discussed in Chapter 16, Ca 2+ is
widely used as an intracellular signal to relay a message from the exterior
to the interior of cells. In muscle, the rise in cytosolic Ca 2+ concentration
activates a molecular switch made of specialized accessory proteins
closely associated with the actin filaments (Figure 17–46A). One of these
inactive T-tubule membrane
activated T-tubule membrane
LUMEN OF T TUBULE
(EXTRACELLULAR SPACE)
Figure 17–45 Skeletal muscle
contraction is triggered by
the release of Ca 2+ from the
sarcoplasmic reticulum into the
cytosol. This schematic diagram
shows how a Ca 2+ -release
channel in the sarcoplasmic
reticulum membrane is opened
by a physical linkage to a
voltage-gated Ca 2+ channel
in the T-tubule membrane.
The T-tubule membrane
and sarcoplasmic reticulum
membrane are drawn in the
same orientation shown in the
micrograph in Figure 17−44B.
CYTOSOL
LUMEN OF
SARCOPLASMIC RETICULUM
sarcoplasmic reticulum
membrane
voltage-gated Ca 2+ channel
ACTION
POTENTIAL
Ca 2+ -release channel
Ca 2+
MYOFIBRIL
CONTRACTION