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Myofibril
Myofibrils
Triad:
Tubules of
the SR
Sarcolemma
Sarcolemma
Mitochondria
I band
I band
A band
H zone
Z disc
Z disc
Part of a skeletal
muscle fiber (cell)
T tubule
Terminal
cisterns
of the SR (2)
M
line
Figure 9.5
Relationship of the
sarcoplasmic reticulum and T tubules to
myofibrils of skeletal muscle.
The tubules
of the SR (blue) encircle each myofibril like a
“holey” sleeve. These tubules fuse to form a
net of communicating channels at the level
of the H zone and saclike elements called
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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