Figure 2­12
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McGraw-Hill
Global
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In this higher magnification of the previous image, the transverse tubule, or T-tubule (T), and components of the
sarcoplasmic reticulum (SR, arrow) are identified. In mammalian skeletal muscle, transverse tubules are located at
the junction of the A and I bands. T-tubules are invaginations of the sarcolemma and conduct the impulse for con-
Holdings,
traction, initiated at the sarcolemma by the nerve fiber, deep into the contractile apparatus. The cell membranes of
T-tubules have a characteristic scalloped appearance. Structures called junctional feet connect T-tubules with the
terminal dilations of the sarcoplasmic reticulum (terminal cisternae).
LLC.
Skeletal muscle is controlled by the nervous system. Axons from motor neurons terminate at the surface of skeletal
Not
muscle fibers, as will be shown in later images in this lesson set. The action potential in the axon stimulates the
to
be
muscle cell to shorten. The initial step of the process within the myocyte is the depolarization of the surface sarco-
redistributed
lemma and the conduction of this wave into the depths of the cell by the T-tubules.
This event is known as excitation and is a part of the contraction cycle of the myocytes. Consult your histology or cell
biology text for further information on the contraction cycle. The signal from the cell surface that passes along the
or
modified
T-tubules is transduced from the junctional feet of the T-tubules to the SR, resulting in the release of calcium ions
stored in the SR. Calcium is released into the myofibrils through calcium release channels, which are also part of the
junctional feet. The presence of calcium ions in the myofibrils controls the interaction of the two major contractile
in
proteins, myosin and actin, as will be described later. The coupling of the wave of excitation with the contraction
any
cycle occurs at the junctional feet.
way
without
permission.
29

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