262
Chapter 9
Sarcoplasmic Reticulum
The sarcoplasmic reticulum in muscle is homologous to the
endoplasmic reticulum found in most cells. This structure
forms a series of sleeve-like segments around each myofi bril
(
Figure 9–11
). At the end of each segment are two enlarged
regions, known as
lateral sacs,
that are connected to each
other by a series of smaller tubular elements. Calcium stored in
the lateral sacs is released following membrane excitation.
A separate tubular structure, the
transverse tubule
(T-tubule),
lies directly between, and is intimately associated
with, the lateral sacs of adjacent segments of the sarcoplasmic
reticulum. The T-tubules and lateral sacs surround the myofi -
brils at the region of the sarcomeres where the A bands and I
bands meet. The lumen of the T-tubule is continuous with the
extracellular fl uid surrounding the muscle fi ber. The mem-
brane of the T-tubule, like the plasma membrane, is able to
propagate action potentials. Once initiated in the plasma mem-
brane, an action potential is rapidly conducted over the surface
of the fi ber and into its interior by way of the T-tubules.
A specialized mechanism couples T-tubule action poten-
tials with calcium release from the sarcoplasmic reticulum
(
Figure 9–12
, step 2). The T-tubules are in intimate contact
with the lateral sacs of the sarcoplasmic reticulum, connected
by structures known as
junctional feet
or
“foot processes.”
This junction involves two integral membrane proteins, one
in the T-tubule membrane, and the other in the membrane of
the sarcoplasmic reticulum. The T-tubule protein is a modifi ed
voltage-sensitive calcium channel known as the
dihydropyri-
dine (DHP) receptor
(so named because it binds the class
of drugs called dihydropyridines). The main role of the DHP
receptor, however, is not to conduct calcium, but rather to act
as a voltage sensor. The protein embedded in the sarcoplasmic
reticulum membrane is known as the
ryanodine receptor
(because it binds to the plant alkaloid ryanodine). This large
molecule not only includes the foot process, but also forms a
Muscle fiber action potential
02
0
40
60
80
100
120
140
Latent period
Time (ms)
+30
0
–90
30
20
10
Muscle fiber
tension (mg)
Muscle fiber membrane
potential (mV)
Muscle contraction
Figure 9–10
Time relationship between a skeletal muscle fi ber action potential
and the resulting contraction and relaxation of the muscle fi ber.
Myofibrils
Cytosol
Sarcoplasmic reticulum
Plasma
membrane
Transverse
tubules
Opening of
transverse tubule
to extracellular fluid
Lateral sacs
Mitochondrion
Figure 9–11
Transverse tubules and sarcoplasmic reticulum in a single skeletal muscle fi
ber.
previous page 290 Vander's Human Physiology The Mechanisms of Body Function read online next page 292 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off