Steps in E-C Coupling:
Troponin
Tropomyosin
blocking active sites
Myosin
Actin
Active sites exposed and
ready for
myosin binding
Ca
2+
Terminal
cistern
of SR
Voltage-sensitive
tubule protein
T tubule
Ca
2+
release
channel
The aftermath
When the muscle AP ceases, the voltage-sensitive tubule proteins return to their
original shape, closing the Ca
2+
release channels of the SR.
Ca
2+
levels in the
sarcoplasm fall as Ca
2+
is continually pumped back into the SR by active transport.
Without Ca
2+
, the blocking action of tropomyosin is restored, myosin-actin
interaction is inhibited, and relaxation occurs.
Each time an AP arrives at the
neuromuscular junction, the sequence of E-C coupling is repeated.
Myosin
cross
bridge
1
The action potential (AP) propagates
along the sarcolemma and down the
T tubules.
2
3
4
Calcium ions are released.
Transmission of the AP along the
T tubules of the triads causes the
voltage-sensitive tubule proteins to
change shape. This shape change opens
the Ca
2+
release channels in the terminal
cisterns of the sarcoplasmic reticulum
(SR), allowing Ca
2+
to flow into the
cytosol.
Calcium binds to troponin and
removes the blocking action of
tropomyosin.
When Ca
2+
binds,
troponin changes shape, exposing
binding sites for myosin (active
sites) on the thin filaments.
Contraction begins:
Myosin
binding to actin forms cross
bridges and contraction (cross
bridge cycling) begins. At this
point, E-C coupling is over.
Ca
2+
Sarcolemma
291
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