Steps in E-C Coupling:
blocking active sites
Active sites exposed and
When the muscle AP ceases, the voltage-sensitive tubule proteins return to their
original shape, closing the Ca
release channels of the SR.
levels in the
sarcoplasm fall as Ca
is continually pumped back into the SR by active transport.
, 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.
The action potential (AP) propagates
along the sarcolemma and down the
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
release channels in the terminal
cisterns of the sarcoplasmic reticulum
(SR), allowing Ca
to ﬂow into the
Calcium binds to troponin and
removes the blocking action of
troponin changes shape, exposing
binding sites for myosin (active
sites) on the thin ﬁlaments.
binding to actin forms cross
bridges and contraction (cross
bridge cycling) begins. At this
point, E-C coupling is over.