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a
c
T-Tubule
Sarcolemma
Junctional SR
Mitochondrion
Mitochondrion
5
m
m
Z-line
Myofibrils
Network SR
b
Fig. 10.3 (a) Schematic diagram of cellular structure of a ventricular myocyte (Katz
2011
). (b). A
T-tubule network image from a rat ventricular myocyte (Soeller and Cannell
1999
). (c) Illustration
of cardiac excitation-contraction system. In a normal action potential, voltage-dependent opening
of the LCCs brings Ca
2+
into the dyadic space (DS), a very small space between the LCC cluster
and jSR (
shaded area
). Elevated Ca
2+
concentration in the vicinity of the LCCs causes their
inactivation. The RyR channels open stochastically and their open probability is sensitive to Ca
2+
in the DS, a process called CICR. Therefore, the RyR channels can be triggered by Ca
2+
entry from
the LCCs, high myoplasmic and SR Ca
2+
, and Ca
2+
diffusing from neighboring CRUs. Ca
2+
entered from the LCCs and released from the SR diffuses to the myofibrils to signal contraction
and participates in many other signaling processes in the myocytes. Ca
2+
is pumped back into the
SR by the sarcoplasmic endoplasmic reticulum Ca
2+
ATPase (SERCA) pump and extruded by
Na
+
-Ca
2+
exchange (NCX). Ca
2+
is also uptaken by mitochondria through the mitochondrial
uniporter and released from mitochondria via NCX in the mitochondrial membrane and opening of
other channels such as the mitochondrial permeation transition pore. LCC and NCX couple Ca
2+
and voltage bidirectionally, but all other currents also affect this coupling either indirectly via their
effects on voltage or directly via Ca
2+
regulation of the ion channels
Ca
2+
release, causing a large release event to result in a spark (labeled as “s” in
Fig.
10.4a
). The Ca
2+
released in a spark may diffuse to cause its neighboring CRUs
to fire, or neighboring CRUs may fire coincidentally together, forming spark
clusters (labeled as “c” in Fig.
10.4a
). When a cluster becomes large enough, it
may propagate as a Ca
2+
wave (labeled as “w” in Fig.
10.3a
), depending on the
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