Biomedical Engineering Reference
In-Depth Information
Table 6.15.
Ionic fluxes that direct the cardiac action potential with depolarizing inward and
repolarizing outward currents (Source: [
466
];
i
K
,
to
: transient outward K
+
current;
i
K
,
r
:rapidde-
layed rectifier K
+
current;
i
K
,
s
: slow delayed rectifier K
+
current;
i
K
,
ur
: ultrarapid delayed rectifier
K
+
current;
i
K1
: inward rectifier K
+
current;
i
K
AT P
: ATP-sensitive channel;
i
K
ACh
: acetylcholine-
activated channel; phase 0: rapid depolarization; phase 1: initial, slight, fast repolarization; phase 2:
slow repolarization (plateau); phase 3: final, rapid repolarization; phase 4: resting potential [gradual
depolarization of pacemaker cells]).
Ion
Channels
Motion
Current
Action potential
phase
Na
+
Inward
i
Na
0 (1-3)
Ca
2
+
Ca
V
1
Inward
i
Ca
,
T
1-3
Ca
V
3
Inward
i
Ca
,
T
1
Na
+
and
NCX
Outward
i
NaCaX
0-1
(Ca
2
+
influx)
Ca
2
+
Outward
2, 4
(Na
+
influx)
K
+
K
V
4.2/3
Outward
i
K
,
to
1-2
K
V
1.5
Outward
i
K
,
ur
0-3
K
V
11.1
Outward
i
K
,
r
1-4
K
V
7.1-minK
Outward
i
K
,
s
2-4
K
IR
2
Outward
i
K1
3-4
K
IR
3.1/4
Outward
i
K
AT P
3-4
(or GIRK1/4)
i
K
ACh
K
IR
6.2-SUR1
Outward
i
K
AT P
3-4
Na
+
,K
+
,
HCN2/4
Inward
i
f
4
Ca
2
+
Cl
−
CFTR, TWIK
Outward
i
Cl
0-3
Non-pacemaker cells have a resting membrane potential (electrical diastole,
phase 4), whereas pacemaker cells generate regular, spontaneous action potentials
(Fig.
6.7
).
6.5.1
Sinoatrial Node Action Potential
Cardiac pacemaker cells exhibit automaticity associated with membrane
depolarization and repolarization. Atrial pacemaker potential is characterized by
2 distinct phases: an initial gradual depolarization (
D1
) followed by a steep slope
(
D2
). The sinoatrial node yields spontaneous action potentials. In comparison with
standard action potentials exhibited by ventriculomyocytes, only 3 phases can
be defined. Spontaneous depolarization (equivalent to phase 4 of standard action
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