Biomedical Engineering Reference
In-Depth Information
Both canine and human sinoatrial nodes have at least 4 preferential conduction
paths, such as the superior and inferior conduction paths, that deliver depolarization
waves simultaneously to different regions of the atrial myocardium. According to
the preferential conduction paths, action potential spreads from the right atria to the
left atria either via the Bachmann's bundle and the coronary sinus myofibers or via
anterosuperior and posteroinferior intra-atrial septum connections [
592
].
In addition to hyperpolarization-activated inward “funny” current (
i
f
) through
hyperpolarization-activated cyclic nucleotide-gated channels, various other inward
currents can contribute to cardiac automaticity, such as
i
NaCaX
current through
Na
+
-Ca
2
+
exchanger (
calcium clock
mechanism). Blockers of
i
f
slow, but do not
stop automaticity, whereas ryanodine that depletes intracellular Ca
2
+
stores can
cause a cessation of automaticity. The Ca
2
+
influx regulates Na
+
-Ca
2
+
exchanger.
Spontaneous, cyclical, local Ca
2
+
release from the sarcoplasmic reticulum can
indeed participate in the generation of both basal and adrenergically stimulated
depolarization rate, in addition to
i
f
current [
593
]. In fact, Ca
2
+
dependence of
SAN automaticity, i.e., the calcium clock, is directly and indirectly linked to
the plasmalemmal ion flux mechanism of SAN automaticity. The sinoatrial node
produces a Ca
2
+
-stimulated adenylate cyclase (AC1 or AC8) rather than the typical
cardiac isoform (AC5 or AC6).
6.2.5.4
Atrial Paths
Since sinoatrial fibers fuse with the surrounding atrial cardiomyocytes, the action
potential spreads through the atria at a rate at least of
∼
0
.
3 m/s and produces atrial
contraction.
However, several nodal bundles conduct action potentials with a greater speed
of about 1 m/s. Three preferred conduction paths have been observed in the right
atrium wall: the anterior Thorel, the mid Bachman, and the posterior Wenckebach
bundles.
6.2.5.5
Atrioventricular Node
The action potential reaches the
atrioventricular node
(AVN;
5 mm long) that
localizes to the right atrium near the lower part of the interatrial septum. The atri-
oventricular node is adjacent to the coronary sinus.
The atrioventricular node is subdivided into 3 regions: atrionodal, nodal, and
nodal-His, according to histological and electrophysiological criteria. Ovoid cells
are observed in nodal and nodal-His regions, rod-shaped cells in the atrionodal
region. The conduction speed is slower in the nodal region.
The atrioventricular node imposes a delay at most of about 100 ms in impulse
transmission to the ventricles, when atria have completed their contraction (Table
6.8
). Hence, it travels at a speed of 1.5 to 4 m/s.
∼
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