Chapter 18
The Cardiovascular System: The Heart
675
18
whole because no other region of the conduction system
or the myocardium has a faster depolarization rate. For this
reason, it is the heart’s
pacemaker
, and its characteristic
rhythm, called
sinus rhythm
, determines heart rate.
2
Atrioventricular (AV) node.
From the SA node, the depo-
larization wave spreads via gap junctions throughout the
atria and via the
internodal pathway
to the
atrioventricu-
lar node
, located in the inferior portion of the interatrial
septum immediately above the tricuspid valve. At the AV
node, the impulse is delayed for about 0.1 s, allowing the
atria to respond and complete their contraction before the
ventricles contract. Tis delay reflects the smaller diameter
of the fibers here and the fact that they have fewer gap junc-
tions for current flow. Consequently, the AV node conducts
impulses more slowly than other parts of the system, just as
traffic slows when cars are forced to merge from four lanes
into two. Once through the AV node, the signaling impulse
passes rapidly through the rest of the system.
3
Atrioventricular (AV) bundle.
From the AV node, the im-
pulse sweeps to the
atrioventricular bundle
(also called the
bundle of His
) in the superior part of the interventricular
septum. Although the atria and ventricles abut each other,
they are
not
connected by gap junctions. Te AV bundle is
the
only
electrical connection between them. Te fibrous
cardiac skeleton is nonconducting and insulates the rest of
the AV junction.
2
Depolarization.
Ultimately, at threshold (approximately
2
40 mV),
Ca
2
1
channels
open, allowing explosive entry of
Ca
2
1
from the extracellular space. As a result, in pacemaker
cells, it is the influx of Ca
2
1
(rather than Na
1
) that pro-
duces the rising phase of the action potential and reverses
the membrane potential.
3
Repolarization.
As in other excitable cells, the falling phase
of the action potential and repolarization reflect opening of
K
1
channels and K
1
efflux from the cell.
Once repolarization is complete, K
1
channels close, K
1
ef-
flux declines, and the slow depolarization to threshold begins
again.
Sequence of Excitation
Cardiac pacemaker cells are found
in the following areas (Figure 18.15): sinoatrial (si
0
no-a
9
tre-al)
node, atrioventricular node, atrioventricular bundle, right and
le± bundle branches, and subendocardial conducting network
(Purkinje fibers). Impulses pass across the heart in order from
1
to
5
following the yellow pathway in
Figure 18.15a
.
1
Sinoatrial (SA) node.
Te crescent-shaped
sinoatrial node
is located in the right atrial wall, just inferior to the entrance
of the superior vena cava. A minute cell mass with a mam-
moth job, the SA node typically generates impulses about
75 times every minute. (Its inherent rate in the absence of
extrinsic neural and hormonal factors is closer to 100 times
per minute.) Te SA node sets the pace for the heart as a
Milliseconds
(b) Comparison of action potential shape at various
locations
(a) Anatomy of the intrinsic conduction system showing the sequence of
electrical excitation
Pacemaker
potential
Pacemaker potential
Plateau
0
100
200
300
400
Internodal pathway
Superior vena cava
Right atrium
Left atrium
Subendocardial
conducting
network
(Purkinje fibers)
Inter-
ventricular
septum
SA node
Atrial muscle
AV node
Ventricular
muscle
1
The
sinoatrial (SA)
node
(pacemaker)
generates impulses.
2
The impulses
pause (0.1 s) at the
atrioventricular
(AV) node.
The
atrioventricular
(AV) bundle
connects the atria
to the ventricles.
4
The
bundle branches
conduct the impulses
through the
interventricular septum.
3
The
subendocardial
conducting network
depolarizes the contractile
cells of both ventricles.
5
Figure 18.15
Intrinsic cardiac conduction system and action potential succession
during one heartbeat.
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