The Cardiovascular System: The Heart
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
, and its characteristic
, determines heart rate.
Atrioventricular (AV) node.
From the SA node, the depo-
larization wave spreads via gap junctions throughout the
atria and via the
, 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.
Atrioventricular (AV) bundle.
From the AV node, the im-
pulse sweeps to the
(also called the
bundle of His
) in the superior part of the interventricular
septum. Although the atria and ventricles abut each other,
connected by gap junctions. Te AV bundle is
electrical connection between them. Te ﬁbrous
cardiac skeleton is nonconducting and insulates the rest of
the AV junction.
Ultimately, at threshold (approximately
open, allowing explosive entry of
from the extracellular space. As a result, in pacemaker
cells, it is the inﬂux of Ca
(rather than Na
) that pro-
duces the rising phase of the action potential and reverses
the membrane potential.
As in other excitable cells, the falling phase
of the action potential and repolarization reﬂect opening of
channels and K
eﬄux from the cell.
Once repolarization is complete, K
channels close, K
ﬂux declines, and the slow depolarization to threshold begins
Sequence of Excitation
Cardiac pacemaker cells are found
in the following areas (Figure 18.15): sinoatrial (si
node, atrioventricular node, atrioventricular bundle, right and
le± bundle branches, and subendocardial conducting network
(Purkinje ﬁbers). Impulses pass across the heart in order from
following the yellow pathway in
Sinoatrial (SA) 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
(b) Comparison of action potential shape at various
(a) Anatomy of the intrinsic conduction system showing the sequence of
Superior vena cava
pause (0.1 s) at the
connects the atria
to the ventricles.
conduct the impulses
depolarizes the contractile
cells of both ventricles.
Intrinsic cardiac conduction system and action potential succession
during one heartbeat.