22
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
myocyte depolarizes, positive charges accu-
mulate just inside the sarcolemma. Because
individual myocytes are joined together by low-
resistance gap junctions located at the inter-
calated disks (see Chapter 3), ionic currents
can flow between two adjoining cells. When
these ionic currents are sufficient to rapidly
depolarize the adjoining cell to its threshold
potential, an action potential is elicited in
the second cell. This is repeated in every cell,
thereby causing action potentials to be propa-
gated throughout the atria. Action potentials in
the atrial muscle have a conduction velocity of
about 0.5 m/s (Fig. 2.10). Although the con-
duction of action potentials within the atria is
primarily between myocytes, some functional
evidence (although controversial) points to the
existence of specialized myocytes that serve as
conducting pathways within the atria, termed
internodal tracts (e.g., Bachmann bundle). As
action potentials originating from the SA node
spread across and depolarize the atrial muscle,
excitation-contraction
coupling is
initiated
(see Chapter 3).
Nonconducting connective tissue separates
the atria from the ventricles. Action potentials
normally have only one pathway available to
enter the ventricles, a specialized region of
cells called the AV node. The AV node, located
in the inferior-posterior region of the intera-
trial septum separating the left from the right
atrium, is a highly specialized conducting tis-
sue (cardiac, not neural in origin) that slows
the
impulse conduction velocity to
about
0.05 m/s. This is one-tenth the velocity found
in atrial or ventricular myocytes (see Fig. 2.10).
The delay in conduction between the atria
and ventricles at the AV node is physiologically
important. First, it allows sufficient time for
complete atrial depolarization, contraction, and
emptying of atrial blood into the ventricles prior
to ventricular depolarization and contraction
(see Chapter 4). Second, the low conduction
velocity helps to limit the frequency of impulses
Atrial Muscle
■ FIGURE 2.10 Conduction system w ithin the heart. Conduction velocities of different regions are noted in
parentheses. Note that Purkinje fibers have the highest conduction velocity and the atrioventricular (AV)
node has the lowest conduction velocity. SA, sinoatrial;
RA,
right atrium;
LA,
left atrium;
RV,
right ventricle;
LV,
left ventricle.
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