cardiac cells. This initial depolarization normally arises in a
small group of conducting-system cells called the
located in the right atrium near the entrance of
the superior vena cava (
). The action potential
then spreads from the SA node throughout the atria and then
into and throughout the ventricles. This pattern raises two
questions: (1) what is the path of spread of excitation, and
(2) what causes the SA node to “ﬁ
re”? We’ll deal initially
with the ﬁ rst question and then return to the second question
in the next section.
Sequence of Excitation
The SA node is the normal pacemaker for the entire heart.
Its depolarization normally generates the action potential that
leads to depolarization of all other cardiac muscle cells, and so
its discharge rate determines the
the number of
times the heart contracts per minute.
The action potential initiated in the SA node spreads
throughout the myocardium, passing from cell to cell by way
of gap junctions. Depolarization ﬁ rst spreads through the
muscle cells of the atria, with conduction rapid enough that
the right and left atria contract at essentially the same time.
The spread of the action potential to the ventricles involves
a more complicated conducting system (see Figure
). The link between atrial depolarization and ven-
tricular depolarization is a portion of the conducting system
atrioventricular (AV) node,
located at the base of
Bundle of His
Left bundle branch
Conducting system of the heart.
Sequence of cardiac excitation. The yellow color denotes areas that are depolarized. Impulse spreads from right atrium to left atrium via the
atrial muscle cells where the atria share a wall. The electrocardiogram monitors the spread of the signal.
Adapted from Rushmer.