Blood, Heart, and Circulation 425cells. Instead, during the period of diastole, the SA node exhib-
its a slow spontaneous depolarization called the pacemaker
potential. Because this pacemaker potential occurs during
diastole, it is also called a diastolic depolarization. The SA
node cells produce this spontaneous, diastolic depolarization
in a clocklike manner through the interaction of different mem-
brane ion channels and transporters.
The production of the spontaneous depolarization, and thus of
the automatic heartbeat, involves ion channels in the plasma mem-
brane and in the sarcoplasmic reticulum. One type in the plasma
membrane is known as HCN channels, which are unique to pace-
maker cells. The “H” in the name stands for hyperpolarization;
these channels—unlike all other voltage-gated ion channels—
open in response to hyperpolarization rather than to depolariza-
tion. When they open, they allow the entry of Na^1 to produce a
depolarization. Because of its unusual cause, the inward flow of
Na^1 though HCN channels is called a “funny current.” The “CN”
part of the HCN channel name stands for cyclic nucleotide; these
channels also open to cyclic AMP (cAMP), produced in response
to stimulation of beta-adrenergic receptors by epinephrine and
norepinephrine.
The “funny current” entry of Na^1 through the HCN channels
is important in producing the diastolic depolarization, but a clock-
like entry of Ca^2 1 into the cytoplasm also contributes significantly.
Once the diastolic depolarization reaches a threshold value (about
2 40 mV), it causes the opening of voltage-gated Ca^2 1 channels in
the plasma membrane. It is the influx of Ca^2 1 at this time—rather
than the more usual inflow of Na^1 —that produces the upward
phase of the action potential in the pacemaker cells ( fig. 13.18 ).
While this upward phase of the action potential is occurring,
the Ca^2 1 that has entered stimulates the opening of Ca^2 1 release13.5 ELECTRICAL ACTIVITY
OF THE HEART AND THE
ELECTROCARDIOGRAM
The pacemaker region of the heart (SA node) exhibits a spon-
taneous depolarization that causes action potentials, result-
ing in the automatic beating of the heart. Action potentials are
conducted by myocardial cells in the atria and are transmitted
to the ventricles by specialized conducting tissue. Electrocar-
diogram waves correspond to these events in the heart.
LEARNING OUTCOMES
After studying this section, you should be able to:- Describe the pacemaker potential and the
myocardial action potential, and explain how the
latter correlates with myocardial contraction and
relaxation. - Describe the components of the ECG and their
relationships to the cardiac cycle.
As described in chapter 12, myocardial cells are short, branched,
and interconnected by gap junctions. Gap junctions function as elec-
trical synapses, and have been described in chapter 7 (see fig. 7.21)
and chapter 12 (see fig. 12.32). The entire mass of cells intercon-
nected by gap junctions is known as a myocardium. A myocardium
is a single functioning unit, or functional syncytium, because action
potentials that originate in any cell in the mass can be transmitted
to all the other cells. The myocardia of the atria and ventricles are
separated from each other by the fibrous skeleton of the heart, as
previously described. Impulses normally originate in the atria, so
the atrial myocardium is excited before that of the ventricles.
Electrical Activity of the Heart
If the heart of a frog is removed from the body and all neural
innervations are severed, it will still continue to beat as long as
the myocardial cells remain alive. The automatic nature of the
heartbeat is referred to as automaticity. As a result of experiments
with isolated myocardial cells and of observations of patients
with blocks in the conductive tissues of the heart, scientists have
learned that there are three regions that can spontaneously gener-
ate action potentials and thereby function as pacemakers. In the
normal heart, only one of these, the sinoatrial node (SA node),
functions as the pacemaker. The SA node is located in the right
atrium near the opening of the superior vena cava, and serves as
the primary (normal) pacemaker of the heart. The two potential, or
secondary, pacemaker regions—the AV node and Purkinje fibers
(parts of the conduction network; see fig. 13.20 )—are normally
suppressed by action potentials originating in the SA node.
Pacemaker Potential
The cells of the SA node do not maintain a resting membrane
potential in the manner of resting neurons or skeletal muscle
Figure 13.18 Pacemaker potentials and action
potentials in the SA node. The pacemaker potentials are
spontaneous depolarizations. When they reach threshold, they
trigger action potentials.–600+20MillivoltsPacemaker potentials
(HCN channels)Voltage-gated
Ca2+
channelsK+ channelsTime