370
Chapter 12
to open, which results in a fl ow of calcium ions down their
electrochemical gradient into the cell. These channels open
much more slowly than do sodium channels, and, because of
the fact that they remain open for a prolonged period, they
are often referred to as
L-type calcium channels
(L = long-
lasting). The fl
ow of positive calcium ions into the cell just
balances the fl ow of positive potassium ions out of the cell and
keeps the membrane depolarized at the plateau value.
Ultimately, repolarization does occur when the calcium
channels slowly inactivate, and another subtype of potassium
channels opens. These potassium channels are just like the ones
described in neurons and skeletal muscle; they open in response
to depolarization (after a delay), and close once the potassium
current has repolarized the membrane to negative values.
The action potentials of atrial muscle cells are similar in
shape to those just described for ventricular cells, although
the duration of their plateau phase is shorter.
In contrast, there are extremely important differences
between action potentials of cardiac muscle cells and those in
the conducting system.
Figure 12–13a
illustrates the action
potential of a cell from the SA node. Note that the SA node cell
does not have a steady resting potential, but instead undergoes
a slow depolarization. This gradual depolarization is known as
a
pacemaker potential;
it brings the membrane potential to
threshold, at which point an action potential occurs.
Three ion channel mechanisms, which are shown in
Figure 12–13b
, contribute to the pacemaker potential. The
fi rst is a progressive reduction in potassium permeability.
Potassium channels, which opened during the repolarization
phase of the previous action potential, gradually close due to
the membrane’s return to negative potentials. Second, pace-
maker cells have a unique set of channels that, unlike most
voltage-gated channels, open when the membrane potential is
at
negative
values. These nonspecifi c cation channels conduct
mainly an inward, depolarizing sodium current, and because
of their unusual gating behavior have been termed “funny,” or
F-type sodium channels.
(Do not confuse this type of sodium
channel with the one that causes the action potential upstroke
in neurons, skeletal muscle, and cardiac muscle cells.) The third
pacemaker channel is a type of calcium channel that opens only
briefl y but contributes an inward calcium current and an impor-
tant fi nal depolarizing boost to the pacemaker potential. These
channels are called
T-type calcium channels
(T = transient).
Once the pacemaker mechanisms have brought the nodal
cell to threshold, an action potential occurs. The depolarizing
phase is caused by calcium infl
ux through the L-type calcium
channels. After a delay, the opening of potassium channels repo-
larizes the membrane. The return to negative potentials activates
the pacemaker mechanisms once again, and the cycle continues.
Thus, the pacemaker potential provides the SA node
with
automaticity,
the capacity for spontaneous, rhythmical
self-excitation. The slope of the pacemaker potential—that
is, how quickly the membrane potential changes per unit
time—determines how quickly threshold is reached and the
next action potential is elicited. The inherent rate of the SA
node—the rate exhibited in the total absence of any neural or
hormonal input to the node—is approximately 100 depolar-
izations per minute. (We will discuss later why a person’s rest-
ing heart rate is typically slower than that.)
Several other portions of the conducting system are
capable of generating pacemaker potentials, but the inherent
rate of these other areas is slower than that of the SA node,
so they normally are driven to threshold by action potentials
from the SA node and do not manifest their own rhythm.
However, they can do so under certain circumstances and are
then called
ectopic pacemakers,
an example of which is given
in the next paragraph.
Figure 12–13
(a) Membrane potential recording from a cardiac nodal cell. Labels
indicate key ionic movements in each phase. (b) Simultaneously
measured permeabilities through four different ion channels during
the action potential of (a).
Figure 12–13
physiological
inquiry
Conducting cells of the ventricles contain all of the ion channel
types found in both cardiac muscle cells and node cells. Draw a
graph showing a Purkinje cell action potential.
Answer can be found at end of chapter.
10.0
1.0
0.1
Relative membrane permeability
Membrane potential (mV)
0
–50
–100
P
Na
+
(F)
Na
+
enters
Ca
2+
enters
Ca
2+
enters
K
+
exits
P
Ca
2+
(T)
P
K
+
P
Ca
2+
(L)
0
0.15
0.30
Time (s)
0
0.15
0.30
Time (s)
(a)
(b)
Threshold
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