Biomedical Engineering Reference
In-Depth Information
equations accurately describe an action potential in response to a wide variety of stimu-
lations. Before presenting these equations, it is important to qualitatively understand the
sequence of events that occur during an action potential by using previously described
data and analyses. The start of an action potential begins with a depolarization
above threshold that causes an increase in
Na
þ
current.
G
Na
andresultsinaninward
Na
þ
current causes a further depolarization of the membrane, which then increases
The
Na
þ
current. This continues to drive
Na
þ
.Asshown
the
V
m
to the Nernst potential for
in Figure 12.26,
G
Na
is a function of both time and voltage, which peaks and then falls
to zero.
During the time it takes for
G
Na
to return to zero,
G
K
continues to increase, which hyper-
polarizes the cell membrane and drives
V
m
from
E
Na
toward
E
K
. The increase in
G
K
results
K
þ
current. The
K
þ
current causes further hyperpolarization of the mem-
in an outward
K
þ
current. This continues to drive
brane, which then increases
V
m
to the Nernst potential
K
þ
, which is below resting potential. Figure 12.27 illustrates the changes in
for
V
m
,
G
Na
, and
G
K
during an action potential.
The circuit shown in Figure 12.16 is a useful tool for modeling the cell membrane during
small subthreshold depolarizations. This model assumes that the
Na
þ
currents are
small enough to neglect. As illustrated in Example Problem 12.6, a current pulse sent
through the cell membrane briefly creates a capacitive current, which decays exponentially
and creates an exponentially increasing
K
þ
and
. Once the current pulse is turned off, capacitive
current flows again and exponentially decreases to zero. The leakage current also exponen-
tially decays to zero.
I
l
0.06
V
m
(V)
0.04
G
Na
(S)
0.02
G
K
(S)
0
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.008
0.009
0.01
−
0.02
−
0.04
−
0.06
−
0.08
Time (s)
FIGURE 12.27
V
m
,
G
Na
, and
G
K
during an action potential.
