Biomedical Engineering Reference
In-Depth Information
Na
þ
into the cell at rest. At
K
þ
force due to diffusion is greater than that
that drives
V
m
, the
K
þ
out of the cell. Space charge neutrality requires
due to drift and results in an efflux of
K
þ
out of the cell. Although these flows cancel
out each other and space charge neutrality is maintained, this process cannot continue
unopposed. Otherwise,
Na
þ
be equal to the flow of
that the influx of
K
½
i
Na
½
i
goes to zero as
increases, with subsequent change in
V
m
as predicted by the Goldman equation.
Any change in the concentration gradient of
K
þ
and
Na
þ
is prevented by the
Na-K
pump.
K
þ
into the cell. Removal of
Na
þ
from the cell is against its concentration and electric gradient and is accomplished with
an active pump that consumes metabolic energy. The biochemical reactions that govern the
Na-K
Na
þ
out of the cell and
The pump transports a steady stream of
pump are given in Chapter 8. The
Na-K
pump is also used to maintain cell volume.
Figure 12.10 illustrates an
Na-K
pump along with passive channels.
pump has been found to be electrogenic—that is, there is a net transfer of
charge across the membrane. Nonelectrogenic pumps operate without any net transfer of
charge. For many neurons, the
The
Na-K
Na
þ
ions for every two
Na-K
ion pump removes three
K
þ
ions moved into the cell, which makes
V
m
slightly more negative than predicted with
only passive channels.
In general, when the cell membrane is at rest, the active and passive ion flows are bal-
anced and a permanent potential exists across a membrane only under the following
conditions:
1.
The membrane is impermeable to some ion(s).
2.
An active pump is present.
Na
þ
con-
centrations that are determined by the active pump. Other ion concentrations are deter-
mined by
K
þ
and
The presence of the
Na-K
pump forces
V
m
to a given potential based on the
Cl
moves across the membrane only through passive
V
m
. For instance, since
Cl
concentration ratio at rest is determined from the Nernst equation with
channels, the
E
Cl
¼
V
m
,or
qV
m
KT
Cl
½
i
Cl
½
o
¼
e
ð
12
:
34
Þ
Na-K Ion Pump
K
+
Na
+
Cl
−
K
+
Na
+
Outside
Cl
−
Inside
Na
+
K
+
K
+
Na
+
FIGURE 12.10
An active pump is illustrated along with passive channels.
