148
Chapter 6
the opposite direction through membrane channels down their
concentration and/or electrical gradients (described collectively
in Chapter 4 as the
electrochemical gradient
). As long as the
concentration gradients remain stable and the ion permeabilities
of the plasma membrane do not change, the electrical potential
across the resting membrane will also remain constant.
Thus far, we have described the membrane potential
as due purely and directly to the passive movement of ions
down their electrochemical gradients, with the concentration
gradients maintained by membrane pumps. However
, the
Na
+
/K
+
-ATPase pump not only maintains the concentration
gradients for these ions, but also helps to establish the mem-
brane potential more directly. The Na
+
/K
+
-ATPase pumps
actually move three sodium ions out of the cell for every two
potassium ions that they bring in. This unequal transport of
positive ions makes the inside of the cell more negative than it
would be from ion diffusion alone. When a pump moves net
charge across the membrane and contributes directly to the
membrane potential, it is known as an
electrogenic pump.
In
mo
s
t
ce
l
l
s
,
the
e
lec
trogen
ic
con
tr
ibu
t
ion
to
the
membrane potential is quite small. It must be reemphasized,
however, that even though the electrogenic contribution of
the Na
+
/K
+
-ATPase pump is small, the pump always makes
an essential
indirect
contribution to the membrane potential
because it maintains the concentration gradients down which
the ions diffuse to produce most of the charge separation that
makes up the potential.
Figure 6–13
summarizes in three conceptual steps how
a resting membrane potential develops. First, the action of the
Na
+
/K
+
-ATPase pump sets up the concentration gradients
for sodium and potassium (
Figure 6–13a
). These concentra-
tion gradients determine the equilibrium potentials for the
two ions—that is, the value to which each ion would bring
the membrane potential if it were the only permeating ion.
Simultaneously, the pump has a small electrogenic effect on the
membrane due to the fact that three sodium ions are pumped
out for every two potassium ions pumped in. The next step
shows that initially there is a greater fl ux of potassium out of the
cell than sodium into the cell (
Figure 6–13b
). This is because
in a resting membrane there are a greater number of open potas-
sium channels than there are sodium channels. Because there is
greater net effl
ux than infl
ux of positive ions during this step,
a signifi cant negative membrane potential develops, with the
value approaching that of the potassium equilibrium potential.
In the steady-state resting neuron, the fl ux of ions across the
membrane reaches a dynamic balance (
Figure 6–13c
). Because
the membrane potential is not equal to the equilibrium poten-
tial for either ion, there is a small but steady leak of sodium into
the cell and potassium out of the cell. The concentration gradi-
ents do not dissipate over time, however, because ion movement
by the Na
+
/K
+
-ATPase pump exactly balances the rate at which
the ions leak through open channels.
Now let’s return to the behavior of chloride ions in
this system. The plasma membranes of many cells also have
chloride channels but do not contain chloride-ion pumps.
Therefore, in these cells chloride concentrations simply shift
until the equilibrium potential for chloride is equal to the rest-
ing membrane potential. In other words, the negative mem-
brane potential determined by sodium and potassium moves
chloride out of the cell, and the chloride concentration inside
the cell becomes lower than that outside. This concentration
gradient produces a diffusion of chloride back into the cell
that exactly opposes the movement out because of the electri-
cal potential.
In contrast, some cells have a nonelectrogenic active
transport system that moves chloride out of the cell, generat-
ing a strong concentration gradient. In these cells, the mem-
brane potential is not at the chloride equilibrium potential,
and net chloride diffusion into the cell contributes to the
excess negative charge inside the cell; that is, net chloride dif-
fusion makes the
membrane potential more negative than it
would otherwise be.
We noted earlier that most of the negative charge in
neurons is accounted for not by chloride ions but by negatively
charged organic molecules, such as proteins and phosphate
compounds. Unlike chloride, however, these molecules do not
readily cross the plasma membrane. Instead they remain inside
the cell, where their charge contributes to the total negative
charge within the cell.
K
+
Na
+
– 70mV
– 70
– 90
0
+ 60
E
Na
E
K
V
m
at rest
Voltage (mV)
Concentration
gradient
Electrical
gradient
K
EY
(a)
(b)
Extracellular fluid
K
+
Na
+
Figure 6–12
Forces infl uencing sodium and potassium ions at the resting
membrane potential. (a) At a resting membrane potential of –70
mV both the concentration and electrical gradients favor inward
movement of sodium, while the potassium concentration and
electrical gradients are in opposite directions. (b) The greater
permeability and movement of potassium maintains the resting
membrane potential at a value near E
K
.
Figure 6–12
physiological
inquiry
Would lowering a neuron’s intracellular [K
+
] by 1 mM have
the same effect on resting membrane potential as raising the
extracellular fl
uid [K
+
] by 1 mM?
Answer can be found at end of chapter.
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