FOCUS
Resting Membrane Potential
Figure 11.8
Generating a resting membrane potential depends
on (1) differences in K
+
and Na
+
concentrations inside and outside
cells, and (2) differences in permeability of the plasma membrane
to these ions.
Finally, let’s add a pump to compensate for leaking ions.
K
+
loss through abundant leakage channels
establishes a negative membrane potential.
K
+
flows
down its large concentration gradient because the
membrane is highly permeable to K
+
. As the positive K
+
ions leak out, the negative voltage that develops on the
membrane interior counteracts the concentration gradient,
pulling K
+
back into the cell. At -90 mV, the concentration
and electrical gradients for K
+
are balanced.
Na
+
entry through leakage channels reduces the
negative membrane potential slightly.
Na
+
flows down
its large concentration gradient, but the membrane is only
slightly permeable to Na
+
. As a result, Na
+
entering the
cell makes the membrane potential slightly less negative
than if there were only K
+
channels.
Na
+
-K
+
pumps maintain the concentration gradients,
resulting in the resting membrane potential.
A cell at
rest is like a leaky boat that is constantly leaking K
+
out
and Na
+
in through open channels. The “bailing pump” for
this boat is the
Na
+
-K
+
pump
, which counteracts the leaks
by transporting Na
+
out and K
+
in.
Suppose a cell has only K
+
channels.
..
Now, let’s add some Na
+
channels to our cell.
..
The permeabilities of Na
+
and K
+
across the membrane
are different.
In the next three panels, we will build the
resting membrane potential step by step.
The concentrations of Na
+
and K
+
on each side of the
membrane are different.
Na
+
-K
+
pumps maintain
the concentration
gradients of Na
+
and K
+
across the membrane.
The Na
+
concentration is
higher outside the cell.
The K
+
concentration is
higher inside the cell.
Na
+
(140 m
M
)
Na
+
(15 m
M
)
K
+
(5 m
M
)
K
+
(140 m
M
)
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
K
+
K
+
K
+
K
+
K
+
leakage channels
Cell interior
–90 mV
Cell interior
–70 mV
Cell interior
–70 mV
K
+
Na
+
Na
+
-K
+
pump
K
+
K
+
K
+
K
+
Na
+
K
+
K
+
K
+
Na
+
K
+
K
+
Na
+
K
+
K
+
Outside cell
Inside cell
Available at www.masteringaandp.com
 
398
previous page 432 Human Anatomy and Physiology (9th ed ) 2012 read online next page 434 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off