C h a p t e r 1
Cell Structure and Function
19
Resting membrane potential
(RMP).
The RMP, which is neces-
sary for electrical excitability, is pres-
ent when the cell is not transmitting
impulses. Because the resting mem-
brane is permeable to K
+
, it is essen-
tially an K
+
equilibrium potential.
This can be explained in terms of
the large K
+
concentration gradient
(e.g., 140 mEq/L inside and 4 mEq/L
outside), which causes the positively
charged K
+
to diffuse outward, leav-
ing the nondiffusible, negatively
charged intracellular anions (A
–
)
behind. This causes the membrane
to become polarized, with negative
charges aligned along the inside and
positive charges along the outside.
The Na
+
/K
+
membrane pump, which
removes three Na
+
from inside while
returning only two K
+
to the inside,
contributes to the maintenance of
the RMP.
3
Outside cell
Inside cell
Concentration
gradient for K +
3 Na +
2 K +
Na + /K +
pump
K +
A -
K +
Action potentials.
Action poten-
tials involve rapid changes in the
membrane potential. Each action
potential begins with a sudden
change from the negative RMP to a
positive threshold potential, causing
an opening of the membrane chan-
nels for Na
+
(or other ions of the
action potential). Opening of the
Na
+
channels allows large amounts
of the positively charged Na
+
ions
to diffuse to the interior of the cell,
causing the membrane potential to
undergo depolarization or a rapid
change to positive on the inside and
negative on the outside. This is rap-
idly followed by closing of Na
+
chan-
nels and opening of the K
+
channels,
which leads to a rapid efflux of K
+
from the cell and reestablishment of
the RMP.
4
Outside cell
Inside cell
Triggering event opening
Na + channels
Na +
Na +
Na +
Na +
Na +
