Neuronal Signaling and the Structure of the Nervous System
149
Graded Potentials
and Action Potentials
Transient changes in the membrane potential from its rest-
ing level produce electrical signals. Such changes are the most
important way that nerve cells process and transmit informa-
tion. These signals occur in two forms: graded potentials and
action potentials. Graded potentials are important in signaling
over short distances, whereas action potentials are the long-
distance signals of nerve and muscle membranes.
The terms
depolarize, repolarize
,
and
hyperpolarize
are used
to describe the direction of changes in the membrane potential
relative to the resting potential (
Figure 6–14
). The resting
membrane potential, at –70 mV, is polarized. “Polarized” sim-
ply means that the outside and inside of a cell have a different
net charge. The membrane is
depolarized
when its potential
becomes less negative (closer to zero) than the resting level.
Overshoot
refers to a reversal of the membrane potential
polarity—that is, when the inside of a cell becomes positive
relative to the outside. When a membrane potential that has
been depolarized returns toward the resting value, it is
repo-
larizing.
The membrane is
hyperpolarized
when the poten-
tial is more negative than the resting level.
The changes in membrane potential that the neuron
uses as signals occur because of changes in the permeability of
the cell membrane to ions. Recall from Chapter 4 that some
channels in the membrane are gated; that is, opened or closed
by mechanical, electrical, or chemical stimuli. When a neu-
ron receives a chemical signal from a neighboring neuron, for
instance, some channels will open, allowing greater ionic cur-
rent across the membrane. The greater movement of ions down
their concentration gradient alters the membrane potential so
that it is either depolarized or hyperpolarized relative to the
resting state. We will see that particular characteristics of these
gated channels play a role in determining the nature of the elec-
trical signal generated.
Graded Potentials
Graded potentials
are changes in membrane potential that
are confi ned to a relatively small region of the plasma mem-
brane. They are usually produced when some specifi c change
Figure 6–13
Summary of steps establishing the resting membrane potential.
(a) Na
+
/K
+
-ATPase pump establishes concentration gradients and
generates a small negative potential. (b) Greater net movement of
potassium than sodium makes the membrane potential more negative
on the inside. (c) At a steady negative resting membrane potential,
ion fl uxes through the channels and pump balance each other.
2 K
+
K
+
Na
+
Na
+
Na
+
3 Na
+
+
+
Extracellular fluid
Intracellular fluid
ADP
ATP
Na
+
/K
+
-ATPase
pump
K
+
K
+
2 K
+
3 Na
+
+
+
+
+
+
+
+
+
+
Extracellular fluid
Intracellular fluid
ADP
ATP
2 K
+
3 Na
+
+
+
+
+
+
+
+
+
+
Extracellular fluid
Intracellular fluid
ADP
ATP
(a)
(b)
(c)
Membrane potential (mV)
+60
0
–70
–90
Depolarizing
Repolarizing
Hyperpolarizing
Resting potential
Time
Overshoot
Figure 6–14
Depolarizing, repolarizing, hyperpolarizing, and overshoot changes
in membrane potential relative to the resting potential.
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