369
CHAPTER TEN
Nervous System I
resting potential is quickly reestablished, and it remains in the
resting state until it is stimulated again
(f g. 10.17)
. The active
transport mechanism in the membrane works to maintain the
original concentrations of sodium and potassium ions.
Axons are capable of action potentials, but the cell body
and dendrites are not. An action potential at the trigger zone
causes an electric current to fl
ow a short distance down the
axon, which stimulates the adjacent membrane to reach its
threshold level, triggering another action potential. The sec-
ond action potential causes another electric current to fl ow
farther down the axon. This sequence of events results in a
series of action potentials sequentially occurring all the way
to the end of the axon without decreasing in amplitude, even
if the axon branches. The propagation of action potentials
along an axon is the nerve impulse
(f g. 10.18)
.
A nerve impulse is similar to the muscle impulse men-
tioned in chapter 9, page 290. In the muscle F
ber, stimula-
tion at the motor end plate triggers an impulse to travel over
instant, briefly increasing sodium permeability. Sodium
ions diffuse inward across that part of the cell membrane,
down their concentration gradient, aided by the attraction
of the sodium ions to the negative electrical condition on
the inside of the membrane.
As the sodium ions diffuse inward, the membrane
potential changes from its resting value
(f
g. 10.16
a
)
and
momentarily becomes positive on the inside (still consid-
ered depolarization). At the peak of the action potential, the
membrane potential may reach +30mV (F
g. 10.16
b
).
The voltage-gated sodium channels quickly close, but
at almost the same time, slower voltage-gated potassium
channels open and briefl y increase potassium permeability.
As potassium ions diffuse outward across that part of the
membrane, the inside of the membrane becomes negatively
charged once more. The membrane is thus repolarized (note
in F g. 10.16
c
that it hyperpolarizes for an instant). The voltage-
gated potassium channels then close as well. In this way, the
(a)
Region of depolarization
Threshold
stimulus
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
(b)
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Region of repolarization
(c)
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
K
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
–70
–0
–70
–0
–70
–0
K
+
channels open
Na
+
channels closed
Na
+
channels open
K
+
channels closed
FIGURE 10.16
At rest (
a
), the membrane potential is about –70 millivolts. When the membrane reaches threshold (
b
), voltage-sensitive sodium
channels open, some Na
+
dif
uses inward, and the membrane is depolarized. Soon aFterward (
c
), voltage-sensitive potassium channels open, K
+
dif
uses out, and the membrane is repolarized. (Negative ions not shown.)
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