Essential Cell Biology 5th edition

Ion Channels and Nerve Cell Signaling
415
membrane
potential (mV)
40
0
-40
closed open inactivated closed
0 1 2
time (milliseconds)
pulse of
electric current
Figure 12–37 Voltage-gated Na + channels
change their conformation during an
action potential. In this example, the
action potential is triggered by a brief pulse
of electric current (arrow), which partially
depolarizes the membrane, as shown in the
plot of membrane potential versus time.
The course of the action potential reflects
the opening and subsequent inactivation
of voltage-gated Na + channels, as shown
(top). Even if restimulated, the plasma
membrane cannot produce a second action
potential until the Na + channels have
returned from the inactivated to the closed
conformation (see Figure 12–36). Until then,
the membrane is resistant, or refractory, to
stimulation.
through these newly opened K + channels—temporarily unhindered by the
negative membrane potential that normally restrains them in the resting
cell. The rapid outflow of K + through the voltage-gated K + channels
brings the membrane back ECB5 to its e12.36/12.37
resting state much more quickly than
could be achieved by K + outflow through the K + leak channels alone.
Once it begins, the self-amplifying depolarization of a small patch of
plasma membrane quickly spreads outward: the Na + flowing in through
open Na + channels begins to depolarize the neighboring region of the
membrane, which then goes through the same self-amplifying cycle. In
this way, an action potential spreads outward as a traveling wave from
the initial site of depolarization, eventually reaching the axon terminals
(Figure 12–38).
QUESTION 12–5
Explain as precisely as you can, but
in no more than 100 words, the ionic
basis of an action potential and how
it is passed along an axon.
Na + CHANNELS
CLOSED INACTIVATED OPEN CLOSED
Na + Na +
axon plasma membrane
+
+
–
– –
+
+
+
+
–
–
+
+ + + –
–
–
+
REPOLARIZED DEPOLARIZED
PROPAGATION
RESTING
–
+
–
+
+ +
–
–
–
–
+
–
time = 0 (action potential triggered)
Na + Na + Na + Na +
+
+
–
– –
+
+
+
+
+
–
+
+
–
+ +
–
–
–
+
+
+
+
Na + CHANNELS
CLOSED INACTIVATED OPEN CLOSED
+
+
+
+
–
– – +
–
–
–
–
+
+ + +
–
+
PROPAGATION
REPOLARIZED
DEPOLARIZED
RESTING
time = 1 millisecond (action potential travels)
Na
–
+ – – +
Na +
+
–
–
+
Figure 12–38 An action potential
propagates along the length of an
axon. The changes in the Na + channels
and the consequent flow of Na + across
the membrane (red arrows) alters the
membrane potential and gives rise to
the traveling action potential, as shown
here and in Movie 12.12. The region of
the axon with a depolarized membrane
is shaded in blue. Note that an action
potential can only travel forward; that
is, away from the site of depolarization.
This is because Na + channel inactivation
in the aftermath of an action potential
prevents the advancing front of
depolarization from spreading backward
(see also Figure 12–37).