Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

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624 Chapter 11: Membrane Transport of Small Molecules and the Electrical Properties of Membranes
PROPAGATION
V 1 V 2 V 3
axon
Figure 11–31 The propagation of an action potential along an axon. (A) The
voltages that would be recorded from a set of intracellular electrodes placed at
intervals along the axon. (B) The changes in the Na + channels and the current flows
(curved red arrows) that give rise to a traveling action potential. The region of the
axon with a depolarized membrane is shaded in blue. Note that once an action
potential has started to progress, it has to continue in the same direction, traveling
only away from the site of depolarization, because Na + -channel inactivation
prevents the depolarization from spreading backward.
V 1 V 2 V 3
(A)
0 1 2 3
time (msec)
axon at time = 0 (triggering of action potential)
Na + CHANNELS
CLOSED INACTIVATED OPEN CLOSED
Na + Na +
axon plasma membrane
+ +
– – – –
+
+
+ +
–
–
+ + + + +
–
–
+
REPOLARIZED DEPOLARIZED PROPAGATION
RESTING
–
–
–
+ +
–
–
–
–
–
Na + Na +
axon at time = 1 millisecond
Na + CHANNELS
CLOSED INACTIVATED OPEN CLOSED
Na +
+ +
+
+
+
–
– – – +
+
– – – –
+
+
+ +
+
–
–
–
REPOLARIZED
–
–
+
DEPOLARIZED
+
Na + Na +
+ + + –
+
PROPAGATION
RESTING
–
–
–
–
–
+
+
–
+
+
+
+
+
–
–
– –
+
(B)
Na +
relevant brain region, they could flash light to specifically activate the channelrhodopsin-containing
neurons to fire action potentials. One group of researchers
expressed channelrhodopsin in a subset of mouse neurons thought to be involved
MBoC6 m11.30/11.32
in aggression: when these cells were activated by light, the mouse immediately
attacked anything in its environment—including other mice or even an inflated
rubber glove (Figure 11–32); when the light was switched off, the neurons fell
silent and the mouse’s behavior returned to normal.
Since these pioneering studies, researchers have engineered additional
light-responsive ion channels and transporters, including some that can rapidly
LIGHT OFF
LIGHT ON
LIGHT OFF
Figure 11–32 Optogenetic control of
aggression neurons in a living mouse.
A gene encoding channelrhodopsin was
introduced into a subpopulation of neurons
in the hypothalamus of a mouse. When the
neurons were exposed to flashing blue light
using a tiny, implanted fiber optic cable,
the channelrhodopsin channels opened,
depolarizing and activating the cells.
When the light was switched on, the
mouse immediately became aggressive
and attacked the inflated rubber glove;
when the light was switched off, its
behavior immediately returned to normal
(Movie 11.11). (From D. Lin et al., Nature
470:221–226, 2011. With permission from
Macmillan Publishers Ltd.)