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Rotation
180°
A
Remove
Reimplant
Neural plate
stage embryo
B
Forebrain
Midbrain
Hindbrain
C
Control
D
Rotated hindbrain
E
Axon adjusts
trajectory
Mauthner
neurons
No
rotation
Rotation
180°
FIGURE 5.3 Mauthner cells grow posteriorly in the hindbrain due to local cues. A. At the neural plate
stage, a segment of the hindbrain region of a salamander embryo is removed, rotated 180°, and reimplanted.
B. shows a dorsal view of a larval brain of such an animal. C. The bilaterally symmetric giant Mauthner
neurons in the normal unoperated larval brain. D. The trajectory of Mauthner axons in an experimental
animal in which a segment of the hindbrain containing the Mauthner primordia is rotated. E. Photo of host
and graft Mauthner cell axons in same animal. (After Hibbard, 1965)
city to another address in another city (Figure 5.4). As
he or she pulls out from home on a particular street,
the driver knows which turns to make, which roads to
get on, the signs to follow, what to avoid, which exits
to take, and how to recognize and stop at the correct
destination. Unlike humans, most axons make these
long journeys without errors. Growing axons are able
to recognize various molecules on the surfaces of other
axons and cells, and to use these molecules as cues to
navigate the sometimes circuitous pathways to their
particular destinations. They can also respond to dif-
fusible molecules such as morphogens (see Chapter
2) that percolate through the embryonic brain and
provide cues about overall orientation. They also need
motor abilities, as they must be able to move forward,
make turns, and put on the brakes when they reach
their target. They may also need to integrate informa-
tion, for cues that have a particular significance during
an early phase of their journey may have a different
significance later on in the context of other signals, or
they may need to adapt their responsiveness so that
they remain sensitive as background levels of particu-
lar guidance cues change. These functions, the sensory,
the motor, the integrative and the adaptive, are all con-
tained within the specialized tip of a growing axon,
called the growth cone . That growth cones are capable
of all these functions is demonstrated in experiments
in which growth cones surgically isolated from their
 
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