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cellular function. Many very adaptive larval behaviors
are lost at this transition while new behaviors are
gained. As would be expected, there are also dramatic
changes in nervous system structure. In both insects
and amphibia, larval neurons die upon exposure to
metamorphic hormone, and some neuroblasts that
have been quiescent throughout larval life begin to
proliferate. Some larval neurons survive the transition
to adult but are drastically reorganized. For example,
the motor neurons that move the abdominal prolegs
of the caterpillar do not die, even though these
appendages are lost, but their axons and dendrites are
remodeled, and old synapses are eliminated to support
new behaviors (Figure 10.11).
In metamorphic insects, there are important transi-
tional behaviors associated with building and emerg-
ing from the pupal state. In moths, the adult motor
system is constructed primarily from remodeled larval
components, whereas the adult sensory system is pri-
marily composed of new neurons. Simple reflexes cor-
relate these neuronal changes with the acquisition or
loss of particular behaviors. The loss of the larval
proleg retraction reflex is associated with the loss of the
dendrites of the proleg motor neurons; the adult stretch
receptor reflex begins when new adult-specific connec-
tions are added to new dendritic growth in an adult
neuron (Levine and Weeks, 1990). Although humans
do not go through metamorphosis in the same way as
flies and frogs, the distinct behaviors of babies and
adults must be largely due to changes in the nervous
system that result not only from experience, but also
from a variety of intrinsic influences, such as hormones
and growth factors that are regulated throughout life.
A6
A
A6 proleg
B
Juvenile horm one
Ecdysteroids
Ecd
JH
4th larva
E
5th larva
E
Pupa
C
AG ganglia
L3
P0
Note dendrite regression
of proleg motorneuron
D
L3
P0
Stimulation of input nerve
Note decrease in EPSP
FIGURE 10.11 Changes in a single neuron during metamor-
phosis. A. The caterpillar of the moth ( Maduca sexta ) showing
abdominal segment 6 and the proleg associated with this segment.
To the right is shown the pupal stage. B. Profile of hormonal changes
through larval life and the transition between the larval and pupal
development where ecdysone rises without an increase in juvenile
hormone. C. Remodeling of the A6 proleg motoneuron during meta-
morphoses. There is a dramatic pruning of the dendritic tree. D. Cor-
related with this dendritic remodeling, there is a decrease in the
activation of this neuron when the sensory input nerve was stimu-
lated. These changes reflect the changing role of this neuron in post-
metamorphic life where there are no prolegs. (Adapted from
Streichert and Weeks, 1995)
BEGINNING TO MAKE SENSE
OF THE WORLD
The nervous system becomes active well before
animals experience the world around them or move
about within it. In fact, this “spontaneous” activity
may initially be necessary for the survival and matu-
ration of synapses (Chapter 9). At some point, the
nervous system begins to sense the world, and some
of the information is of immediate use. Although
neonatal mammals are helpless in many ways, their
survival usually depends on perceptual abilities that
lead them toward their mother's nipples and motor
skills that allow them to ingest the milk. Many animals
begin to hear, smell, see, taste, and feel well before the
sensory epithelia (e.g., hair cells, photoreceptors, etc.)
and central nervous system connections are mature.
Therefore, it is important to understand the relation-
ship between neural tissue development and percep-
tion. How is visual detection limited by neonatal
retinal ganglion cell physiology and morphology?
Why do infants rarely enjoy espresso or extra hot
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