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behaviors are the ones done almost a century ago by
Harrison (1904). He raised some salamander embryos
in an anesthetic solution throughout the period of
bending, coiling, “S” movements, and early and sus-
tained swimming. He then transferred the embryos to
anesthetic-free solution. He found that the long-term
anesthetized embryos were able to begin to swim as
soon as the anesthetic was washed out. In a few
minutes, they were behaviorally indistinguishable
from the controls.
More recently, these types of experiments have been
done with other drugs that block synaptic transmis-
sion or action potentials in combination with more
quantitative behavioral measurements (Haverkamp
and Oppenhein, 1986; Haverkamp 1986). The results
are essentially the same (Figure 10.8). For the devel-
opment of coordinated movements of the limbs in
chicks or swimming movements in amphibians, activ-
ity in the nervous is not crucial. In other words, the
earliest movements that an animal makes are not nec-
essary stepping-stones to the development of at least
some simple behaviors. Thus, it seems that many early
motor patterns are determined by the molecular
signals that direct the formation of neural connections
(Chapters 5 and 6).
STAGE-SPECIFIC BEHAVIORS
If early motor behaviors serve no particular purpose
in the building of the nervous system, then one might
expect to see many such behaviors only in the embryo.
Indeed, such embryonic specific behaviors are seen in
the leech (Reynolds et al., 1998). For example, one
behavior is called lateral ridge formation and is the result
of the contraction of dorsoventral “flattener” muscle
at a time in development when the embryo is still
essentially a germinal plate. The contraction of these
muscles lifts the boundary of the future dorsal and
ventral territories. Another embryo-specific behavior
is called circumferential indentation and occurs when an
embryonic leech is prodded on one side (Figure 10.9).
An adult leech, when presented with a similar stimu-
lus, will usually exhibit a local bend away from the
stimulus-contracting muscles on one side and relaxing
those one the other. However, the embryo excites all
the muscles in those segments causing a circumferen-
tial contraction. It is likely that circumferential inden-
tation is a behavior that simply occurs at an incomplete
stage of neural circuit formation.
A
B
A
B
Normal
Anesthetic Reared
Stage 17
Stage 17
Prod
Prod
Circumferential indentation in
a leech embryo in response
to prod
Local bend away from prod
in mature leech
Stage 35
Stage 35
C
100
Local Bend
Wash out anesthetic
Circumferential
indentation
Record from ventral roots
Record from ventral roots
50
0
Normal Swimming
Behavior
Normal Swimming
Behavior
40
50
60
% Embryonic Development
70
80
90
100
FIGURE 10.8 Swimming out of anesthesia. A. A set of normal
Xenopus embryos put in a culture dish at the late neural plate stage
and raised till the swimming stage in normal pond water. They swim
normally, and electrophysiological records from ventral roots on
opposite sides of the spinal cord show the expected alternating
pattern of activity. B. A set of sister embryos raised in an anesthetic
solution until stage 35, at which point some the embryos were imme-
diately put into a recording chamber. The pattern of activity as
the swimming behavior looks essentially normal. (Adapted from
Haverkamp, 1986)
FIGURE 10.9 A transient embryo-specific behavior in a leech
gets replaced. A. At 50% of embryonic development, most prods to
one side of the mid-body of a leech embryo result in circumferential
indentation of the body due to the contraction of muscles all around
the body at the level of the poke. B. At 80% of development, a similar
prod leads to local bending away from the prod, resulting from dif-
ferential contraction of muscles on one side of the body versus the
other. C. Graphic illustration of the transitory embryonic behavior
with the more mature behavior. (Adapted from Reynolds et al., 1988)
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