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down the motor columns and up the sensory columns,
but the commissural interneurons appear only in the
rostral cord and hindbrain. Thus, a neural signal from
a touch anywhere on the skin travels ipsilaterally up
the sensory path, crosses over in the neck region, and
stimulates motor neurons in that region of the other
side, causing a bending of the neck away. The signal
then proceeds down the motor pathway on that side
of the spinal cord involving successively more poste-
rior segments.
Although coiling movements do not propel the
animal forward, the rostrocaudal propagation of a con-
traction along the body is a progression that is seen in
swimming behavior, which develops next. So how
does swimming arise? After the animal is capable of
coiling, the next component of new behavior is the
“S” phase. This arises, as the coiling movements
become faster and alternate from side to side as new
interneurons are added that communicate between the
left and right sides. As the sustained swimming system
develops in fish, more spinal neurons are added to the
circuit such as the CiA interneurons in the spinal cord
of zebrafish. These are an important component of the
early rhythmic swimming circuit and provide all the
ipsilateral glycinergic inhibition. The development of
these cells can now be easily followed as they express
the transcription factor Engrailed-1 (Higashijima et al.,
2004) (Figure 10.5). If a right-hand coiling movement
proceeds only halfway down the body before a left-
hand coiling movement starts at the neck region, the
result is a “S”-shaped wave proceeding caudally, and
the propagation of the animal forward: the beginning
of swimming. Similarly in humans, the precursors to
mature locomotion can be seen long before infants take
their first steps. One can see left-right motor coordina-
tion in the crawling movements that babies make even
at pre-crawling stages, when they are put on a gentle
downhill slope.
Coghill saw many behaviors develop in this inte-
grated way, and one of his other great contributions
was his correlational study of nervous system anatomy
with the behavior: he was able in some cases to attrib-
ute the origins of particular behaviors to newly added
neuronal connections. An example of such a correla-
tion occurs for the Mauthner neurons—very large cells
in the hindbrain of tadpoles and fish. Mauthner
neurons receive auditory input ipsilaterally and
project posteriorly onto contralateral motor neurons.
In the embryonic zebrafish, as early as 40 hours after
fertilization, the Mauthner cells and homologous
neurons in other hindbrain segments can initiate a
directional escape response away from a stimulus fol-
lowed by a series of strong tail flexures. This system is
probably involved in evoked hatching behavior, as the
response is sufficient to rupture the egg membrane and
FIGURE 10.5 The morphology of En1 interneurons from live
transgenic zebrafish expressing GFP under the control of the En1
promoter. A. The region of the spinal cord imaged below is shown
in green. B. A CiA interneuron is shown in green, while in red ret-
rogradely labeled motor neurons are shown. The ventral processes
from the CiA interneuron extend among the motoneurons and
appear to contact their somata. C. Three segmentally successive CiA
internerons are shown, revealing the detail in which single neurons
can be visualized in live fish. (From Higashijima et al., 2004)
allow the animal to emerge. The Mauthner cell some-
times fires spontaneously, which suggests that it might
function also in spontaneous hatching behavior. The
transparency of larval zebrafish has enabled physiolo-
gists to use calcium imaging in the intact fish to
observe the activity of the Mauthner cell during behav-
ior (Figure 10.6). Such work shows that during an
escape, these cells are indeed activated in patterns that
are exactly predicted by behavioral studies (O'Malley
et al., 1996).
In the chick embryo, Hamburger (1963) describes
the early movements as “uncoordinated twisting of the
trunk, jerky flexions, extensions and kicking of the
legs, gaping and later clapping of the beak, eye and
eyelid movements and occasional wing-falling...per-
formed in unpredictable combinations.” The integra-
tion of movements between limbs, such that the left leg
alternates with the right during walking, or the right
and left wings beating synchronously during flight, do
not emerge until later in development. Thus, the
random thrashings and reflexes of individual parts of
the chick embryo, as well as the mammalian embryo,
are brought under control as more circuitry develops.
After the chick hatches, circuits across the midline syn-
chronize the right and left wings so that they beat
together, and if one wing is weighted down so that it
moves more slowly, the contralateral wing will follow
the slower pattern (Provine, 1982). This imposed coor-
dination of elemental movements may be carried out
according to anatomically organized central pattern
generators. Thus, if the brachial cord that drives wing
movements in chick embryo is exchanged with the
lumbrosacral cord that drives leg movements, the
result is a very mixed up chicken in which the wings
flap alternately and the legs hop synchronously
(Figure 10.7) (Straznicky, 1967; Narayanan and Ham-
burger, 1971). The descending pathways from the
brain clearly help integrate movements and bring
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