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and feedback GABAergic input onto flexor motor
neurons (Wenner and O'Donovan, 2001). In the adult,
this is a negative feedback loop that prevents motor
neurons from firing too much. But in the embryo,
GABA is excitatory (Chapter 8), and any depolariza-
tion leads to massive excitatory activity among all the
connected neurons, which gradually tapers off. Block-
ing both the GABAergic and glutaminergic synapses
in an isolated cord preparation stops the rhythmic
activity (Chub and O'Donovan, 1998). However, such
experiments also show that when the activity is
blocked, excitability gradually increases. This ensures
that in normal spinal cords each quiescent period is
followed by an active period. The rhythmic bursting
pattern is thus an intrinsic property of the developing
network.
Repeated oscillations of
downward moving flexures
A Early behaviors in sequence
Contraction
spreads down
body
Second
flexure
Contraction
here
First
flexure
Non-motile
Early flexure
Coil
“S-phase”
Swimming
B Coil circuit
C Swim circuit
Commissural
neuron
Brain
Brain
Ear
Ear
EMBRYONIC MOVEMENTS:
UN COORDINATED OR INTEGRATE D?
1
Muscle
segments
1
2
2
Commissural
interneurons
3
3
How coordinated is embryonic motor behavior? Do
the episodes of trunk and limb movement represent an
integrated behavior that needs only to be improved, or
are the behaviors essentially spastic and random due
to the immaturity of the circuit? The pioneering work
of Coghill in the 1920s favored the notion that behav-
ior develops in an integrated fashion (Coghill, 1929).
Behaviors build upon each other as the circuitry
matures and new components are added. Coghill
began to study the first movements of the salamander
embryo because, like all amphibian embryos, they
grow from shell-less eggs in water and are accessible
for observation from the earliest stages of develop-
ment. Also, there was an extensive history of embry-
ological and neuranatomical studies on these animals.
By looking at large numbers of such embryos, Coghill
found that slow bending of the head to one side was
the very first movement executed by the salamander
embryo (Figure 10.4). The movement involves the
trunk muscles situated immediately behind the head.
As development proceeds, muscles further and further
down the body become involved so that the “bend,”
which started as a slow movement at the neck region,
becomes a coiling of the entire body.
Careful examination of this fully developed coiling
behavior reveals that movements start at the neck
region and then proceed down the body (Figure 10.4),
such that the sequence of each movement recapitulates
the developmental progression of the movement as
well as the progression of neuronal maturation (e.g.,
first in the hindbrain and then down the spinal cord).
Both bending and coiling can be stimulated by a light
touch of the skin on the side opposite the contraction.
4
4
5
5
Sensory
neurons
Motor
neurons
6
6
Ascending
sensory
pathway
Descending
motor pathway
Left
descending
motor
pathway
Right
descending
motor
pathway
FIGURE 10.4 Coghill's sequence of early amphibian behavior
and its proposed neural basis. A. An axolotl embryo at five stages of
behavioral development. B. The neural circuit for the coil response.
A stimulus anywhere on one side of the body is transmitted to the
contralateral spinal motor pathway by commissural cells in the ante-
rior cord or hindbrain where the neural signal descends, stimulat-
ing primary motorneurons of the cord. C. The early swim circuit (the
sensory mechanism) is omitted but is the same as in B. Motor exci-
tation travels down one side of the cord, but by this stage in devel-
opment, some reciprocally exciting commissural neurons that cross
the floorplate in the hindbrain have developed so that excitation on
one side at the neck region can cross over after a delay to excite the
contralateral motor pathway leading to coiling on one side being
quickly followed by coiling on the other side. (Adapted from
Coghill, 1929).
Coghill's anatomical explanation for this chronology
is that sensory and motor neurons, which innervate
the skin and muscles, are present in prereflexogenic
stages. However, at the time that bending away from
a light touch emerges, a set of interneurons appear to
form the first commissural pathways from sensory
neurons on one side to motor neurons on the other
(Figure 10.4). Longitudinal ipsilateral tracts extend
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