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neurons have already made a functional synapse onto
a muscle, the result is a spontaneous movement.
Reflexive movements, those elicited by sensory stimu-
lation, also emerge early in development. All that is
needed in addition to the above is a synapse between
the sensory neuron and the motor neuron to complete
a reflex arc. A question that has intrigued those who
study the origins of behavior is whether the very first
skeletal movements of an animal are spontaneous and
involve no sensory input, or whether they are reflexive
movements in response to sensory stimulation. Some-
times it is difficult to tell when a movement is truly
spontaneous. For example, leech embryos show move-
ments in their egg cases, and the frequency of these
unprovoked movements increases during the latter
half of embryonic leech development (Reynolds et al.,
1998). This increase corresponds to the time of eye for-
mation, suggesting that the embryos may actually
be responding to light. Indeed, if the embryos are
observed in red light, rather than white light, this
increase in “spontaneous” behavior is not observed. If
the first behaviors are always reflexive, then sensory
input must have a dominant role in establishing
behavior. If, however, truly spontaneous movements
appear first, then the motor system is probably matur-
ing independent of sensory input.
By shining light through chick eggs, candling them
as it is called, it is possible to see the embryo moving
inside of its shell. If one observes the later stages of
development, chick embryos are very active, moving
their wings and legs within their shell (Preyer, 1885).
Careful studies of chick embryos raised in glass dishes,
rather than shells, reveal that totally undisturbed
animals exhibit a variety of behaviors (Figure 10.2).
Neuroanatomical studies done at the same stages
suggest that the earliest of these behaviors occurs prior
to the establishment of any sensory input to the spinal
cord. And indeed, for several days after their first
occurrence, sensory stimulation does not change the
frequency of these behaviors, nor is sensory stimula-
tion able to evoke a motor response at all. One could
argue, however, that there are subtle stimuli to which
the chick can respond, or that the sensory inputs
are themselves firing spontaneously. Perhaps, if all
sensory stimuli were removed, the embryo would not
move. To resolve this issue experimentally, Victor
Hamburger, an extremely insightful developmental
neurobiologist interested in the ontogeny of behavior
(Oppenheim, 2001), surgically deafferented chick
embryos by removing the neural crest cells that give
rise to sensory DRG cells (Hamburger et al., 1966). In
such embryos, which were sensory-deprived ab initio ,
he saw movements that began at the same stage as
in unoperated controls. Moreover, these movements
Remove dorsal
Spinal cord
Leg Activity
FIGURE 10.2 Spontaneous movements are generated in the
absence of sensory input. A. The cross-hatched region of the lumbar
spinal cord of this premotile chicken embryo is shown in cross
section in (B), which shows the operation done in ovo, which
removes the dorsal spinal cord and neural crest containing all the
sensory neurons in this region. C. Activity monitor of average
normal leg movements (line) and leg movements generated by legs
without any sensory input in operated animals (circles). (Adapted
from Hamburger et al., 1966)
were indistinguishable in frequency or quality for
several days (Figure 10.2). These experimental findings
are supported by electrophysiological studies of the
spinal cord which reveal no detectable synaptic input
onto motor neurons when sensory neurons are stimu-
lated during this prereflexogenic period of behavior.
The establishment of motor programming that is inde-
pendent of sensory input is strongly reinforced by
studies in Drosophila (Suster and Bate, 2002), which use
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