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a genetic technique that exclusively silences the entire
sensory nervous system from the earliest stages. Such
mutant larvae hatch and crawl in a coordinated
manner in the complete absence of sensory input. The
larvae often crawl backward instead of forward, but
that may be because sensory input is needed to give
directional purpose the movement. Thus, the motor
circuit is assembled correctly and begins to drive
behavior in the absence of such inputs.
that closely mimic their output in an intact animal
that is walking or moving its wings (Figure 10.3).
In both intact animals and isolated embryonic chick
spinal cords, one observes alternating bursts of action
potentials from extensor and flexor motor neurons.
Electrophysiological records from the neurons them-
selves reveal that there are depolarizing synaptic
inputs onto both flexor and extensor motor neurons.
Both neuronal populations begin to depolarize toward
threshold at the same time and begin to fire synchro-
nously. However, at the peak of their depolarization
phase, the flexor motor neurons receive synaptic input
that causes a large shunt conductance, and as a result,
these motor neurons stop firing just when the extensor
motor neurons are firing most rapidly. This helps
explain the mechanisms that lead to oscillations in
flexor and extensor motor patterns.
But how do such bursts of activity first arise?
Removing the dorsal half of the cord, where sensory
axons travel and many interneurons reside, does not
affect the rhythmic motor episodes. The episodes can
also occur in bisected cords, implying that right-left
communication is not essential. Local interneurons in
the venral cord called R-interneurons , or Renshaw cells ,
receive cholinergic input from motor axon collaterals
THE MECHANISM OF
SPONTANEOUS MOVEMENTS
If sensory stimulation is not involved in early motor
behavior, then how are these spontaneous movements
generated? Provine (1972) was the first to appreciate
that the beginnings of electrical activity in the spinal
cord of a chick were connected with the development
of spontaneous behavior. To study this in detail, the
chick embryonic spinal cord can be isolated and the
activity patterns of particular nerve cells can be
recorded (O'Donovan et al., 1998). The motor nerve
roots of such isolated cords produce activity patterns
A
Sartorius
(in ovo)
2 min
5 sec
B
Sartorius
(in vitro)
FIGURE 10.3 In vitro motor development physiology. A. Electromyographic recordings from the sorto-
rius muscle of a chick embryo in ovo. B. A piece of the embryo kept in a culture dish. Very similar sponta-
neous movements begin to occur in these two situtations, although the bursts of activity in the in vitro
preparation are shorter and less frequent. (Adapted from O'Donovan et al., 1998).
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