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experiments show that synaptic transmission can
influence the process of synapse elimination at the
NMJ. In fact, plasticity at the NMJ is a lifelong matter.
Adult motor terminals will sprout to innervate adja-
cent muscle cells when neuromuscular transmission is
blocked, and this polyneuronal innervation can be
reduced when the muscle cells are stimulated directly
(Jansen et al., 1973; Holland and Brown, 1980).
layer IV
If synapse strength and elimination are influenced
by neural activity, then we would expect sensory
coding properties (see BOX: Watching Neurons Think)
to be altered when the pattern of environmental stim-
ulation is manipulated. In fact, there are many exam-
ples of animals being reared in an altered sensory
environment, and an influence on neuronal function is
often found. Even when the sensory environment is
well defined, it is practically impossible to figure out
how a stimulus will affect the neural activity pattern
produced throughout the nervous system. Neverthe-
less, this issue has been addressed successfully by two
experiments in the central auditory system, one using
sound stimulation and the other using electrical stim-
ulation of the cochlea. Central auditory neurons
usually respond to a limited range of frequencies
because the auditory nerve fibers from the cochlea
project topographically in the central nervous system
(cf. tonotopy). A range of sound frequencies plotted
against the sound intensities at which each frequency
evokes a threshold response from a neuron, called a
frequency tuning curve , provides a good measure of
afferent innervation. When many areas of the cochlea
project to a central neuron, then its frequency tuning
curve is broad. When only a small region of the cochlea
projects to a neuron, then its frequency tuning curve
is narrow.
To test whether the timing of neural activity influ-
ences the development of frequency tuning, mice were
reared in a sound environment consisting of repetitive
clicks for a few weeks (Sanes and Constantine-Paton,
1985b). This type of sound evokes synchronous activ-
ity in a large population of cochlear nerve axons
(Figure 9.16A and B). When frequency tuning curves
were obtained from the inferior colliculus of normal
mice and compared to those reared in repetitive clicks,
the latter group had significantly broader curves
(Figure 9.16C). A similar experiment has been per-
formed in rats using pulses of noise, and the normal
layer IV
FIGURE 9.13 Visual experience influences intrinsic cortical pro-
jections. A. When dye injections were made into superficial layers of
the visual cortex of normal cats, the label was retrogradely trans-
ported by neurons in both ocular dominance columns. B. In cats
reared with artificial strabismus, the label was retrogradely trans-
ported only by neurons that shared the same ocular dominance.
(Adapted from Löwel and Singer, 1992)
obtained in the developing cat visual pathway, where
TTX blocks the segregation of retinal afferent in eye-
specific layers of the LGN, and also the segregation of
LGN afferents in the cortex. To test whether the tem-
poral pattern of activity is important (as suggested by
the strabismus results), two sets of motor axons inner-
vating the same muscle were stimulated stimulated in
synchrony (Busetto et al., 2000). This manipulation
preserved polyneuronal innervation of muscle fibers.
If too many synapses remain when activity is
blocked or synchronized, then one might predict that
unsynchronized postsynaptic activity could speed up
the process of synapse elimination. In fact, direct elec-
trical stimulation of the muscle induces the early loss
of motor synapses (Figure 9.15). This result is particu-
larly intriguing because it suggests that postsynaptic
electrical activity can determine whether presynaptic
terminals survive (O'Brien et al., 1978). Together, these
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