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Non-deprived eye
( ) occupies all
of layer IV
Responds to non-
deprived eye only
Non-deprived eye
( ) occupies half
of layer IV
Responds equally
to both eyes
Age of Monocular Deprivation (months)
FIGURE 9.22 The effects of monocular deprivation are age-dependent. One eye was sutured shut in
macaque monkeys at different postnatal ages. The sutures were removed after several months, and ocular
dominance columns were examined with anatomical (left) and electrophysiological techniques (right). When
deprivation began between 0 and 2 months of age, most neurons subsequently responded to the open eye
only, and the open eye occupied much more of Layer IV. The effects of deprivation declined with age. The
anatomical effect of deprivation on ocular dominance columns seemed to decline more rapidly than the phys-
iological effect on ocular dominance histograms. (Adapted from LeVay et al., 1980)
Synaptic terminals can influence neighboring con-
tacts on the same postsynaptic cell but only within a
finite distance. When two motor nerves were grafted
onto the same muscle in adult rats, both of them were
able to maintain functional contacts over several
months, even though muscles are normally innervated
by one axon. The trick was to place the two motor ter-
minals at least several millimeters from one another
(Kuffler et al., 1977). If the terminals were placed
within a millimeter or two, then one of the contacts
was eliminated within about three weeks (Figure 9.23).
In fact, some animals have muscle fibers that are nor-
mally innervated by more than one motor axon. In one
polyneuronally innervated muscle in chicks, the dis-
tance between terminals can be reduced when synap-
tic transmission is blocked, presumably because
competition between active terminals normally keeps
them separated (Gordon et al., 1974).
The terminal endings from a single axonal arbor
seem to innervate a continuous region of the post-
synaptic cell, whether it is a primary dendrite or a
small area of muscle cell (Forehand and Purves, 1984;
Glanzman et al., 1991). For example, when sensory
neurons from the sea slug, Aplysia , are grown in
culture along with a common target motor neuron,
their terminals come to occupy separate regions of the
postsynaptic cell. However, if the two sensory neurons
are grown without a target, then they grow extensively
along one another. That is, the sensory neurons do
not display contact inhibition, as discussed above
(Chapter 4). Therefore, the antagonistic relationship
between synapses must somehow be mediated by the
postsynaptic neuron. Even if we know the amount and
pattern of synaptic activity in a set of inputs, this may
not be enough to predict whether one synapse will dis-
lodge a second one. One must also know the spatial
arrangement of these terminals.
Synapses that are capable of function can form in
the absence of synaptic transmission (Cohen, 1972;
Duxson, 1982). Presynaptic cholinergic terminals are
even able to differentiate in a zebrafish mutant lacking
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