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Refinement of Synaptic Connections
The process of development would appear to be
complete once all neurons are born, differentiate, and
connect with one another. Yet even as the number of
synapses increases in the target region (Chapter 8), a
separate event is set in motion that leads to the elimi-
nation of some existing synapses. For example, thala-
mic projections arborize in the kitten visual cortex
before birth, and a postnatal burst of synaptogenesis
leads to an increase in the number of synapses per
neuron from a few hundred to about 12,000. As new
synapses are added, individual afferent projections
from the thalamus begin to retract some of their
branches from neighboring regions of the cortex
(Cragg, 1975; LeVay et al., 1978). This eventually leads
to a “striped” pattern of innervation that has been
studied intensively. Some afferent connections are
eliminated entirely during development. Turning again
to the kitten visual cortex, it has been shown that com-
missural afferents from the opposite side of the brain
are lost in great numbers during the first three post-
natal months.
Why are synapses being assembled and disbanded
at the same time, particularly when the pathfinding
and mapping mechanisms produce such accurate
results? The central nervous system is a tissue
designed for continuous modification, from birth into
adulthood (cf. learning and memory). The addition
and loss of synapses may reflect a major goal of devel-
opment: that is, to optimize behavioral performance in
a particular environment. One might argue that there
is no better time for learning and optimizing perform-
ance than during development (see Chapter 10). There-
fore, both synapse addition and synapse elimination
can improve the specificity of neural connections. The
correct complement and strength of synaptic connec-
tions should optimize the computational properties of
each neuron.
What distinguishes developmental plasticity from
learning and memory? The clearest difference is that
the developing nervous system is altered permanently
by some manipulations that have little effect on the
adult. As discussed below, there is often a critical
period of development during which synaptic con-
nections or function can be altered by manipulations
to the sensory environment. Experimental manipula-
tions of this sort probably alter the normal amount or
the pattern of synaptic transmission and action poten-
tials, and this altered activity state somehow influences
the growth and differentiation of synaptic connections.
Even though the immature nervous system is particu-
larly sensitive to these manipulations, we will see that
developmental plasticity and adult learning share
several molecular mechanisms.
Three general patterns of innervation distinguish
the developing nervous system from that of the adult.
First, individual axons that arborize in the correct topo-
graphic position (see Chapter 6) may spread out
further than they do in the adult, perhaps a few tens of
microns past their proper boundary (Figure 9.1A).
While this may seem to be a trivial distance, if billions
of neurons make projections of this sort, then neural
computations could be adversely affected. A second
way in which innervation may be immature occurs
when a postsynaptic neuron receives synapses from
more afferents than in the adult (Figure 9.1B). The ratio
of innervating afferent axons per postsynaptic neuron,
called convergence , varies greatly in the nervous system.
At the mammalian nerve-muscle junction there is one
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