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A
Refinement of topography
B
Refinement of convergence
C
Refinement of postsynaptic compartment
FIGURE 9.1 Three kinds of immature afferent projections during development. A. The projection of three
afferents to the cortex is shown, and each one centers its arborization at the topographically correct position
in the target. However, one of the arbors initially extends too far, and three local branches are eliminated
during development. B. A single neuron is shown to receive input from three afferents initially, and two of
these inputs are eliminated during development. C. A projection is shown to innervate the soma and den-
drite of a postsynaptic neuron initially, but the dendritic innervation is eliminated during development.
motor axon synapse per muscle cell. In the cerebellum,
each Purkinje cell receives innervation from a single
climbing fiber axon (cf. climbing fiber convergence is
1), but is contacted by thousands of parallel fiber
synapses on its dendritic tree (cf. parallel fiber conver-
gence is ยช200,000). As we shall see, many postsynaptic
neurons attain the adult number of afferents only after
a fraction of the functional contacts are eliminated. A
third way that innervation may become more specific
during development is through the elimination of ter-
minals from one region of the postsynaptic neuron
(Figure 9.1C). In one auditory brainstem nucleus,
inhibitory terminals are eliminated from the dendrite
and gradually become restricted to the cell body.
The addition or elimination of synapses during
development is the most extreme way to modify a
neural circuit. However, the postsynaptic response
magnitude that is produced by an individual synapse
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