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tors on their surfaces so that they respond differen-
tially to the different target cells. These gradients of
ligands and receptors are often the result of very early
patterning events in the embryo, such as those that lay
down rostrocaudal and dorsoventral patterns (see
Chapter 2). Molecular cues, in the form of homophilic
or heterophilic adhesion molecules, especially mole-
cules that can show great diversity, are often involved
in the final cellular and synaptic levels of targeting.
The molecular nature of target recognition leads one to
appreciate how a nervous system can wire up to a
fairly high degree of precision in the absence of func-
tion. As we have also seen, however, the fine-tuning of
neural connections is heavily dependent on synaptic
activity, and before the final connections are made,
synapses are tested. In Chapter 9, we shall learn much
more about the role of neural activity on synapse
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