A scientific principle that has brought together some diverse findings in neuroscience is “parsimony in nature.” Simply put, this is the idea that natural processes or systems first observed in one context, for which they appear beautifully fitted, tend to turn up again in other contexts, sometimes filling other functions—to which they seem equally well suited. The neurotransmitters illustrate this notion: for instance, norepinephrine dilates the blood vessels in muscle tissue but causes the opposite effect, constriction, in the blood vessels of the skin. Thus it appears that nature has conserved the one transmitter and thriftily made it over to another use by having it interact with different receptor sites in the two contexts. Another striking instance of this principle is found in certain physical changes in the nervous system that accompany learning in very simple animals. In the marine snail Aplysia, a newly learned behavior is associated with the growth of signal-transmitting elements from its nerve cells. At the time of this new growth, the levels of cell-adhesion proteins drop briefly but significantly. This change in levels suggests to researchers that the proteins may actually serve to inhibit such extra growth much of the time, when learning is not taking place in the animal. By contrast, outside the brain, cell-adhesion proteins are much better known for their crucial role in the immune system, where they aid in the attachment of disease-fighting antibodies. In yet another context, cell-adhesion proteins may play a still different role in the development of a baby's brain, by guiding the migration of nerve cells to the six-layered cerebral cortex that covers most of the brain like the bark of a tree.

“When neuroscience works well, it begins to unify data,” says Dominick Purpura. Such indeed was the effect of a recent feat in the research world: the mapping of the precise connections in the basal ganglia, deep inside the brain. In primates, this region was known in relatively little detail until recently. The basal ganglia are important for the control of movement, for which they receive signals from the cerebral cortex; electrical recordings show both these areas to be active a fraction of a second before a movement takes place. To map the pathways of these nerve signals has called for a solid foundation of anatomy, highly refined techniques for the selective staining of particular cells, and close studies of signal-carrying agents such as



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