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What goes on behind drawn curtains in the tight community of the brain? Consider the lacy, branching appendages of nerve cells known as dendrites. Dendrites have long had a reputation as solid but dull citizens of the neural metropolis: bean counters who passively add up the information they receive through the synapses dotting their surface. But it isn't easy to study dendrites directly, and some researchers have suspected that, like what goes on in the banker's basement at midnight, the secret lives of dendrites are more complex and interesting than their public image suggests.
Recent results indicate that those suspicions may be right. In the past few years, neurophysiologists, aided by new techniques that allow direct examination of dendrites, have begun to penetrate the mystery, producing some of the clearest insights yet into how dendrites work. Their studies, two of which appear in this issue of Science, mark the beginning of a "new era in dendritic biology," according to Columbia University neuroscientist Eric Kandel.
In that new era, researchers are confirming long-held suspicions that dendrites, far from being bean counters, play an active role in shaping the life of the neuron. Their finely branched network acts as a two-way highway, not only conveying incoming messages to the cell body, but also relaying information from the cell body back to their own outer reaches--information that may modify their responses to future signals. This ability of the dendrites to react and adjust their activity is likely to play an important role in such mental processes as learning and memory. The work has "really put the spotlight on the dendrites," says neuroscientist and neural modeler Terrence Sejnowski of the Salk Institute.
The "bean-counter" image of dendrites stems from the fact that their job as information receivers requires that they pass along to the cell body the synaptic potentials, the little dollops of charge that enter the dendrites when neurotransmitter molecules activate synapses. During the journey to the cell body, each synaptic potential adds up with all the others moving through the dendrites. If their sum is large enough when they reach the cell body, they trigger an action potential by causing sodium ions to flow into the cell. The action potential then sweeps down the axon, the part...