Neurons, the living cells that are "wired" by a vast network of branching fibers to form the brain and other parts of the nervous system, don't just sit there like so many components of a computer.

Instead, research has shown, neurons continually rewire their own circuitry, sprouting new fibers that reach out to make contact with new groups of other neurons and withdrawing old fibers from previous contacts.

The research could give clues as to how a damaged brain recovers following a stroke or other damage. This rewiring process may account for how the brain improves one's abilities, such as becoming proficient in a sport or learning to play a musical instrument. Some scientists have suggested that the brain may use this method to store facts.

Many neuroscientists had suspected this behavior by neurons, but had not been able to prove it because they lacked a way to map the fibers from the same neuron at different times.

Now two researchers from Washington University in St. Louis have done just that and discovered that a neuron's fibers can change significantly in a few days or weeks, presumably in response to changing demands on the nervous system. They reported their findings in Nature, the British scientific journal.

The research was on adult mice, but since all mammalian nervous systems appear to behave in similar ways, the researchers assume that the findings also apply to human beings.

The scientists, Dale Purves and Robert D. Hadley, studied nerve fibers called dendrites (from the Greek for "tree") because their heavily branched shape resembles a leafless tree.

Every neuron consists of a central blob, called the cell body, from which two kinds of fibers emerge: dendrites, which receive signals from other neurons, and axons, which send signals to other neurons' dendrites. Typically a neuron has many short dendrites and one long axon, sometimes several inches long or, in the case of the spinal cord, several feet long. Near the far end, axons branch and often touch hundreds of other cells.

Biologists have long known that during embryonic development, dendritic branches grow in complexity, increasing their links to other neurons and thus increasing the capabilities of the nervous system. Biologists have also known that once the nervous system is fully developed -- in early childhood among humans -- neurons stop multiplying.

The same thing happens in mice. Because the new findings were about adult mice, they suggest that even though an adult brain cannot increase its number of cells, it can increase or reroute its internal connections.

Purves and Hadley demonstrated changes in dendrite branching by surgically opening mice to expose a cluster of nerves in the neck and injecting a single cell body with a fluorescent dye that spread into the dendrites, making them show up in a special photographic technique. The mice were stitched up and kept for varying periods before a second operation to reexamine the neuron.

Modest dendrite changes showed up in as little as four days, more extensive changes in a month.