One of the great medical mysteries is the source of phantom-limb syndrome. Doctors have been puzzled for centuries by patients who, after losing an arm or a leg, continue to feel sensation--and even excruciating pain--in the lost limb. For some time, neuroscientists have suspected that the pain was a side effect of the brain's attempts to reorganize itself following a major disruption in sensory input. But researchers at Vanderbilt University in Nashville report the surprising discovery that this reorganization is causing significant growth of new brain cells in amputees.

Their finding, reported in the Proceedings of the National Academy of Sciences, builds on the discovery in the past year that adult mammals can grow new brain cells, an activity that scientists once thought ceased after childhood. If this neuronal growth can be harnessed, cures may be possible not only for phantom-limb pain but for severed spinal cords.

The Vanderbilt researchers based their studies on monkeys that had had an arm amputated. Knowing that the nerves of each body part connect to a specific region in the brain, the researchers injected tracer elements into the animals' chins. When they examined the brains, they found the tracer in brain regions connected to the hand and arm as well as to the face. There was clear evidence, they say, that neurons from the face area of the brain had grown extensive new connections to the hand area. This, they say, may explain why people who lose an arm often report that when they are touched on the face it feels as if the sensation comes from the missing limb.

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