WINSTON-SALEM, N.C. -- The thalamus, the brain's central switching center for relaying sensory information to the brain's somatosensory cortex, "remodels" after sensory nerves are severed, scientists from Wake Forest University School of Medicine and the University of California at Davis report in the Nov. 6 issue of Science.
"This is exciting because it could be the underpinning of the neurobiological basis for recovery of function after stroke or damage to the nervous system," said Tim P. Pons, Ph.D., professor of surgical sciences (neurosurgery) and professor of physiology/pharmacology at Wake Forest.
Pons and Edward C. Jones, Ph.D., director of the Center for Neuroscience at the University of California-Davis, said that a portion of the thalamus in nonhuman primates was completely reorganized after the nerves relaying sensory information from the arms were severed.
The new study follows a 1991 report from the team, also in Science, that the somatosensory cortex itself remodels after injury.
Both reports add to mounting evidence that the brain is not fixed and unchanging after infancy, as had been scientific dogma, but in fact can make new connections. Pons uses the term "plasticity" to describe these changes.
"We had shown that at least one-third of the entire somatosensory cortex is capable of reorganization and this latest work shows that at least one-third of the thalamus is also capable of a similar type of reorganization."
The somatosensory cortex is the brain's processing center for sensory information.
The actual work involved experiments touching the face after the entire upper arm and hand were deprived of their normal inputs. The thalamus had rewired the portions of its surface that previously hooked into the sensory arm and hand nerves so that these portions activated dormant nerves in the face. These nerves were stimulated with a fine glass probe or a camel hair brush.
Pons reported there are measurable -- but tiny -- electrical discharges at that point in the thalamus. Adjacent points on the skin activate adjacent points in the thalamus.
"When the face takes over the hand representation, the brain still interprets the impulses as coming from the hand," he said.
The work was strikingly similar to studies of people who had undergone upper arm amputations. When those investigators touched these amputees in certain parts of the face, the patients had sensations that seemed to be coming from the missing limb. "This is a very plausible explanation for phantom limb sensations and especially phantom pain sensations," Pons said.
"We have identified the thalamus as being a critical component for that plasticity that is exhibited at the cortex. We're not sure that these changes don't also occur at the spinal level," Pons said, "but we doubt it is a factor because of the severe degenerative changes seen anatomically in these animals."
While he said it was premature to speculate about direct applications of the research to people, the researchers will be trying to harness the plasticity of the brain when it helps -- such as after a stroke -- and halt the plasticity when it causes problems, such as epilepsy, Parkinson's disease and Alzheimer's disease.
Materials provided by Wake Forest University Baptist Medical Center. Note: Content may be edited for style and length.
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