Download Neuronal Growth In The Brain May Explain Phantom Limb Syndrome

Neuronal Growth In The Brain May Explain Phantom Limb Syndrome
ScienceDaily (May 2, 2000) — One of the most troubling aftereffects of an arm or leg
amputation is the phantom limb syndrome, in which the person reports receiving
sensations from the lost limb. Neuroscientists at Vanderbilt University report the first
direct evidence that significant growth and reconnection of neurons in the brains of
amputees may be at the root of this problem. The finding may ultimately lead to a
treatment for phantom limb sensation. It also raises the hope that it may become possible
to repair severed spinal cord injuries as scientists find ways to promote and regulate such
growth.
For some time, neuroscientists have known that the phantom limb syndrome and its
close companion, phantom limb pain, are an unpleasant side effect of the brain's attempt
to reorganize itself following a serious disruption in the sensory information that it
receives from the rest of the body. The specific regions of the brain in the cerebral
cortex, thalamus and brainstem that process sensory information from the central
nervous system-called somatosensory regions-are highly organized, and this
organization begins to change after an amputation or major spinal cord injury.
Writing in the April 25 issue of the Proceedings of the National Academy of Science
(PNAS), Assistant Professors of Psychology Neeraj Jain and Sherre L. Florence,
Research Associate Hui-Xin Qi, and Psychology Professor Jon H. Kaas report that
neurons in adult brains of monkeys grow and make new connections in somatosensory
areas when they are massively deprived of sensory input. This strongly suggests that
neuronal growth underlies the brain's reorganization following such injuries, they argue.
"We have suspected for some time that this is the case," says Jain. "But, until recently,
the prevailing view has been that this kind of regenerative growth is unlikely to occur in
adult brains. Hopefully, this new insight will suggest ways to stop or reverse phantom
limb sensations, which tend to become more real over time."
Phantom limb syndrome is the most dramatic and mysterious example of a phenomenon
called neuropathic pain, pain that does not seem to have a physical cause because it is
produced by a malfunctioning nervous system. Neuropathic pain responds poorly to
standard pain treatment and may get worse instead of better over time. For some people,
it becomes a serious disability.
In the PNAS paper, the Vanderbilt researchers report on the results of a series of studies
of the brains of adult monkeys who had sustained spinal cord injuries or had an arm
amputated for therapeutic reasons.
The nerve endings in the hand, arm, face and other parts of the body are connected to the
brain through the spinal cord. Sensory information from each part of the body is
localized in specific areas of the brainstem, thalamus and cortex. These areas show up
much more clearly in the cortex of monkeys than in those of humans because the
monkey cortex is smooth, not highly convoluted like the human cortex. This has allowed
researchers to map these somatosensory areas extensively and they have found that the
areas connected to the face are adjacent to those connected to the hand and arm.
"The human brain is organized in much the same fashion. People who have lost an arm
frequently report that when they are touched on the face they feel as if the sensation
came from the missing limb," Jain says.
To determine how the brains of the monkeys with spinal cord injuries or amputated arms
had changed as a result of their loss, the researchers first injected a tracer compound into
their chins. When their brains were examined, the scientists found evidence for the tracer
not only in the regions of the brain associated with the chin, but also in the areas
associated with the hand and arm.
"This shows that the brain does not stay still, but it reacts to major changes," Jain says.
When the sensory input from part of the body suddenly vanishes, the brain reacts by
reprogramming the area that is no longer serving a useful function. This is a very slow
process, taking months to years. Also, the sensory loss has to be massive to trigger such
changes: the brain has other ways of responding to smaller insults, such as the loss of a
finger, the scientist says.
In order to determine if neuronal growth was involved in the reprogramming process, the
researchers turned to the brainstem, where the somatosensory areas are much more
compact. They hypothesized that even modest neuronal growth in this part of the brain
would have significant consequences.
The researchers found clear evidence that neurons from the face area in the brainstem
had extended axons and made a number of connections in the hand area. Although the
number of such connections was limited there were enough to activate many of the
neurons from the hand area, the researchers found.
"We conclude that the adult primate [central nervous system] is capable of extensive
new growth and that the growth of even a few new connections can have a major impact
on the functional organization of the brain," they conclude.
Questions for Consideration:
1.
Cite the suspected root of phantom limb syndrome.
______________________________________________
______________________________________________
______________________________________________
2. In your own words, define neuropathic pain.
______________________________________________
______________________________________________
______________________________________________
3. How does neuropathic pain respond to typical treatment (i.e. pain killers)?
______________________________________________
______________________________________________
______________________________________________
4. Explain what features of the monkey made it a great candidate for this
study?
______________________________________________
______________________________________________
______________________________________________
5. Describe the process used to prove this hypothesis.
______________________________________________
______________________________________________
______________________________________________
______________________________________________
______________________________________________
______________________________________________
______________________________________________
______________________________________________
______________________________________________
6. Summarize the process of neural reprogramming and its effect on amputees.
___________________________________________________
___________________________________________________
___________________________________________________
___________________________________________________
___________________________________________________
___________________________________________________
___________________________________________________