pain centers? (Phantom Limbs)

falvarez at desire.wright.edu falvarez at desire.wright.edu
Thu Mar 23 14:17:26 EST 1995


I already discussed this two days ago, but probably a repetition is needed in 
view of the follow up on this issue:

When a limb is amputated the sensory neurons *are not* destroyed. Their cell 
bodies are in the dorsal root ganglia and start to react to the injury. Only 
their normal peripheral terminations have dissapeared and now the peripheral 
branch of the dorsal root ganglion neuron ends in a neuroma. This is the 
cut end of the nerve and contains all the peripheral branches ending now in 
an abnormal fashion. However their central connections still map to the 
central representation of the missing limb. One thing neuromas are well 
known for is producing an unregulated tonic discharge of APs in the fibers 
terminating in them. So the spinal cord *does* receive a signal and this is 
very abnormal and unregulated and can produce all short of long term changes 
in the spinal cord network and also in the networks of upper brain relay 
centers. Much of this mechanisms are as yet not exactly known, but clinically 
manipulations of the stump (e.g. with anesthetics, ion channel blockers....) 
or of the peripheral sensory fibres relieve phantom limb pain. In 
other occassions the process has already gone very far centrally and the 
central networks spinal and/or supraspinal are sensitized to the extent that 
they also contribute to form the phantom limb sensation even with little 
peripheral input. Similar central sensitization mechanisms also occur and 
contribute to other hyperalgesic states where an abnormal peripheral input 
occurs (e.g. peripheral neuropathies, immflamation....) Much is still under 
research. If you are  interested in the phantom limb phenomena I suggest this 
reading:

 By the way, not al phantom limbs are necessarily painfull.

Phantom limb and other phenomena after amputation
by T.S. Jensen and P. Rasmussen
In Textbook of Pain, 1994 (ed Wall and Melzack) Chapter 36 pp 651-666

Francisco J. Alvarez
Wright State University






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