Biology 202
2002 Second Paper
On Serendip

It has been found that it is not necessary to have had an amputation to experience the phantom limb phenomenon. It has been reported that after avulsion of the brachial plexus of the arm, even though no injury to the arm itself occurred, that there is extreme pain felt in the arm. For surgery, patients that receive an anesthetic block of the brachial plexus experience phantom arm. It also occurs in the legs when there is an anesthetic block of the lower body, and when there is a block of the spinal cord at the thoracic level, patients have experience phantom body. (2).

Not only can one experience phantom limb due to an anesthetic block, but it has been reported that people born without limbs also experience it as well. Ronald Melzack, psychologist from McGill University in Montreal found that children born without arms or legs experience phantom limbs. He studied 125 people, mainly teenagers, who had been born with a limb below the elbow. Melzack found that the phantom limb occurred by rain or by dreams where it was thought that there were two normal hands. An 11 year old girl reported that she felt pain in her fingers (she had no arm below the elbow) if she bumped her funny bone. Others have said that they felt tingling, itching or even numbness in arms or legs that they did not have. (3).This study concur the findings of a Swiss group who had a woman with no limbs that still felt movement when they stimulated parts of her brain that usually sense limb movement magnetically. (4).

Learning that people who were born without limbs can experience phantom limbs have made scientists reevaluate the theories they have about how the nervous system develops. The Swiss group showed through experience performed on this woman that her brain still has a sensory map for the limbs that she does not have. If was believed before that if a person did not have an arm that they did not develop the neurons to feel an arm. It is believed that for amputees that the neurons that used to receive feelings from the limbs are still firing which is the cause of phantom limbs. But with this woman, magnetic resonance imaging showed neuronal activity in parts of the brain, when she said she was moving her phantom limb. (4).

Phantom limb occurs not only in those who lose a limb or were never born with one, it can occur in people who are paraplegic that have a complete break in the spinal cord. Even though no signals can get through the break in the spinal cord, patients still report feeling in their legs and lower body. (2).

For years, scientists have been trying to figure out why phantom limbs occur. In the 1930s, it was discovered that the surface of the cerebral cortex had a map of the human body called the homunculus. The sensory stimuli are close to the cortex of the brain so that the primary sensory regions are excited by stimuli. This occurrence is that scientists refer to as a map. In the recent years, it has been found that this cortical map is very plastic (the ability for neurons to adapt to change) in humans and in animals. There is one theory that believes that if part of the somatosensory cortex has no input then the map will reorganize itself so that the unaffected part of the cortex represents a different part of the body surface. This idea is known as cortical reorganization. There is a big debate within the scientific world on whether or not the cause of the phantom limbs is due to cortical or non-cortical contributions. (1).

For the cortical model side of the debate, it is thought that the cortex has the ability to reorganize itself so that the signals received from residual limbs are perceived to be coming from the lost limb. To support this theory, Ramachandran studied the localization of touch sensations on the body post amputation. He found that when he lightly touched certain parts of the body, a sensation was felt in the phantom limb. He found that there were two specific regions (the face and a line above the amputation) that evoked sensations in the phantom limb. Ramachandran theorized that in the homunculus, the area for the hand is surround on both sides by areas for the face and the line of amputation. (1).

Although some believe that cortical model is correct, many believe in the non-cortical model. It is thought that perhaps it is peripheral signals or signals at the level of the spinal cord are the cause of phantom limbs. Katz believed that spontaneous input from the cortex increased the speed of firing of the post-ganglionic sympathetic fibers of the spinal cord. The increased firing causes these fibers to be excited which activates the primary afferent fibers which then discharges. These discharges ultimately cause the phantom sensations. Non-cortical theory believes that spontaneous activity at the edge of the amputation causes phantom limbs. (1).

There is still a debate going on as to why phantom limbs occur. Only through further study of the nervous system can we hope to fully understand and hopefully one day stop its occurrence. Until then, we can only try to find ways to ease the pain of amputees, paraplegics and people who were born without limbs.

References

2) http://cognet.mit.edu/MITECS/Entry/melzack

3) http://www.findarticles.com/cf_0/m1511/n2_v19/20159526/p1/article.jhtml

4) http://www.sciencenet.org.uk/slup/CuttingEdge/Jun00/phantom.html


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