Phantom Limbs: A Neurobiological Explanation

Biology 202
2002 Second Paper
On Serendip

Phantom Limbs: A Neurobiological Explanation

Tara Rajan

Severely injuring a digit or limb can result in unrepentant damage and amputation. However, the painful sensations experienced in regard to that limb do not necessarily cease after amputation. The concept of feeling sensations in a limb that is no longer attached to the body is referred to as feeling a "phantom limb." This phenomenon is experienced by approximately 80%-100% of all patients who have lost a limb (1), and has therefore sparked wide interest in scientific community.
Phantom limb sensations are common for people with missing limbs or digits. In most cases, a phantom arm hangs straight down at the side when the person sits or stands (2). The subject feels as though the limb is there even though he or she is consciously aware that it is no longer attached to the rest of the body. For example, a story by S. Weir Mitchell was written in 1866 in which a civil war soldier undergoes amputation of both legs after a battle. Before the patient knows that his legs have been amputated, he asks the nurse to scratch them for him as he feels discomfort. The nurse and doctor are surprised and dumbfounded since they both are aware that the soldier no longer has these limbs and therefore should not being feeling any pain or discomfort stemming from them (2).
While the story of this soldier is fictitious, similar events occur constantly in patients who suffer the loss of a limb. There are phantom sensations that are not painful, including warmth, tingling, itching, movement, and uncomfortable positioning (1).. However, phantom pains are also common in which the subject feels stabbing, cramping, burning, and shooting pains in the phantom limb (1).
The biology behind phantom limbs is especially important in understanding and trying to treat the phenomenon. Since the senses are prompted by sensory neurons, phantom pains must also be the result of action in sensory neurons. However, if the leg is no longer attached to the body, then how can the neurons in the leg transport signals to the nervous system in order for the body to feel sensations?
One possible explanation is the existence of neurons from the phantom limb even after amputation. Even though the neuron's axons were cut, the neurons could still be present due to regeneration of broken axons. This would allow for an action potential to travel back to the spinal cord even though the leg itself is no longer attached. The sensory neurons are still in tact with the central nervous system and the body can therefore still feel sensations coming from the missing limb (4).
A study was performed by Merzenich in 1986 in which the index finger of a monkey was amputated, and signals were monitored in the corresponding part of the monkey's corticol map (3). Since the monkey's finger was no longer attached to the body, the logical hypothesis is that there would be no signals coming from the finger's area to the nervous system. However, every time the two fingers adjacent to that of the amputated one were touched, there were nerve impulses in the spinal cord. This led the scientists to believe that there are existing axon branches that become unbranched after normal input ends.
Similarly, Pons et al. demonstrated in 1991 that in adult monkeys in which one or more arms had been removed, the representation of the arm could be occupied in other representations of the body (1). In essence, if one part of the somatosensory cortex is deprived of input, the somatosensory cortex will reorganize the inputs so that the idle areas of the cortex become active.
Support of this theory includes a study done by Romachandran in 1992 that studied a subject soon after losing an upper limb and one digit. The subject was lightly touched in varying places to try to localize where phantom pains originated (1). It was found that certain regions, called reference fields, evoked phantoms pains for a specific limb or digit.
Since the senses are actually felt in the thalamus and cerebral cortex, it also makes sense that certain areas also experience more intense phantom sensations than others. For example, the most distal areas are the thumb and index fingers, which have the most extensive representations in the thalamus and cerebral cortex (1). This fact shows that phantoms pains correlate with the firing of sensory neurons to the brain, and those appendages with more complicated representations in the central nervous system mean more intense and more frequent phantom feelings.
Further support for the hypothesis axons exist in areas after normal input ceases is that amputation is not necessarily required for phantom pains. In an accident in which the shoulder is strongly thrown forward and all of the neurons are ripped from the spinal cord, phantom pains are also experienced (2). In this example, the axons of the neurons are torn, but the neurons are still present and can still pass action potentials to the nervous system which lead the brain to believe that it can feel the limb, even though it brain cannot physically move the limb.
Phantoms limb pains are experiences that biology and neurology are together beginning to uncover and understand. Perhaps after finding a definitive explanation for the sensations, subjects experiencing uncomfortable phantom pains will be able to seek some kind of therapy or treatment. The phenomenon is an excellent example of the interesting ways that neurons and the nervous system function, and it continues to question the concepts of knowing and consciousness.

References

1)Biological and Neural Network Models of Phantom Limbs

2)Scientific American: April 1992. Phantom Limbs

3)Macalester University, Pain of the Phantom Limb

4)Macalester University, Monkey Manipulation