Phantom Limb Pain example essay topic

For more than a hundred years, physicians have published accounts of people who perceive an amputated arm or leg as if it were still there. Many amputees feel burning, cramping, or shooting pains in these phantom limbs. Doctors explained this by saying that the patients went through a denial period due to the trauma of loosing a body-part. Experts have discovered in the last decade that the sensations which the amputees have been reporting, is due to a manifestation called Phantom limb phenomena. The neurobiologists have been chasing the phantom. Their first attempts were to figure out were do the sensations originate.

When a limb is amputated, the severed nerves that formerly carried messages of touch, temperature, and pain from the skin, form nodules on their cut ends, called neuromas. For years, the favored explanation was, that the cut nerve endings continue to send impulses up the spinal cord to the brain. This explanation was repealed when the neurosurgeons cut just above the neuromas. The proof was in the fact that the relief was temporary and therefore that the sensation must be emanating from the brain. The discovery, that the sense of touch emanates from our heads rather than our fingertips, leads the researchers onto a new dimension in neuroscience. It is known to us that when the receptors of the neurons are stimulated, an electrical impulse is generated.

It travels through the spinal cord, to the brain stem. This electrical impulse travels further to the relay station thalamus, and from there to the somatosensory cortex. This area has been accurately mapped out. Each area of skin has its lot in the cortex.

The more receptors active in an area of skin, the bigger will be its allotment. With this in mind, scientists outlined the diagram of correlation between the areas of the body and cortex. For example, the hands would have a significantly larger representation, the the back. They constructed a model of a greatly exaggerated person drawn out on the surface of the brain, a homunculus.

The scientists are now challenging the theory of brain plasticity. It has been always believed that the brain is plastic during infancy, but after this critical period of flexible growth, the brain's nueronal circuitry becomes hardwired. The homunculus was believed to be part of that hardwired system. Therefore, once a limb is amputated, the allotted area in the cortex should be forever silent.

Since the 1960's, neuroscientists have been doing experiments which help us understand what help us understand what is happening in the brain of an amputee who is experiencing the phantom limb pain. Two of the most prominent of these experimenters are Michael Merzenitch and Timothy Pons. In 1980's, Michael Merzenich and his colleagues initiated a study in this field. They worked on a series of experiments which produce curious results. In one such experiment, they amputated a finger of an adult monkey. After a few weeks, they read the signals of the monkey's cortex.

The expected results was for the hardwired brain (in the area of the missing finger) to be silent. They were amazed to find that the neurons within the region, fired, each time the adjacent fingers were touched. They explained this by the fact that the remapping was only a millimeter or two, not significant amount because that is the approximate length of a neuronal axon. The explanation (one which would not demolish the "hardwired brain theory") was that the existing, unused branches of the axons were ready in contact across the borders of the cortical regions.

When normal input from one amputated finger ceased, these dormant connections were invading the vacated region, but only as far as an axon could reach. In 1991, Timothy Pons experimented on a group of macaques that, in an unrelated experiment, 12 years earlier, had had the sensory nerves from one arm cut where they entered the spinal cord. Pons attached electrodes in the cortexes of the monkeys, to investigate what has 12 years of dormancy had done to the part of the monkeys homunculus, where the limbs have been unplugged. Expecting to find just a few millimeters of encroachment, he was shocked to find the face region had completely invaded the neighboring cortex. In all four animals that were studied, the entire hand and arm region responded when the face was stimulated. Because the numbed limb was not sending any messages to the cortex, a massive remapping occurred.

Pons explains that in the thalamus, location of the chin and arm lots are next to each other. When the arm stops sending mesages to the cortex, the chin takes over. What is the pu For more than a hundred years, physicians have published accounts of people who perceive an amputated arm or leg as if it were still there. For example, the hands would have a significantly larger representation, then the back.

For years, the favored explanation was, that the cut nerve endings continue to send impulses up the spinal cord to the.