The Neural Correlation Behind Phantom Limb Pain
The psychological and physiological ailments that phantom limb pain causes is undeniable; however, the question of why it occurs connects to the intricate processes of the brain. The brain receives information about external happenings thanks to sensory neurons. These sensory neurons fire information to the brain through electrical impulses with the occurrence of normal stimulus. Sensory neurons thus cause motor neurons to be triggered as a reaction to present stimuli. These motor neurons relay information from the brain through similar electrical impulses back to the source of the stimulus. With normal stimulus, this can be an acute reaction. However, with deep neuron damage as presented with phantom limb pain, further reactions are met due to the partnership of sensory and motor neurons in the brain.
When damage occurs to a specific area such as with the amputation of a limb, the neurons at the site are inevitably damaged. These neurons are described as end neurons, and the damage to this area causes the constant firing of sensory neurons to the brain, triggering the engagement of motor neurons, as similar with presentation of normal stimulus. They rush to the source of the neuron damage, causing pain. However, due to amputation, the origins of these neurons are inevitably cut off. Theoretically, there should be no sensory and motor neuron engagement; however this is not the case. In a study conducted in 1986, a scientist named Merzenich amputated a test monkey’s finger, creating neuronal damage. In order to monitor the neuron activity of the severed finger of the test monkey, Merzenich utilized cortical maps. These maps demonstrate the relationships between neuron activity and external stimulus at the site. Through monitoring the cortical maps of the monkey, it was assumed that no activity would occur from the amputated fingers, however when neighboring fingers were stimulated in addition, there was presence of neural impulses. This experiment shows that to some extent, the sensory and motor neurons were still engaged despite the neural damage done.
For those with phantom limb pain, it becomes challenging to distinguish their amputated limbs from their intact limbs. The physiological effect of phantom pain seems to be overall correlated to the disrupted activity in the sensorimotor cortex. Due to the trauma that amputees have experienced, phantom limb pain causes reorganization in the brain. Reorganization occurs in a region of the brain called the somatosensory cortex, and there has been research pointing to additional reorganization of the larger primary motor cortex as well. The muscles at the site of amputation present a greater amount of activation of the motor neuron pool than that of the intact side. There is also an association with increased levels of phantom limb pain and the magnitude of cortical reorganization. From the results of a fMRI study done to examine the neural correlates of phantom pain, there was a profuse sign of activation in the motor cortex and supplementary motor area during induced hallucinations. This thus supports the proposal that motor performance and imagery overlap in areas of the brain. There is a strong association between the motor cortex and somatosensory cortex activation due to a patient’s phantom pain because they perceive this pain as real rather than imaginary.
A clinical trial evaluating phantom limb pain through a more psychological approach was through the use of mirror therapy. The results of this study published in the New England Journal of Medicine showed that mirror therapy in fact is effective in patients with multiple types of amputation, especially lower leg amputation. The study targeted three ways to induce mirror therapy, one with partial mirror representation (through the use of an opaque sheet over the mirror), mental visualization, and with actual exposure to the mirror. The patients’ pain levels were recorded and the patients from all groups were evaluated. According to the study, 100% of patients in the mirror group reported decrease in pain regarding their phantom limb pain, allowing for relief. This study produced results pointing towards the activation of mirror neurons in providing relief to the patient, or the idea that seeing the amputated limb essentially reattached through the mirror dulls the protopathic pain registered in the brain.
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