Nearly all amputees feel their missing limb as if it still existed, and many experience chronic phantom limb pain. The going theory is that this pain is triggered by the brain. But scientists have now located and blocked these sensations in the body itself — a finding that upends conventional thinking.
According to the popular top-down theory, phantom limb pain happens when there's a sudden loss of sensory input and the brain hasn't had a chance to adapt — what scientists call maladaptive cortical plasticity. And indeed, neuroscientist Vilayanur Ramachandran's Mirror Box experiment — in which two outward-facing mirrors alleviate phantom limb pain by "tricking" an amputee into thinking their missing limb is still there — would seem to indicate that something's going on in the brain.
A Bottom-up Process
But a recent experiment by Marshall Devor of the Hebrew University of Jerusalem's Department of Cell and Developmental Biology and Center for Research on Pain indicates that the primary source of phantom limb pain resides in nerves near the spine, likely the dorsal root ganglion (a cluster of of neurons that carries signals from the body to the spinal cord, which in turn gets routed to the brain).
What's more, Devor's team managed to alleviate the associated pain. Consequently, their work shows that phantom limbs are not imagined in the brain, but felt in the body; it's bottom-up and not top-down.
The researchers came to this conclusion by injecting 31 leg amputees suffering from PLS with a local anesthetic near where the nerves from their amputated legs enter the spinal cord in the lower back. After a few minutes, the phantom limb sensation and pain were temporarily reduced or eliminated.
It's thought that these neurons "terrorize" the brain with abnormal signals when a limb is lost. So, it's not that PLS happens as a result of loss of input — it happens as the result of exaggerated input generated by neurons in the dorsal ganglia that used to innervate the limb.
"The dismantling of phantoms via the silencing of these 'terror cells' is in my mind a death blow to the brain theory of phantom pain and a call to industry to join forces with scientists to find new treatments for neuropathic pain," said Dr. Haim-Moshe Adahan in a Haaretz article.
And indeed, this research could make life considerably more bearable to the millions of amputees currently suffering from PLS.
Read the entire study at Pain: "Peripheral nervous system origin of phantom limb pain."
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