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It was stated in class that we can only experience the world through the signals that we have receptors for.  We have a fairly limited amount of auditory receptors in our ears, photo receptors in our eyes, olfactory receptors in our noses and various pressure receptors covering our bodies.  We discussed the fact that activation potentials are generated in axons just under the skin when pressure is applied. 

At the time, all of that made sense to me.  If someone were to pinch the skin on my arm then the axon membrane in that area would change shape to allow sodium from the outside to flow in, thus starting the action potential that would propagate until it reached my brain and I said, ouch stop! 

However, after reading an article about phantom limbs, I’m perplexed.  How does one with no arm experience the sensation of that non-existent arm being pinched?  If the nerve endings have been severed in the amputation then where are the action potentials that signal pain in the brain coming from? 

The fact that people continue to feel pain in amputated limbs suggests to me that the brain is divided into specialized sections that are in some way hardwired to process specific inputs.  It also makes me think about the fact that we have so many sensory inputs and that the brain is only so large.  The specialized areas must be very close to one another.  This could offer another explanation for phantom limb pain.  Perhaps the area of the brain programmed to process sensation from the arm is right next to the area programmed to process sensations originating in the foot.  If this were the case then it may be possible that a few neuron bodies from the foot crossed over the boundary and implanted in the area meant to experience arm sensations.  The pain that the amputee experiences as arm pain is actually originating in the foot!

Just an idea, but it does help to explain two things: Where action potentials in amputees are generated and how the brain is organized to handle the vast amount of inputs that it receives.