Phantom pain

Phantom pain, also called  deafferentation pain, anesthesia dolorosa, or denervation pain, is pain that is felt in a part of the body (usually an extremity) that either no longer exists due to amputation or is insensate as a result of nerve severance. It is often described as a burning sensation, though individual accounts vary. This pain does not originate from the limb itself, as such would be impossible in these cases, but is instead the result of the brain receiving messages from the spinal cord which it interprets as pain coming from the affected limb.

Physiology
Pain usually begins when nerve endings relay information to nerves, which in turn relay it to ganglia just outside the spinal vertebrae, which send it to nociceptive (pain-sensing) neurons inside the spinal cord, which finally send the information to the brain. If the nociceptive neurons cease to receive information to pass on, they will instead send the neurological equivalent of white noise, which the brain then interprets as pain.

In cases in which a traumatic event causes nerve severance and also detatches the nociceptive neurons from the rest of the spinal cord, phantom pain will not develop. Some patients who do not receive adequate relief from nerve blocks, narcotic painkillers, or other methods of pain management may undergo a procedure called dorsal root entry zone (DREZ) lesioning. DREZ lesioning is a form of neurosurgery in which the wayward nociceptive neurons are destroyed.

Phantom pain from amputation
One of the oldest and more common beliefs of the cause of phantom pain involved the cut nerve endings in the stump. The cut nerve ends grow nodules which are known as neuromas. These neuromas continue to generate impulses. The impulses flow through the spinal cord into parts of the brain. These brain areas include the thalamus (a central way station of the brain); then onto the somatosensory areas of the cortex, the presumed centers for sensation.