All the information which comes into our sensory system such as feelings like touch and pain is transmitted up to the sensory cortex of the brain, where the parts of the body have specific parts of the cortex dedicated to them. The diagram which results when the body parts are illustrated against the sensory cortex is called the homunculus, in which certain parts such as the lips and hands have much larger areas of brain devoted to them than average. This is because the importance for daily life of those areas is greater and more information is required from them to figure out what's going on.
In an acute injury the injured part goes through an inflammatory process, with the soup of irritating chemicals waking up normally silent pain nerves and a stream of nerves impulses making their way upwards. These enter the spinal cord and pass the messages on to the next stage nerves, which become highly stimulated by the incoming impulse streams as they start to amplify the incoming pain levels. This increases and passes on higher pain intensities up to the brain, forcing us to take corrective actions.
Pain needs to get through up to the brain and force its way into our conscious minds until we feel it, as our pains are always in our minds strictly speaking. Pain is never imaginary but our brains construct a virtual sensory reality so we can make sense of the world which includes touch, pain and visual realities. It is important to realise that the brain builds the pain experience that we endure and that this is not made up by our injured neck, disc prolapse or torn ligament.
When we have an amputation our Nervous system is divided as well as our limb. The bone, muscle and ligaments which are cut through are easy to envisage, but we do not think about the nerves which have to be cut, the consequences of which can be very important. The nervous system does not like parts of it to be removed, it does not like it when an area which normally sends in loads of information suddenly stops doing so. When this occurs odd things start happening in the nervous system, things which can have unpleasant results.
When incoming impulses are completely prevented from reaching the second stage nerves, these nerves react by rapidly increasing their excitability. With no incoming messages due to the amputation or nerve transection, the second stage nerves start to fire off spontaneously, that is for no particular reason but just because they are over-excited. The leg nerves may be missing but all the central nervous system transmission nerves for the leg still exist. The areas of the brain looking after the missing part are still present and still capable of creating pain in that missing part.
When someone has an amputation the pain problem which is often generated afterwards in known as phantom pain, a pain syndrome which appears gradually over a period of months or weeks and which can be very difficult to manage. An example of a very unpleasant type of pain called neuropathic pain, phantom pain can be very deep or stabbing and is very hard to cope with as a sufferer. These neuropathic pains are being generated inside the nervous system and are not related to normal, tissue injury pain.
Morphine related drugs such as codeine, Tramadol, fentanyl and morphine may not be very effective against neuropathic pain so pain clinics more commonly prescribe drugs which affect nerve transmission of pain. These include gabapentin, pregabalin and amitriptyline, which are typically used for depression and epilepsy but have been found to have activity against pain. TENS (transcutaneous electrical nerve stimulation) can be useful, sending electrical stimulation in through electrodes on the skin. Coping with a long term pain problem can be addressed by cognitive therapy.
Phantom pain can be an intractable, serious problem for anyone with an amputation, and having significant pain before the amputation may make the likelihood of phantom pain greater. A multidisciplinary approach involving a pain clinic is most likely to be helpful.