Chronic phantom pain from nerve injury
If an injury to a nerve completely interrupts the flow of information to the nervous system, common sense says you should feel nothing in the area where the damaged axons have peripheral terminals.
This thinking is true, but only to a point. If you stimulate skin in the damaged area, you may have no sensation of touch, cold, heat, or pressure. However, people who have lost their arms or legs report that phantom sensations occur where the limb used to be. For example, after amputation of a limb, most people feel or imagine that the limb is still present, sometimes in a distorted form. Then after some time, the phantom sensation actually can become painful.
Paradoxically, the healing process itself may create pain. When peripheral axons are damaged, the portion closest to the spinal cord is still connected to its cell body, and it survives, but the part close to the skin that’s separated from the cell body deteriorates (degenerates). Like a plant seeking the sun, the axons attempt to grow toward their old target from the injury, and they can ordinarily reach it, make new receptors, and reestablish nearly normal sensations.
This process, called regeneration, works best when a nerve is crushed rather than cut across, because the nerve’s coverings are preserved; providing channels for growth. However, in the worst case, when the nerve is cut and its normal target is gone, as in limb amputation, a tangle of regenerating axons, called a neuroma, forms. Neuromas can create the sensation of pain.
Normally, axons in sensory nerves don’t conduct impulses unless their receptors are stimulated. Axons in nerves are insulated from one another, and no synapses communicate between axons or cell bodies in the dorsal root ganglion. In other words, they don’t usually talk. However, this system in a neuroma goes crazy, and two things happen:
· The ends of axons in the neuroma start acting wildly, which is called spontaneous activity.
· The axons start talking to each other (even though, normally, they give each other the silent treatment).
Sensations generated by spontaneously active nerves are felt at (referred to) the sites where the terminals and receptors used to be (such as an amputated leg). This situation is because the brain is fooled into thinking that the damaged axons are responding to the same stimuli that normally activated them before the nerve was injured. The activity within a neuroma can even be interpreted by the brain as feeling like the pain of the original injury that set off this unfortunate series of events in the body. Neuromas can form at the site of a nerve injury even if the nerve is not severed and the limb is not amputated. If this occurs, pain is felt in that nerve’s innervation territory (where it’s peripheral receptors are). And — here is the really, really important thing to understand — activity within the pain pathways produced by some abnormal source is what you’re usually dealing with when you have chronic pain.
Adrenaline is always present to some degree in your body. Adrenaline is usually a good thing because it mobilizes our resources to deal with dangerous and injurious situations. However, following a nerve injury, even if the nerve is not severed, it becomes very sensitive to any release of adrenaline (also known as epinephrine) in the surrounding area, which is the case for axons that have been cut as well as those that are damaged but still in one piece. This situation is called sympathetically maintained pain because release of adrenaline normally occurs with activation of the sympathetic nervous system.
If nerves are stretched or pinched continually, axons can be damaged and can become spontaneously active. This condition is called nerve entrapment. For example, sciatica is usually caused by pressure on the sciatic nerve from osteoarthritis or disc protrusion in the lumbar spine.
Nerve entrapment is an example of how any condition that irritates a nerve can cause chronic pain. Also, diabetes can lead to pain due to constant nerve trauma caused by the disease and made worse by inflammation.