neuroplasticity and chronic pain -- 3/30/17

Today's encore selection -- from The Brain's Way of Healing by Norman Doidge. It is the brain, not the body, that experiences pain. Repeated trauma can cause the brain to experience more pain than is warranted, a phenomenon referred to as chronic pain. This gives hope that chronic pain can be remedied through treatment of the brain:

"Touch a part of the body's surface, and a specific part of the brain map, devoted to that spot, will start to fire. These maps for the body's surface are organized topographically, meaning that areas that are adjacent on the body are generally adjacent on the map. When the neurons in our pain maps get damaged, they fire incessant false alarms, making us believe the problem is in our body when it is mostly in our brain. Long after the body has healed, the pain system is still firing. The acute pain has developed an afterlife: it becomes chronic pain.

"To understand how chronic pain develops, it's helpful to know about the structure of neurons. Each neuron has three parts: the dendrites, the cell body, and the axon. The dendrites are treelike branches that receive input from other neurons. The dendrites lead into the cell body, which sustains the life of the cell and contains its DNA. Finally, the axon is a living cable of varying lengths (from microscopic ones in the brain to others that run down to the legs and can be three feet long). Axons are often compared to wires because they carry electrical impulses at very high speeds (from 2 to 200 miles per hour) toward the dendrites of neighboring neurons. A neuron can receive two kinds of signals: ones that excite it (excitatory signals) and ones that inhibit it (inhibitory signals). When a neuron receives enough excitatory signals, it will fire off its own signal. When it receives enough inhibitory signals, it becomes less likely to fire.

Electrodes placed on the surface of the cortex can be used to stimulate the brain
of a conscious patient or record its activity. Photograph: Eric C. Leuthardt

"Axons don't quite touch the neighboring dendrites. They are separated by a microscopic space called a synapse. Once an electrical signal gets to the end of the axon, it triggers the release of a chemical messenger, called a neurotransmitter, into the synapse. The chemical messenger floats over to the dendrite of the adjacent neuron, exciting or inhibiting it. When we say that neurons 'rewire' themselves, we mean that alterations occur at the synapse, strengthening and increasing, or weakening and decreasing, the number of connections between the neurons.

"One of the core laws of neuroplasticity is that neurons that fire together wire together, meaning that repeated mental experience leads to structural changes in the brain neurons that process that experience, making the synaptic connections between those neurons stronger. In practical terms, when a person learns something new, different groups of neurons get wired together. As a child learns the alphabet, the visual shape of the letter A is connected with the sound 'ay.' Each time the child looks at the letter and repeats the sound, the neurons involved 'fire together' at the same time, and then 'wire together'; the synaptic connections between them are strengthened. Whenever any activity that links neurons is repeated, those neurons fire faster, stronger, sharper signals together, and the circuit gets more efficient and better at helping to perform the skill.

"The converse is also true. When a person stops performing an activity for an extended period, those connections are weakened, and over time many are lost. This is an example of a more general principle of plasticity: that it is a use-it-or-lose-it phenomenon. Thousands of experiments have now demonstrated this fact. Often the neurons that were involved in the skill will be taken over and used for other mental tasks that are now being performed more regularly. Sometimes one can manipulate the use-it-or-lose-it principle to undo brain connections that are not helpful, because neurons that fire apart wire apart. Suppose a person has formed a bad habit of eating whenever he is emotionally upset, associating the pleasure of food with the dulling of emotional pain; breaking the habit will require learning to disassociate the two. He might have to actively forbid himself from going to the kitchen when he is emotionally upset, until he finds a better way to handle his emotions.

"Plasticity can be a blessing when the ongoing sensory input we receive is pleasurable, for it allows us to develop a brain that is better able to perceive and to savor pleasant sensations; but that same plasticity can be a curse when the sensory system that is receiving ongoing input is the pain system. That can happen when a person slips a disc, which then presses repeatedly on a nerve root in her spine. Her pain map for the area becomes hypersensitive, and she begins to feel pain not only when the disc hits the nerve when she moves the wrong way, but even when the disc is not pressing hard. The pain signal reverberates throughout her brain, so that pain persists even after its original stimulus has stopped. ...

"[Researchers Patrick David] Wall and [Ronald] Melzack showed how a chronic injury not only makes the cells in the pain system fire more easily but can also cause our pain maps to enlarge their 'receptive field' (the area of the body's surface that they map for), so that we begin to feel pain over a larger area of our body's surface. ...

"Wall and Melzack also showed that as maps enlarge, pain signals in one map can 'spill' into adjacent pain maps. Then we may develop referred pain, when we are hurt in one body part but feel the pain in another, some distance away. Ultimately, the brain maps for pain begin to fire so easily that the person ends up in excruciating, unremitting pain, felt over a large area of the body -- all in response to the smallest stimulation of a nerve.

"Thus, the more often [a person feels] twinges of neck pain, the more easily his brain's neurons recognized it, and the more intense it got. The name for this well-documented neuroplastic process is wind-up pain, because the more the receptors in the pain system fire, the more sensitive they become.

"[One patient we were studying] realized that he was developing a chronic pain syndrome and was caught in a vicious cycle, a brain trap: each time he had an attack of pain, his plastic brain got more sensitive to it, making it worse, setting him up for a new, still worse attack next time. The intensity of his pain signal, the length of time it lasted, and the amount of space in the body it 'occupied' all increased."
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