NeuroScience of Chronic Pain Part III

By: admin Published: April 18, 2013

by Jay McCallum, PT, DPT, OCS

How Does Chronic Pain Change the Sensory System?

In my last post I tried to outline the super-complex process that the brain goes through to generate the experience of pain, and what I want to try to do today is talk about what changes when the nervous system is bombarded by pain impulses over a long period of time.  This can be a challenging idea to transmit to people who have had pain for may years – they have nearly all heard some refrain on the idea that ‘your pain is in your head,’ with the subtext being ‘your pain is not real’ or, worse yet, ‘you’re faking it.’  I have had patients become angry and leave care, or become tearful or defensive when I try to bring this topic up, which is why I wanted to spend the time writing in my last post about how the experience of pain is generated by the brain.  What we are talking about here is not ‘the subconscious’ or anything like that – it’s about structural changes in the parts of the brain responsible for processing various inputs and generating the output or experience of pain.

There are several ways that the nervous system adapts over time to pain, and unfortunately the idea that we get tougher or desensitized is not at all accurate.  Remember that our nervous system is “plastic”, that it, it changes its structure in response to stimulus.  Generally, the more of a stimulus you give the nervous system, the more ‘resources’  it devotes to that stimulus.  Think of practicing a skill like hitting a golf ball – to break down the details of that, we improve our skill at that because with practice we devote more neurons to the motor program.  Or think of a sommelier who can identify many nuances of a wine — again, more neurons devoted to that process.  With pain, what we see is that the spinal cord itself ‘turns up the gain’ on its pain pathways, devoting more neurons to nociception (pain stimulation) than ‘normal.’  It looks a bit like this diagram, if you think of the vertical axis as being basically loading to a tissue, with the peak being the point at which injury would occur:

Normal Protective Pain

Normal Protective Pain

This is how pain is supposed to work – the person feels pain just a little before the point at which tissue damage occurs.  But when the spinal cord is turning up the gain, it looks more like this:

Abnormally early onset of pain in relation to tissue load

Abnormally early onset of pain in relation to tissue load

In this diagram you see that there has been a change in true tissue tolerance related to the original injury but the nervous system begins to generate pain far before any tissue damage begins to occur.  Let me emphasize here for a minute again that this is TOTALLY REAL PAIN, it just isn’t calibrated well to tissue damage anymore.  This change appears to happen primarily in the spinal cord, and as such is often referred to as “central sensitization.”

Central Facilitation

As if this isn’t enough, the brain itself has to get into the act.  As with other things I’ve described with this series of posts, the changes are complex, but there are two main themes – loss of precision and a process called facilitation.  We’ll start with the loss of precision.  As neurons in the sensory cortex of the brain are continuously bombarded with stimulation the stimulation basically starts to bleed over to neighboring neurons, to the point that brain imaging scans show overlap between areas of activation in the brain with stimulation of different parts of the painful region.  Patients experience a sense that their pain is spreading over time, sometimes even spreading to the opposite side of the body, and simultaneously becoming more difficult to describe.  Rather than pointing at the pain with a finger they wave at a whole area of their body, and they describe a sensation that the pain is ‘moving around.’  This loss of precision is easiest to ‘see’ experimentally in people who have a single extremity involved, say a very painful right arm.   We use our sensory cortex to help us identify whether we are looking a picture of a right arm or a left arm, basically sort of superimposing that picture onto our sensory cortex to see which side it ‘fits.’  Those patients with painful right arms, for example, are significantly slower to identify pictures of right arms than left arms, which tells us that their sensory cortex is not processing information quickly or accurately.

Facilitation basically means ‘priming’, and to understand this remember that the brain is absorbing information about many different things as it creates the experience of pain, and over time it no longer needs the entire set of stimuli to create the experience.  In our course, the analogy was to ask a group of attendees to stand up, then sit, then stand, then sit, etc.  Each time the speaker gestured along with giving a verbal command, until the last time when all he did was gesture.  And everyone stood.  A partial stimulus – had he just walked up to them and gestured initially they would have just stared at him – leading to the output of standing.  With pain, it might be the stimulus of an environment or activity that has historically provoked pain.  A case study of a bicyclist who had chronic pain with hill climbing was presented – she was placed in a setting where screens to either side of her could be tilted to make it look and feel like she was climbing a hill, and her pain could be brought on (and relieved) simply by tilting those screens, with no change in the actual intensity of her riding.  A partial stimulus of hill climbing leading to the experience of pain.  (You can read more about her here).

So…to recap:  Nociception (remember, that’s the name we use for nerves in the body that carry a painful stimulus to the brain) gets amplified by the spinal cord over time.  The sensory cortex of the brain that is responsible for generating the sensation of pain and sending it to our consciousness becomes somewhat sloppy (very like what happens with the motor cortex as I described in my first post on this subject).  And the pain system begins to leap to conclusions based on incomplete data.  Finally, just because I can’t say it enough times, the pain is REAL.  It’s just no longer accurately representing tissue damage like it is supposed to.

I have one more post in the pipeline, which will deal with what we can do in physical therapy to address this process, but the short version is that it is very much a thing that physical therapy can impact because it’s about retraining the brain, and PTs work with changing brain function every day.  Lots of things can change the way the brain functions, including everything from trauma (a head injury or a stroke) to what might amount to an overuse injury to part of the brain (chronic pain), and PTs treat all those things.  So if you have a history of longstanding pain that has not always ‘made sense’ to your medical providers, then consider giving us a call at 928-556-9935 or an email at [email protected].

 

 

Neuroscience of Chronic Pain Part II

By: admin Published: April 2, 2013

by Jay McCallum, PT, DPT, OCS

 

The Sensory System

In the first post of this series I focused on how the motor system changes in response to longstanding pain, basically with a shift in strategies towards more of a mass bracing pattern that over time tends to perpetuate the pain.  But that’s only half the story, at best – the way we perceive pain also changes in response to a chronic painful stimulus.  And, as you can imagine, those changes are not particularly helpful.  But, before we launch into a discussion of how the system changes, let’s start with talking about how it normally functions.  As with everything else our brains do, it’s an amazingly complicated process, so I’m just going to hit some of the highlights.

How Does the Brain Create an Experience?

We all like to think that we are directly connected with the reality of the world around us.  We use our eyes to see what’s there to see, our ears to hear what there is to hear, and we feel things based also on the stimulation of our nerves.  But the process is really not nearly that straightforward.  optical illusion

This is an example of an optical illusion, where the viewer is asked to count the number of black dots.  Problem is, they keep moving around – white when you look right at them, black when in the periphery of your vision.  In reality, they’re all white, but the brain does not accurately present that information to your consciousness because it is affected by the dominance of the black squares.  There are also examples of auditory illusions, tactile illusions, even smell illusions!  And let’s be careful about this term ‘illusion’ – the perception is quite real – I really do see little black dots in that picture.

So, the best way probably to think about how we experience the world around us is to envision a product that has been presented to our consciousness by our brains, after taking multiple inputs into account.

So What Goes Into the Experience of Pain?

Quite a few things.  The first, obvious one is stimulation of a nerve or nerves, at least in most cases.  That input to the brain is called nociception.  But we know that nociception is neither necessary nor sufficient for the experience of pain.  For example, consider a person with phantom limb pain after an amputation.  The sensory nerves and the tissues that they are responsible for no longer even exist, but the person nevertheless experiences extremely real pain.  We’ve all heard stories about people with terrible injuries who didn’t realize that they even had the injury until later, because they were busy dealing with the situation at hand.  Or, on a more daily level, that cut or scratch that didn’t start to hurt until you saw it sometime later.  Some of the other inputs the brain takes into account in generating an experience of pain are things like our sense of body position, what we’re seeing and hearing, and what our prior experiences and beliefs surrounding similar circumstances are.

The key thing to remember here is that the purpose of pain is to alert us of a threat, and that the brain is attempting to make a threat decision with what it presents to our consciousness.  The more threatening the combination of inputs is, the greater the pain is.  Lorimer told a great story that summarizes this well.  He was walking in ‘the bush’ as they call it in Australia, and felt something catch his foot momentarily.  There was no pain, just a catch, and he continued with his walk.  Then he woke up in the hospital – he had been bitten by an eastern brown snake, a terribly venomous snake with an extremely painful bite.  But his brain had not reported pain to him, because he had such a large experience of walking in the bush and scratching his leg on twigs, so the very similar input of the snakebite was not judged as dangerous.  Months later, he was again walking in the bush and felt something catch his leg, but this time it was hideously painful, with persistent pain for a week afterwards.  But, you guessed it, scratched by a twig.  But that combination of inputs – walking in the bush, stimulus to the outside of the leg, etc – was now on the ‘really dangerous’ list as far as his brain was concerned, in that last time it nearly killed him.

Say Again?

Let’s pause here to summarize all this.  Pain is an experience, not a stimulus.  It is generated as a result of many inputs, including both things happening to our bodies and around us as well as our past experiences and beliefs.  Nociception (that stimulus of a nerve fiber in a tissue in the body somewhere) is neither sufficient nor even necessary for the experience of pain to occur.  There is, rather, an extraordinarily complex orchestra of events that are oriented around risk assessment that leads to the experience of pain.  And, like anything complicated, sometimes that orchestra starts to malfunction.  In my next post I’m going to talk about how that happens, but the really short version is that the more the orchestra plays the pain tune, the better it gets at it, until it’s going on and on in a way that is partially or even completely disconnected from the input its getting from the tissues.  It looks a bit like this:Pain diagram

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