Nociceptors: how pain is transmitted in the body

Hi spoonies, welcome to Chronic Insights, the podcast for people who take so many medications they need one of those pill organiser things so you know what to take on which day, the podcast for people who take so many pills we rattle when we get shaken. I’m James Allen, I have axial spondyloarthritis, it’s been quite bad the past couple of days, but right now my pain is about 5 out of 10. So, just about manageable at the moment. How are you? I hope you’re doing reasonably well today, but if you’re not: I’m sending you some hugs, and spoons, and love, and inviting you to - notice your breathing, try to slow it down a little bit - and try to relax as best you can, and know that - you’re not alone. You’re with your old friend James, who knows what it’s like.

So, I had quite an intense pain the other night in my pelvis, right on the - you know that curved bit of bone at your waist, the sort of - top of your pelvis, called the iliac crest - I’ve gotten to know the names of quite a lot of bones and joints over the years because I find it easier to talk to doctors and physios if I can just say - iliac crest instead of - that jutty out bit of bone!

So, sometimes I get this intense throbbing that just feels like it’s right inside the bone. And it just kind of sits there, throbbing away, and I can’t really massage it because it’s sort of, too deep inside. And it was - kind of scary. You know, I felt this - fear, because I’ve felt this pain before, and it usually happens when my AS gets really bad, when I go through a big flareup. And so I felt my anxiety going up, I couldn’t stop thinking about it - and then I remembered something I learned a while ago that I think actually helps a bit, and - maybe it will help you too. And that’s to actually learn more about how pain works, what’s going on in my body. So, that’s going to be the main topic for today’s episode - to actually learn some basic stuff about the neuroscience of pain, because I actually find - and some studies have shown, experimentally, that - learning about pain, about what it really is, how it works, can actually make it easier to cope with emotionally. So, I’ve done some research and I’m really looking forward to sharing that with you. I’ll probably break this across multiple episodes, depending on how long it takes to cover it, and depending on what you think - is it useful? Is it boring? I don’t know, let me know: you can email me at james@chronicinsights.com or DM me on Instagram, @chronicinsights. And at the end of the episode, I’ve got some news about my book which is coming out soon, Poetry for Spoonies.

But before that, just a bit of housekeeping to do - I’ve decided to do an Instagram post of the week, just my favourite spoonie related post - not sure it works over audio, but I thought, maybe I can describe the post in words, so - okay, let’s try it. Post of the week this week is titled: Me if I had a badge for every medical adventure I’ve survived, and it’s a photo of this guy - who’s just got - okay, going from the top down, he has a hat, this wide brimmed hat with a line of little tiny medals dangling from it, his chest is covered in medals and those little ropes and tassels you get on really high up military ranks, he’s got medals and patches on his arms, and it doesn’t stop there! I’ve never seen this, he has trouser medals, it’s a shame the photo is cut off because I really need to know if they extend down to his shoes - I really hope they do. So, I thought that was a good one.

Okay, last bit of housekeeping, some quick updates on the Chronic Insights symptom diary app. I had one user write in to ask, how do you log into the app? And the answer is - you don’t! Which I know is weird, it’s like - where’s the login button? Because almost all apps make you create an account. Chronic Insights doesn’t, because it’s 100% private - there’s no data collection, none of your symptom diary is sent off your phone, which means: no need to log in! So they were trying to figure out, how do I transfer a symptom diary to a new phone? Good question, there’s a backup feature which (and i think this is quite clever, if I do say so myself), it allows you to backup your diary automatically to your own private Dropbox folder, so if you lose your phone or get a new one, you can just restore it from the backup, and it remains 100% private because only you have access to your Dropbox folder. The only downside is that you can’t sync between devices, so you can’t have the app on both a phone and a tablet and sync the same data, but I decided that was a tradeoff worth making for guaranteed privacy.

And finally thanks to Matthias for helping me to fix a bug which some people have been having where the 3D model would disappear when you rotated it - so when you’re recording pain on, for example, a leg, and then rotated the model around to record pain on the back, it would disappear - I was trying to fix this for months and I just couldn’t figure it out, because it wasn’t happening on my phone. It turns out - it was only a problem if you were using the German version of the app, and possibly other languages. The problem was with how the app stores numbers, it was using a comma as the decimal separator if the language was set to German, which is how it is conventionally done in Germany - so like, 3.1 would be written 3,1. And that was really messing things up. So, thanks so much to Matthias for writing in to help me with that, I’m working on a fix and it should roll out in a few days.

Okay, with that out of the way… I've lived with chronic pain for over 20 years, and in all that time, I’ve never really understood how pain works. It was just something that was there, and far from wanting to understand it, I did everything I could to ignore it, distract myself from it, or wish it would just go away.

Which is understandable - there’s a reason why chronic pain sucks so bad, because pain is supposed to be temporary. It evolved in our bodies for a purpose. It 's nature's way of telling us that something is wrong. So, when we get some kind of injury, we know about it. We stop what we're doing, we stop moving the part of our body that's in pain, because pain is deliberately unpleasant, it's very hard to ignore, it makes us alert, it stimulates our brain and our body to get our attention. And normally, this is useful, because it means we attend to the injury, we heal, we get better, the pain goes away. If we're walking around in the wild, and we cut our foot on a rock somewhere, if we didn't know about it, that would be really bad - not knowing we were injured could actually kill us. That's why it exists.

But when pain goes wrong - and there are lots of ways it can go wrong, as we spoonies know all too well - when it goes wrong, and becomes chronic, those bodily reactions are no longer useful. When pain is chronic, it still creates this alert, stressful emotional state, but it doesn't help us - because by definition we have a chronic health condition which isn't healing and going away. But it makes us feel like there is danger, there is something wrong. It has become mal-adaptive, meaning that it has become more harmful than useful. And this is what makes chronic pain so hard to live with, and why it contributes so much to our fatigue.

But research has shown that learning about how pain works can actually help. I did a brief survey of recent medical journal papers on PubMed which asked the question - does learning about pain help - 10 out of 11 found some evidence that it does in fact help, and these papers studied a range of health conditions including fibromylagia, migraine, plantar fasciitis, chronic neck pain, spinal pain, and inflammatory arthritis. I’ll put a link to these studies in the show notes so you can read them for yourself.

Why is this? Well, the thinking is basically this. When we feel pain, our natural, innate reasoning for this is that our bodies are actively being damaged, that our bodies are under attack, and this understandably creates a sense of fear, anxiety, it activates our fight or flight response, and in chronic pain this actually exacerbates the pain. But in many chronic conditions, like with fibromylagia, the pain isn't being caused by our bodies actually being damaged like they are when we break a leg or stub our toe. And in other cases like in my condition, axial spondyloarthritis, there is some damage caused by the immune system attacking cells in my body, but due to something called "central sensitisation" (which we'll learn about later) the amount of pain I experience and the amount of damage that it feels like is happening is exaggerated quite far beyond what is actually happening. So by learning why this is, by learning what is actually going on, and teaching our brains that it's okay, you don't need to be on high alert anymore, maybe chill out and relax a bit because we're not being attacked, the pain itself isn't a threat to our lives - research shows that people who learn about the neuroscience of pain actually feel less afraid of their pain, feel less anxious, feel like they are able to live more peacefully alongside their pain. And by learning that some chronic pain isn't always caused by actual damage to your body, it can become easier to move and stretch and exercise. I don't know about you, but with axial spondyloarthritis, movement and exercise does help. But it's just so hard to motivate myself when I'm tired and I'm in pain all the time. But just knowing that actually, the pain is being amplified by my brain and by moving I'm actually not damaging my joints, even though it feels like I am, is just a little extra thing that can help me decide - okay, yeah lets move a bit.

I should also point out that one study out of the 11 I looked at did not find that learning about pain helped, and some of the 10 that did showed only slight changes. So, it's important to let go of any particular expectations of your pain significantly reducing or changing in any way by learning about pain. But I can tell you this. I have found that it helps me live with chronic pain, and many other people have too. And regardless of whether it actually effects our pain, I really believe that if you live with chronic pain, it can be so empowering to learn more about your condition, to learn the real science behind what's going on in your body, because it makes you feel more in control, instead of pain being in control of you. It also makes us better patients. So when our doctors offer us a new treatment, or a new medication or exercise comes along, we're better informed to make a decision about what's right for us. And it means we can take more of an active role in research, and understanding what scientists are doing so that, maybe for example we can help raise money for research into better pain relief for example.

So, what's not to love here? We've got nothing to lose, right? So let's get started with some science!

Audio snippet: Yes, Science!

We'll start of by looking at the basic mechanism of pain. The normal sequence of events that's supposed to happen when we hurt ourselves, and the body lets us know by making us feel pain.

It all begins with the millions of sensors that we have all over our body - they're called sensory receptors, because they're designed to receive stimuli from our environment, and they are part of the somatosensory system, somatosensory because soma means body in ancient Greek. For example, we have touch sensors. This is one of my favourite facts about the body -

did you know that we actually have different kinds of touch sensor? There's one which detects light touch, pressure and tiny amounts of slippage, very quickly, giving you immediate feedback (they're called Tactile corpuscles), there are slower ones for detecting how long a touch is held for (they're called Merkel nerve endings), and there are ones deeper inside the body which detect vibrations, and stretching and rotating of your body (they're called Bulbous corpuscles and Pacinian corpuscles). There are thermoreceptors, which detect heat and cold. In fact, there are two different kinds - one that just detects cold, and one just detects heat. There are more of these in your face and ears than anywhere else, which is why they feel cold more easily than the rest of your body. All combined these sensors give our brains an extraordinary amount of information about what's going on in our body.

And then there's pain receptors, and there are a lot of them. They're called nocireceptors, nocireceptor because "Noci" is latin for hurt. Now, I couldn't find any hard numbers on how many we have, but we have more of them than other types of sensor, and women have more than men, which might partly explain why women in general are more likely to have chronic pain, and experience that pain more than men. This is going to be a topic of a future episode on gender differences in pain.

So, these nocireceptors respond to things which happen which we need to know about urgently, like being hit, an abrasion or cut, extreme heat, and it sends an electrical impulse along a nerve (or "neuron"), first to the spinal cord, then up the spinal cord to our brain. So this is an actual electrical impulse. It's powered by the neuron itself, which contains electrically charged particles called ions, made from things like potassium, sodium, calcium and chloride. Some are positively charged, some are negatively charged, and neurons are really, really good at separating these charged particles. And what happens when we separate a positive and negative charge? We get an action potential - it's how batteries work (that's why you have a positive and a negative end), it's how lightning works, it's how our power sockets work (where we have a positive and a negative wire), and electricity is when electrons are allowed to flow between the two sides. So our neurons have this amazing ability to create a difference in charge inside it, so that when something happens at one end to trigger it, the neuron opens up these channels along it's cell wall (called ion channels) which allows these ions to flow, and this happens in a wave which starts at one end of the neuron and ends at the other end, thus transmitting the signal. There is actual electricity happening in your body all the time! Isn't that amazing? Now, it's not a lot - we're talking about 100 millivolts, or 0.1 of a volt. You get about 1.5 volts from a AA battery. But, fun fact - the electricity in your nerves travels at about 100 meters per second!

So this electrical impulse starts in the nocireceptor that was triggered by some kind of injury, something like this:

Audio snippet: Austin Powers: Dr Evil getting a planet in the balls

and that neuron travels to the spinal cord first. Then another neuron takes the impulse up to the brain, to an area of the brain called the thalamus, which is like a junction box which is in charge of dispatching signals onwards to the cortex, which is at the outer layer of the top of the brain, and that is where the actual experience of pain is processed.

But before we get to that, we just need to say a bit about how these neurons pass on their messages from one to the next, because this is going to be important later when understanding chronic pain. Neurons connect to each other by connectors called synapses. A synapse is where two ends of a neuron meet, but the electrical impulse doesn't just travel automatically jump over the gap between the two ends. This is where important chemicals called neurotransmitters come into play. Now you've probably heard of neurotransmitters like dopamine and serotonin, but there are actually over 100 of them. When an electrical impulse travels down one neuron to the synapse, these neurotransmitters are ready and waiting at the end of the neuron to jump into the gap. On the other side, the next neuron detects these neurotransmitters. Now, depending on what kind of neurotransmitter it is, and how much of it jumps into the gap, the next neuron may or may not continue the electrical signal. Some neurotransmitters increase the likelihood of the signal continuing, but some actually decrease the likelihood - they're called inhibitory neurotransmitters. Now, at first this might seem strange - why would a neuron bother transmitting a signal, only to then prevent the signal continuing over the synapse to the next one? What you've got to realise is that usually, there are usually aren't just 2, but many, many neurons all acting together, and so the body can create these really complicated junctions where some neurons are working to increase the signal, and others are working to decrease the signal. And this is important, because as we will see, the ability for the body to dampen down signals is just as important as the ability to send signals.

So, this impulse goes from your nocireceptor, up the spinal cord, to the thalamus, and there it gets routed to the cortex. And this bit is another one of my favourite facts about the body. The outer layer of the top part of our brain is called the cerebral cortex. Neuroscience research has revealed that sensations from each part of our bodies maps to a specific part of the cortex, so for example touch from your arm is processed by this part of the cortex, and touch from your left cheek is processed by another part of your cortex. And this mapping is basically the same in most people, so that neuroscientists have created an actual brain map which shows which part of the body are connected to which part of the cortex. If you haven't seen this, it's fascinating. It's called the cortical homonculus - homonculus meaning a little model of a person. And someone also created an actual model of a human where the size of the body parts is in proportion to the surface area of the brain devoted to that area - and when you see it you're going to go: WHAT the hell is this?

It looks really weird. I’ll put a link in the shownotes. For example, the hands and lips are massively out of proportion, showing that we have a lot more touch sensitivity in those areas than we do anywhere else.

So when you get an injury in your foot, for example, the message gets routed to a particular part of the brain that deals with the foot. By the way, this is why phantom limb pain exists. So, if you don't know about phantom limb pain, it's if someone loses a limb in an accident for example, sometimes they continue to experience pain which feels like it's coming from the missing limb. That's because there is still this area of the brain which is expecting signals from that particular limb. And because it's not there anymore, that area of the brain, instead of just switching off, it doesn't know that the limb isn't there anymore - and so it gets confused and starts misinterpreting signals.

So, the signal arrives at this area of the cortex. And really, these signals going into the brain isn't really "pain" yet, it's just information. The actual experience of pain comes from the brain's processing of this information, so actually we should think of pain as an output from our brain, not the input. And the output, the experience, depends not just on the signals that arrive from the body from our nocirecptors, but, as we will see in just a bit, a whole bunch of other things combined. Memory. Emotional state. Alertness. Things that have happened previously, all these things combined determine what we actually experience - how much pain, the type of pain. This is why, in fibromyalgia for example, we can experience pain without anything actually happening in the body which should normally cause us to feel pain.

And this is something that has been really really difficult for people talking to their doctors about their pain, because in the past, luckily it, well it shouldtn be, today, but in the past, doctors have sometimes kind of refused to believe people, or maybe not believe, but to dismiss, pain which doesn’t have a physical thing you can point to, like on an xreay or a scan, which oyu can point to and say, this is why your arm hurts because you have this thing. That used to be the case, that doctors didn’t really, fully, understand the whole picture of what pain is and how it’s caused, and what goes on in the brain. Luckily, today, doctors do know a lot more about it, or at least they should do. And that’s why if you are ever dismissed or you’re not believed, if the pain that you feel is dismissed by people just know that maybe it’s just that you need to find another doctor who understands pain, who understands the neuroscience of pain better, because the real experts, the current research, the current scientific thinking is that you don’t have to have anything physically obviously wrong with the body to experience chronic pain. There’s so much complicated stuff going on that can go wrong and can cause pain.

So, talking of pain, I’m quite exhausted, which I normally am after talking for a while, and I’m a bit achey and a bit sore. So, I’m going to have to stop here, but I’ll pick this up next time. Because I’ve only really just started to describe the normal pain process, I haven’t started to talk about what goes on in chronic pa-in, which is quite different and quite interesting, and obviously that’s most relevant to us, but to understand that , we first have to understand what happens in the basic, the normal pain pathway, so next time I’ll start to talk about chronic pain specifically and how that actually works.

So, let me know - was that useful? Should I do more spoonie science? I’d love to know what you think.

To sign off today’s episode, I just wanted to let you know that - my first test print of Poetry for Spoonies came through the letterbox this morning, so I’m really excited to have a scan, see what it’s like, check it for spelling mistakes, check the printing quality, and see if it’s ready to go on sale. So excited!

So I’ll leave you with a poem from the book. I came up with the basic idea for the poem while I was on the train back home after a book binding class, when I was, just, completely exhausted, almost unable to think. It’s called Matchbox Train.

a matchstick figure is seated on the train home
staring inanimately at an open notebook
of staring, empty pages

exhausted

fatigue and pain have peaked
to a point in unison such that
they are no longer distinguishable

the figure is dredging thought from molasses
looking for words to lay between the lines
to parallel the depth of what it feels

the tracks on the page remain empty
as the glue joining its matchstick legs
seeps slowly into the seat

then the diesel engine roars to life
and with brutal mechanical force
eases the train from its platform

the abrasive fury of that heavy machine
vibrate through the figure, matching the feeling
of splintered, rough-hewn aches

and the gentle rocking of the empty carriage
carries through the figure, in harmony
with its swaying, drugged languor

somehow the train itself has become
a surrogate of this thin being’s soul

so the figure puts the notebook away
and sets its matchsticks length-ways

settling into this snug-fitting box
with eyes closed now

existing only as wood and glue and pain
and a soporific pendular motion

object and scene merging
into a coalesced whole

This poem and 33 others will be available in my book Poetry for Spoonies, coming out soon. Watch this space for updates. Until then, thanks for being here. Thanks for the love and support. Sending you spoons and energy and my very best wishes. See you next time. Bye bye, I love you.