Sean Mackey, M.D., Ph.D.
speaker
373 appearances
1 recordings
1 series
first heard Apr 2025
last heard Apr 2025
Sean Mackey, M.D., Ph.D.’s voice in public audio — every appearance, attributed to the second.
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A-beta fibers are your touch fibers. When you touch or stroke your skin, those get activated. When you stand up and you stand on one leg, they're also responsible for position sense. They have a heavy coat of insulation around them and they are wicked fast. C-fiber is one meter a second. A-delta fiber is 10. A-beta fiber is 100 meters a second. Fast. That's why you can dance.
That's why you can walk because you've got those fast reacting A-beta fibers. Now, let's go back to your thumb, Peter. You just hit your thumb with a hammer. Sharp jolt of pain goes to your brain. Got a little delay. Oh damn, this is really going to hurt. Hot burning flooding sensation comes over your thumb. What is the next thing that you do? Everybody does this a little differently.
Before or after swearing? There you go. A lot of people swear, so it's after swearing. You swear and then- Shake it. Shake. Okay. You're a shaker. Or a squeeze. Squeeze. Proximal to it. There you go. You squeeze, you shake it. Sometimes you run it underwater. What are you doing when you're squeezing it and shaking it?
Yeah, that's beautiful. And you're talking about longer term things which are all perfect. Because cold does those things, and also cold, by the way, reduces action potential velocities and firing in those A-delta C fibers. But what you're doing most of all when you rub it is you're activating A-beta fibers. you're actually not influencing much your A delta or C fibers, those nociceptive fibers.
You're not really impacting it there. That horse left the barn. That horse left the barn. You got horses still running out the barn. You can squeeze all you want for the time being and the horses are still heading out and hitting your spinal cord. But where things get interesting is the A beta fibers, those touch fibers,
They're coming into a slightly different area of your spinal cord, and they're sending over projections into where those nociceptive fibers are in your spinal cord. And they have an inhibitory role. That's the take-home message. So the A-beta fibers are inhibiting the signals coming in from where you hit your thumb with a hammer and preventing them from going to your brain.
It's a beautiful example of neuromodulation. You're doing your own neuromodulation with that. And we're all hardwired to do that thing. And there's a medical device that takes advantage of that. You're familiar with the TENS unit? TENS is T-E-N-S, Transcutaneous Electrical Neural Stimulation.
Now, what it originally did, there's been modifications of it, is what you do with the TENS, I know you know this, is typically little black pads that you put over the area that hurts. You put an electrical stimulation through these pads. They're activating A-beta fibers. And so you do them over here and it's having a neuromodulatory effect back in the spinal cord. Pretty cool when it works.
I would just build on that, that I would say my job, our job as pain docs is to help reduce the pain and help them down a path of functional rehabilitation. So absent that second piece, I typically fail. I'm leading them down a road of functional rehabilitation, which involves physical, psychological, social, emotional health, all things you talked about beautifully in your book.
Somebody with, I think, more, you know, there's no susceptible musculoskeletal pain. I tend to think of somebody for whom TENS is more likely to work, something that has more of a classic nociceptive type of pain problem. Beyond that, Peter, it's a trial and error.
That's part of the frustration in pain management and in healthcare in general is the lack of a precision approach and the very frustrating, laborious trial and error process until we get something that works. So we talked about the gait control. We talked about TENS.
You're right with that individual perception. variability, and pain. We haven't talked about the brain's role in the gait control, which we're going to get to, but just getting to your question, there's been some elegant studies. A guy named Kim, years ago, did a beautiful study where he applied a 48 degrees Celsius stimulus to 500 people. 48 degrees Celsius, I think it's 121 degrees Fahrenheit.
Somebody will look it up. And you apply it to the arm, the hand, and then ask, what's your pain score? And what he found was perfect distribution of people who said, nah, this ain't painful. Ain't nothing. That's like zero, one out of 10. Some were like, yeah, it's a little painful, two or three. And others were like, yeah, a little more moderate, four, five, six.
And you got all the way up to some people saying, oh my God. You're burning me. You're burning me. Take that off immediately. 10 out of 10. I do the same thing in a medical school demonstration. Can't call it an experiment because I'm not getting IRB. But when I teach the neuroscience class around pain, I bring in a circulating ice water bath.
And you want it to circulate because if you just stick your hand and leave it in still, you get a boundary. It's warmer.
So circulating ice water bath. And I asked them to dip their arm in for 15 seconds, pull it out, whisper in our research assistants here what their pain score was. We tabulate that all up. And at the end of the class, I show the medical students. And it looks just like that line I showed you, I mentioned to you before. You got some people in the class who say, I keep my hand in there all day.
And there was others who were like, oh my God, I couldn't even keep it in there at all. 10 out of 10. The whole point of that is to drive home one of the key messages in our discussion. The amount of stimulus or nociception may have little to nothing to do with your experience of pain.
And why that is so important for healthcare professionals to understand is because for so long, we have projected our own experiences onto everybody else.
You're absolutely right. And maybe that's a good opportunity to build on that and introduce more of the brain. So we've talked about the functions of the brain, the cingulate cortex being some of the more emotional, the primary somatosensory cortex, the homunculus being more sensory, the insular cortex has an interoceptive state. It's like our internal awareness of our bodily state.
We talked about the amygdala. Now let's introduce also the prefrontal cortex. The prefrontal cortex, the big thinking part of our brain up here, both the ventral medial and the dorsal lateral, play a key role in our modulation, our cognitive control of pain.
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