The nerves that allow us to enjoy a cuddle
The nervous system carries information about what touch from the skin to our brains. And different types of stimuli are carried in different classes of nerve fibres. Pain and temperature sensations are conveyed in one group of nerves, while fine touch and itch sensations are signalled by others. And we have a pretty good idea how this information is relayed to the brain. But there’s also one more recently identified class of nerve fibre - responsible for the pleasant sensations that accompany being stroked or massaged - that neuroscientists assumed, on the basis of the characteristics of the nerve cells involved, would travel alongside the pain and itch sensations. But when they did the experiment in humans to prove it, they got a surprise. Speaking with Chris Smith, Liverpool John Moores University's Francis McGlone…
Francis - Well, our research for a number of years now has been characterising the functional properties of a particular class of sensory nerves that innervate the skin called C fibres. And C fibres are classically understood for code for pain and itch. There's been another C fibre identified relatively recently that responds specifically to gentle touch. And this nerve fiber we hypothesise is the basis for why people like a cuddle, or they like to be stroked in, in consensual sort of relationships.
Chris - So in this case it's C for cuddle! But what was the question that you had to get at then?
Francis - Well, the question was its pathway centrally. So we record from these nerve fibres in humans using an amazing technique developed by the Swedes called microneurography. We can put a very fine electrode through the skin into an underlying nerve bundle and we can listen in specifically to particular classes of nerve fibres. So we know this nerve is in the skin and we know its response properties, which are really quite fascinating because this nerve fiber is tuned to respond optimally to the stroking velocities that people would report as most pleasant.
Chris - So you know a lot about basically what excites these nerve cells, but exactly how I become consciously excited when I'm stroked in this way, that that was the unknown?
Francis - Well, we've got the two ends of the, of the equation. We've got the first order neuron in the skin. What we don't know is the second order pathway - i.e. which spinal pathway is a C tactile afferent moving up towards the brain.
Chris - Is that not the same as the pathway that these other C fibres, these very small unmyelinated with very fine calibre nerve fibres take? They classically - we understand from, you know, physiology going back a long, long time - they go into the spinal cord, make some connections and then they go across to the other side of the spinal cord and whiz up to the brain. Is that different than you think?
Francis - Well, that was our major hypothesis. That was the whole basis of this study was to basically establish that the C tactile afferent took the same spinal pathway as the other classic C fibres, the ones that code for itch and the ones that code for pain. So that was our hypothesis that, when that is cut, response to pleasant touch would have disappeared.
Chris - Now when you say when it's cut, would this be in humans who are having either trauma to the spinal cord or purposeful interruption of those pathways for some reason?
Francis - Yes. So I should explain that in chronic neuropathic pain patients, the problem with chronic neuropathic pain, it is very difficult to switch it off, so, some years ago, neurosurgeons came up with an operative procedure where they basically put a needle into the spinal cord and the spinal cord has got loads of pathways coming up it, but there's a specific pathway called the spinothalamic tract, where we know all the C-nociceptors and the itch nerves are. So the neurosurgeon will put a needle into that and then they heat that needle up so that it basically cauterizes the spinothalamic tract and then it's quite incredible actually that, of 20 patients that took part in the current study, I think 18 or 19 after this operation, we're completely pain free. Their itch had gone, and of course we fully expected when we tested their response to present touch, that would have been absent as well - and it wasn't. We're having a real rethink now as to what's going on in terms of how this nerve fiber codes for the pleasantness of touch.
Chris - Do you think they've got a unique fiber bundle: these aggregate in some way within the substance of the spinal cord; they don't join those pain and itch and other pathways; they stay isolated and go up anatomically in a distinct area. And so when you do that lesion, you just miss them?
Francis - Uh, there is that possibility that we may, they may have a sub sort of pathway that travels up the spinothalamic track that we missed. But these operations completely removed itch. They completely remove pain, so they'd have to be hiding somewhere in order for us not to have detected them. There is just one maybe niggling problem is that when the surgeon is stimulating through that needle to make sure that he's in the right part of the spinal column, the patient is brought out of anaesthesia. And they're asked to respond when they feel anything. And of course if you're in the spinothalamic tract and you electrically stimulate it, clearly these patients will respond with "yeah, that's really painful!" So we're looking specifically with the surgical procedure for a response of pain from the participants. Now, it's just vaguely possible that, by doing that, we missed out another sort of sub-tract in the spinothalamic tract that has all the CTs. That's a possibility, but it's unlikely.
Chris - Is it possible that these slightly extraordinary nerve fibres are taking a completely different path entirely? Could it be that they're going up, say, the back part of the spinal cord, what we call the dorsal column pathway, which traditionally there's very fine touch in there, isn't there? If I want to tell which coin I've got in my pocket, when I reach in with my fingers, I'm using that pathway. Is it possible they're going up there?
Francis - Absolutely. I mean that's when we reasoned the um, results in our paper, we did make that point, that it's possible that these fibres are also going up the dorsal columns. And as you say, there is some evidence and some animal literature that that may be the case. It's surprising how little we really understand about the complexities of these afferent pathways coming up in the spinal cord. And what we're going to be doing next now is to let the brain tell us what's going on rather than somebody subjectively reporting whether that touch feels pleasant or not. So these patients, the next cohort of patients, we'll be running an fMRI analysis on them and stroking them again and see what the brain's telling us...