What is touch?
What exactly is the sensation we call touch? How do we feel things? Katie Haylor spoke to neuroscientist Francis McGlone from Liverpool John Moores University, asking him firstly to explain how the human body percieves an action like touching a table...
Francis - I think the easiest place to look at that is the skin has lots of little microphones in it, and those little microphones are basically encoding a mechanical stimulus that’s happened on the skin surface. They’re transducing that into an electrical signal which then passes up that nerve fibre. Now these nerve fibres are myelinated so the touch that most people know about is transmitted through myelinated nerve fibres. So that information is travelling faster than a Formula 1 racing car into your brain and the second you touch something or something touches you, you feel it immediately, and these are low threshold mechanoreceptors.
Katie - What is this myelination?
Francis - Myelination is an evolutionary trick, if you like. If you need to do something quickly then you need a myelinated nerve that basically allows that signal to get to the brain very quickly. Motor nerves are the densest myelinated nerves because if you wanted to move, then you move instantaneously. If you didn’t have myelin then there’d be a delay between your intention to move and moving. So we see the consequence of that, of course, with diseases where there’s demyelination such as multiple sclerosis.
Katie - I see. So what about these different inputs then? How about temperature, there’s pain, or even itch for example?
Francis - The somatosensory system, the skin senses are, in fact, multisensory. Most of us feel touch as just a mechanical sense, but there’s about 20 different types of receptors in the skin that respond to temperature, that respond to itch, and that respond to pain. So there’s an array of information coming in from the body that basically tells you the quality of something on your skin. Then, in muscles and joints we have more mechanoreceptors that basically tell your brain that a muscle’s moved or that a limb has moved.
So pain and itch are transduced by a different class of nerve fibre called C fibres. Now, C fibres are unmyelinated so they basically send information to the nervous system very slowly. So one has to ask well, why would these systems have any functions if they are transmitting information so slowly. Well, they’re moving that information into emotional systems to govern behaviours that have more of an affective quality to them. In fact, we have two pain system by the way: we have a fast one which gets you out of there quickly so if you put your finger on a hot plate you immediately pull back. That’s the first pain system protecting you to get away from that tissue threatening stimulus. But if you’ve ever done that, you know that a couple of seconds later that emotional throbbing, burning pain comes in - now that’s C fibres.
Katie - Talking of pain: what about nerve damage? How does damaging this particular system of affect our sense of touch? In the wider sense, I guess, touch I mean somata senses in general.
Francis - The classic loss of C fibres we see with diabetic neuropathy, so patients that have diabetes. These long nerve fibres, particularly the ones that innovate the feet are the first ones to get damaged, and diabetic patients can lose their sense of touch and, in fact, they lose their nociceptors sense of pain as well. So these nerve fibres can be damaged by conditions such as diabetes.
Katie - Okay. So inputs like temperature, pain, itch, they’re all coming through a similar system but it’s different fibres that are responding in order for these signals to get up the nervous system, if you like, into the brain?
Francis - Yes. Again, we go back to that point that C fibres play a fundamental role in protecting us and that protection is mediated by the behavioural state, so that’s accompanied by that affected state so it’s either rewarding or it’s punishing.