Temperature and touch

What exactly does the peripheral nervous system do? Katie Haylor recruited a few unsuspecting colleagues to find out. First up, Katie challenged intern Hannah to, solely by touch, pick out a two pence piece from a pocket full of different coins...

Hannah - This first one feels quite smooth, a bit too small for 2p to think it's a 1p.

2p is one of the larger ones I think I'm gonna go for this one…. No!

Katie - So you picked out the 50p. How come?

Hannah - It felt like it was the right  one, but no….

Katie - Harder than it sounds, right? Now Hannah was integrating temperature differences, size and shape differences, whether they were ridges on the coins and so forth, all in order to help her make that admittedly wrong decision. So what are Hannah’s peripheral nerves actually doing?

Well sensory nerve endings stretch out into the skin and there are different types of receptors on the end ones that respond to temperature, pain and pressure. For instance when Hannah feels for example the ridges on a five pence piece compared to no ridges on a 1 pence piece, those signals rocket up the nerve cables called axons to the sign ups where chemicals move across the gap and the electrical signal continues on to other nerve cells up to the brain, where that information is interpreted. But just how good is the neural resolution in our peripheral nervous system? Time for another test. And this time it’s on Adam.

Katie - I'm going to touch your finger and I want you to tell me how many different points are touching you -  ready?

Adam - That was one, and that felt like one too. That was two.

Katie - Okay now I’m going to do the same thing on your leg. Ready? Yeah.

Adam - One, one and one too.

Katie - Hmm. Well he got it right for the finger touch but completely wrong on the leg. I was using a pen and a pencil. So two points of contact every time and they weren't even that close together. So what's going on? Well nerves are concentrated differently in different areas of the body. Fingertips or lips for instance have lots of nerve endings and therefore a much higher resolution than say the back of the leg. And this makes sense from an evolutionary point of view as you don’t routinely use the back of your leg to pick berries from a bush or to kiss your loved ones.

Time to terrorize my colleagues a little more. Some aspects of the peripheral nervous system can be tricked. Let's take temperature for instance. Now have you ever wondered why chilli tastes hot? In order to see how well Adam and Hannah could discriminate spicy food from hot food, I challenged them to, with eyes shut, taste and identify microwaved pieces of red pepper against pieces of room temperature, but seriously spicy, chilli pepper.

Katie - Pick up the fork. There we go.

Hannah - Where is it!?.....

Katie - Hannah’s doing pretty well, Adam, you’re doing badly… OK you've both got some, now have a taste. Is this red pepper or chilli pepper?

Hannah - I would say the regular pepper.

Adam - Yeah I'd agree, that one doesn't feel particularly spicy.

Katie - OK, next try the other one….what do you think?

Adam - Well my face hurts now, so that’s spicy!

Katie - I'm not sure Adam will be going near a vindaloo any time soon. So I failed to fool them but it seems like the chilli didn't. Spicy food like chilli contains a chemical which binds to a receptor responsible for detecting temperature, in this case high temperatures. So activating this receptor causes our bodies to interpret chillies as being hot, when really they're not at all. But knowing that won't stop me from downing water or better still milk after taking on a particularly serious curry. So that's hot. What about cold? Well it seems a crafty chemical called menthol is also rather good at this molecular trickery. Menthol  - you guessed it -  can also bind to a receptor normally responsible for detecting temperature. In this case the cold. And this is why menthol-containing mints can make your mouth feel cool.

Hannah - If you just chew it, doesn't feel cold but when you breathe in, you get like a cold breeze coming in so it feels like a gust of wind in your mouth. It's like a whole storm happening in your mouth like a cold air.

Katie - And how does this compare to ice chips because they're genuinely cold? So if you can fish around in that bucket, there should be some ice and there's a tiny little ice chip. Adam's going for it, off we go...

Adam - Really cold!

Hannah - It’s more all-encompassing cold!