Science Interviews

Interview

Sun, 7th Dec 2008

The Benefits of Blinking

Dr Paul Murphy, Cardiff University

Listen Now    Download as mp3 from the show The Science of Vision

Paul - It's good for our eyes because the surface of your eye needs to have moisture on it to keep the cells healthy. They add little nutrients to the cells. They provide oxygen to the eye and they also provide a nice smooth optical surface. That helps us see nice and clearly.

Meera - how does that happen when we blink? How is the health of our eye maintained?

Slow motion blinkPaul - There are different parts of our tears. there is a little bit at the bottom which is like a mucus layer. That comes from the little cells in your eye. They secrete that all the time. That helps the tears stick to the eye. Then you have a watery bit that comes from glands tucked up under your eyelid. They're secreeted gradually all the time. That adds the watery bits. When your eyes are watering those cells are being squeezed and more water's coming out.  We've also got an oily layer at the top to help two things. They help tears spread each time we blink. The blinking squeezes all the tears down and then they have to spread back out and that oily layer makes that easier. They also stop the evaporation. You want your tears to stay there and you don't want them to wash away or evaporate away. That evaporation is part of the secret of what we think is going on with blinking. If you keep your eyes open for too long it hurts. Everyone has staring competitions but that hurts if you keep your eyes open for too long. So you need to blink before that happens. You don't want to spend your life blinking so there must be somewhere in the balance between the two. If you have evaporating coming off with the tears even with that oily layer there, that's producing temperature change and maybe a localised temperature change. The evaporation isn't always equal in the different places it's happening. We've got a thermal energy camera.

Meera - Actually yes. We've got one here with us. it's playing a recording of one of the visitors here today blinking. You can see what the temperatures are for all the various regions of the eye.

Paul - yes that's right. You can see the different temperatures of the eye. We've got hot and cold temperatures represented by different colours. The central part of the eye, the clear cornea that we're all seeing through has no blood vessels. That's important. It can't have blood vessels otherwise you couldn't see through them. That's a bit colder, it doesn't have the blood supply heating that. The tears are formed by the eyelids and also warmed by the body as they're being produced. Each time we blink those warm tears are spread back over the eye. When you keep the eye open you're evaporating the tears slowly. Once they're over that colder, central part you get the temperature change going on. These really sensitive corneal nerves can detect that temperature change. Out of that temperature change the cornea says, right, we've got a temperature change going on. That's sent to the blink centre of your brain and triggers another blink. The whole thing starts again.

Meera - Does the composition of our tear films vary greatly throughout the general public?

Entombment of Christ, 1672, in Saint-Martin Church in Arc-en-Barrois (Haute-Marne, France). Mary Magdalene.Paul - Yes. We're all different, aren't we? We're all the same but we're all different. From an overall point of view we're supposed to blink maybe ten or twelve times a minute. Some of us blink more. Some of us blink less. That's partly due to the tear film. Some of us have better quality tear film. Some of us poor quality tear film. Similarly, looking at that is actually just to take a little tear sample. Put it in a glass slide. Allow it to dry there for ten minutes. It forms this beautiful crystal pattern. If it's a cold winter's morning and you look at the crystals on the windscreen of the car or something like that. It's that kind of thing that you're looking at. If you've properly balanced tear film you've a beautifully, neatly-packed, tight, small furling pattern that looks really regular and neat and tidy. If you start to have an imbalance where there's not enough salts or there's too much or there's too many proteins or that mixture's wrong the pattern changes quite dramatically. It's a really easy way for us to start judging quality of tear film.

Meera - Having looked at this in a patient's eye would you then be able to tailor eye drops more specifically to what their eye needs?

Paul - Yes. That's what's starting to happen. The manufacturers of all these drugs and drops have begun to look at these issues. Now there are quite a number of drops out there. We're still only at the beginning of this, unfortunately. We're still trying to find the correct mixture. That's what we're hoping our work will do to try and answer some of those questions.

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