Dr Peter Brennan, Bristol University
Part of the show Science of Seduction, Pheromones and the Food of Love
Chris - So Peter, take us through it. What actually is a smell?
Peter - A smell is a collection of molecules. Something like coffee has many different molecules that make up the smell. What the brain has to do is identify that specific combination of molecules that contribute to coffee odour, and distinguish it from the complex combination of molecules that gives you something like strawberry odour, and other odours.
Chris - So these molecules are drifting around in the air and go up your nose. What happens then?
Peter - They go up the nose and dissolve in the mucus up your nose. They then bond to receptor proteins. These are molecules which have some sort of complementary site that the odour molecules fit into.
Chris - Like a docking station.
Peter - Yes, that's right. What happens then is that they activate that cell in the nose which then depolarises, or changes its electrical potential, and that signal is then sent down a long process of the cell to the brain. Here it triggers changes of other cells in the brain.
Kat - So do you have a different receptor up your nose for every different kind of smell molecule? And are some people missing some types of smells?
Peter - That's a really good question. There are lots of these different receptors. Humans have about three hundred and fifty different types of functional receptor, but that doesn't mean that humans can only smell 350 different types of molecule. These receptors will all bind to a range of different molecules and so what the brain has to do is to look across the whole population of these receptors and identify the pattern of cellular activity.
Chris - It's a bit like light. The screen in front of me is making a range of different colours, but all the colour son the screen are produced by just three colours: red, green and blue. However, I'm not restricted to seeing just red, green or blue. Depending on how much of each of those is present, I see a different colour.
Peter - That's right, yes. If the brain was to look at a signal from any one receptor type, it wouldn't know which of five or six molecules was out there. By looking across a whole population of these receptors, then it can pick up this pattern.
Kat - How do humans compare in the grand scheme of smellers in the world? We all hear about how dogs have an amazing ability to smell. Where are we on the scale?
Peter - Quite poor really. There are animals that don't have a sense of smell like whales, for instance. But things like dogs or mice have much greater abilities to both detect both really faint smells and to distinguish lots of fine differences in the sense of smell that we would just think smelt the same.
Kat - Can you get humans that are super-smellers? Do you get people who have very special smelling ability?
Peter - That's a good question. People who have lost the use of other senses, such as by going blind, can sometimes have very acute hearing or sense of smell. As far as people with the whole range of sensory abilities are concerned, then certainly people who have trained as perfumers or wine tasters or tea tasters, have much greater ability to certainly talk about smells and to put labels on them.
Chris - Is it because they've spent their life doing that and they've got better at doing it, or is it that they've gone into those jobs because they have such a superior sense of smell?
Peter - That's a good questions and I don't really know the answer. It could work either way. Certainly learning and memory are very important for actually distinguishing smells and the way the way the whole of the smell sense works.
Chris - An intriguing thing is that smell seems to be very powerfully linked to memory. So one whiff of one particular odour, such as coffee, sets off a whole train of thoughts. Why is that?
Peter - Smell is quite an ancient sense. It's been around for many millions of years. The big brains we've got at the moment, most of the cortical area developed from areas of more primitive brains devoted to the sense of smell. And so the sense of smell is quite primitive in brain terms and feeds into lots of the emotional areas much more directly than some of our other senses.
Chris - But it's also one of the most underrated of our senses. If you do a survey of people and ask them which of their senses they'd give up if they had to, an enormous number of people choose smell.
Peter - But a lot of people do lose their sense of smell for periods of time when they have a cold or an allergy. If the epithelium swells up and blocks air from flowing through it, you lose your sense of smell. What you will notice is that their sense of smell will also be diminished.
Kat - Is that true of smokers as well?
Peter - It's certainly diminished in smokers, but normally they wouldn't lose it altogether.
Kat - It just becomes numbed a bit.
Peter - Yes.
Chris - It's nearly Valentine's Day and we did say that we'd talk a bit about the science of seduction. So let's move on to another family of molecules: pheromones. These are of dubious significance in humans, aren't they?
Peter - There's more and more evidence accumulating at a low level that humans are producing chemicals which can have an effect on other humans, either in terms of their reproductive state or their menstrual cycle, or in terms of their behaviour or mood. It's quite difficult to actually quantify the effect because it's normally quite a subtle effect. Whether it plays that much of a role in today's society is very much open to question.
Chris - Do you know why it is that women living together end up with their menstrual cycles synchronising? Do you have nay feel for why that happens? Do you know what the molecule is that's driving that?
Peter - There are some molecules which Martha McClintock, who's a researcher in the States, has found in the underarm secretions of women which can have an effect on the menstrual cycle length of other women. If she takes these odours from under the arm just before ovulation, then they can shorten the length of the menstrual cycles of other women, and if these odours are taken from under the arm after ovulation, then they tend to lengthen the cycle. This seems to act to co-ordinate the cycles.
Chris - One quick question then. That's all very nice, but what's the point of it?
Peter - You can make up all sorts of stories but no-one really knows and I don't have any great idea as to why this might be important now.
Kat - If we discover a human pheromone, is it going to smell like sweaty armpits?
Peter - One of the main roles of the underarm secretions is probably pheromonal communication. We have lots of glands that produce particularly smelly compounds under our arms. All these hairs under our arms are acting as wicks, which draw out the secretions and release them into the air. So it might well smell sweaty and horrible, but some people might find it attractive. Maybe if they don't have a sense of smell they might!
Kat - What about the role of smell in mother and baby bonding? I remember reading a paper about mice in which a part of their brain involved with smell gets bigger when a female mouse gets pregnant. Is that the same case in humans?
Peter - I don't know about the brain getting bigger, but certainly smell does play a role. If you take newborn babies, then just the day after they're born you can present them on one side of their head with breast pads from their mother, and on the other side of their head breast pads from another mother in the same hospital. They turn their head towards the breast pad of their mother.
Chris - So they've almost certainly learnt to recognise their mother from a very early age?
Peter - That's right. And they may well be doing this either from compounds in the mother's milk or in the uterus as they've been developing.
Chris - How does the nose get used to smells, because that's a characteristic of the nervous system and the way it works, isn't it?
Peter - Yes, and it's particularly important for the sense of smell because there are molecules given off all around us by the carpets, furniture, the walls, other people, and yet we have to get used to them so that we can respond to new odours that come in from the cup of coffee or the strawberries. It's actually some connections between the nerve cells in our brain between the first and second stages of processing which actually get reduced in their efficiency, and they therefore don't transmit the signal as efficiently after they've been stimulated for a certain amount of time.