Sniffing around for clues on olfaction

A rose by any other name would smell as sweet,wrote Shakespeare in a romantic flourish,not suspecting that generations of scientists would come to ponder this statement. How does olfaction,the sense of smell,work? Living and non-living things release certain chemicals that upon entering the nose dissolve in the mucus inside. Beneath the mucus is a membrane called the olfactory epithelium,home to olfactory receptor neurons that can detect thousands of odours. These receptors transmit information through the olfactory nerve to the olfactory bulb,which in humans is located in a rather inaccessible region at the back of the nose. The bulb in turn communicates signals to the olfactory cortex,hippocampus,amygdala,and hypothalamus,among other regions of the brain. Thanks to this shortcut to the cortex,the sense of smell travels to the brain very fast compared to other senses. Some of these areas are implicated in memory and emotion,thereby aiding discrimination of odours based on past experience.

This deep connection with the limbic system is also the reason that smells from the past often bring back evocative memories. For Upinder Bhalla,at the National Centre for Biological Sciences,Bangalore,Its the smell of the first rain of the monsoon in Delhi,where he grew up,thats most evocative. I never forgot that smell,in all my years abroad, he says. Through experiments on rats and by computer modelling,Bhallas lab tries to understand how the brain encodes smell. In many of these behavioural studies,rats are challenged to recognise certain odours and respond to themsomething they seem to do very well and without much delay. You have to see a rat sniff a trail of chocolate in the dark to believe it. The human nose has about 350 types of receptors,each of which recognises a set of odours. Rats and dogs have almost four times as many receptors,many of them lost to humans in the course of evolution, says Bhalla,adding,To a dog,the smells of a room may be just as rich and layered as the sights of the room would be to us.

In a paper published in the journal Science in 2006,Bhalla and his colleagues showed that rats smell in stereo. To explain,a rats nostrils,though only about 3 mm apart,sample slightly different spaces and thus are able to effectively localise smells. Training six rats to follow odour cues,the researchers found that performance drops significantly below criterion when either of the nostrils is stitched shut and recovers immediately after removal of the stitches. Is this true of humans? Despite having a relatively weak sense of smell,we do a reasonably good job of recognising and remembering odours. In fact,our sense of taste and our enjoyment of food are largely hinged on olfaction. Recent studies,however,have yielded mixed results. One such study has proposed that humans can smell in stereo only when the odour involved is one that stimulates the trigeminal nerve,a cranial nerve responsible for sensation in the face that is irritated by unpleasant smells. Others have asserted that humans recognise some odours better than dogs.

Recent studies at Bhallas lab have yielded other interesting results,even if in rats. All real-world smells are a combination of compoundsthe scent of a rose,for instance is made up of over 300 different compounds,some of which may smell unpleasant taken alone. Bhalla has found that olfactory neurons understand this. They dont attempt to categorise odours,instead smoothly shifting gears from one intermediate odour to the next,and responding accordingly. The next time you wrinkle your nose at the garbage truck,here is something to think about.