|MadSci Network: Neuroscience|
Dear Gary, This is a biochemistry question which is not yet fully answered. Your question is at the cutting edge of research! Both smell and taste rely on protein receptors (in the nose) recognising molecules. The lining at the roof of the nasal cavity has a small area of mucous membrane where specialised nerve cell endings are positioned. The cells of these receptor nerves have several hair-like extensions that dangle down to the surface of the mucous membrane where they are exposed to molecules in the inhaled air. The sensitivity and the selectivity of smell are astonishing. Humans can tell the difference between several thousand smells. For some smells just four or five molecules of the material are enough to be noticed. Although scientists have yet to work out the details, it is likely that the membranes of receptor nerve cells carry proteins with receptor sites able to recognise specific smelly molecules. Binding of the molecule to the receptor site results in a message being transmitted to the brain. Esters are organic compounds arising from a combination of a carboxylic acid and an alcohol, giving the R-CO-O-R linkage. Low-molecular weight esters give fruits their characteristic odours and flavours. It is interesting that almost all the low-weight esters smell pleasant, while their acid counterparts are acrid and repellent. This is simply an artifact of our evolution; not surprisingly, a selective advantage was gained by organisms which turned up their noses at rotting foods but ate fresh ones. The coevolution of flowering plants (angiosperms) and humans is an interesting story in its own right. The relation of the smell brain (rhinencephalon) to intelligence is an old story. It is argued that one reason for mammals developing their reasoning powers was that our verminous ancestors, small rats rummaging around for dinosaur eggs, were primarily nocturnal. Along with the development of sensitive olfactory organs - tiny molecular analysers - they had to interpret the smells. This is a lot harder on a neurological level than the task the brains of the dinosaurs were charged with, which amounted to "chase and eat." The development of the cerebrum directly parallels that of the smell brain. To this day, smells remain extremely strong triggers of emotions, more so than sights or sounds. Best wishes, Sean Thanks to: Dr T.V Padma, Unilever and Pharmcentral.
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