How important is the sense of smell?

Just think about the power of scents that evoke ‘the vast structure of recollection’. Think about salmon that find its way back to the spawning grounds through its sense of smell. Or imagine clouds of volatile compounds created by plants that signal opportunity to insects.

Different scents are detected by the specialized receptor cells in the olfactory epithelium. Each cell expresses only one olfactory receptor and the estimated number of odorants is much larger than the number of odorant receptors in any species (about 10,000 different scents vs about 350 human olfactory receptors). The questions are: How do we recognize this vast array of molecular structures with a limited repertoire of odorant receptors? How do we discriminate between them? Co-activation of few receptors, in defined combinations, gives rise to a large olfactory repertoire. These combinations contribute to a range of olfactory percepts, together with coordinated activation of the associated group of neurons in the central nervous system. This is known as a combinatorial coding strategy.

Although humans have a significantly diminished olfactory receptor repertoire relative to other mammals, the oft-ignored importance of the sense of smell is undeniable. There is growing evidence for olfactory impairments in the normal ageing process, Parkinson’s, Alzheimer’s and Korsakoff’s disease. On the other hand, there is evidence for an increase in olfactory neurons during pregnancy.

Olfactory receptors constitute the molecular basis for the sense of smell. They are the interface between external and internal environment. Energy released by the binding of the odorant to the receptor translates into electrical activity in the brain, thus representing the first step in perception of scent.

I am interested in the molecular architecture of these receptors and how they bind odorant molecules. Is there a common mechanism of activation of these receptors? How can there be a common mechanism of activation given the huge genetic diversity of olfactory receptors?

Now that there is a complete olfactory genome available, it seems that receptor-driven perspective is sine qua non for the future in olfactory research.