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Review Article
Introduction
'Providence made us noses to smell with', Socrates declared circa 400 BCE,1thus showing a basic comprehension of olfaction - one of the oldest senses, phylogenetically speaking.2Olfaction affects all aspects of life that can be thought of as the 'seven stages of man', from infant to old age or second childhood 'sans everything', as described in Shakespeare's As You Like It.3Olfaction also plays a key role in an individual's decline in health either from ageing or a specific disease process affecting the olfactory cleft or the projection pathways of the brain. In addition to the chronological influence of olfaction, life is a rich tapestry with multiple interwoven experiences, decorated by odours that evoke memories and emotions, thereby influencing the overall quality of life.
The sense of olfaction is ubiquitous in the animal kingdom; it arises from the nasal cavity in vertebrates and the sensory cells of the antennae in invertebrates. Humans are capable of detecting more than 10 000 different odours and distinguishing between 50004odours at low concentrations - an acuity that exceeds manufactured analytical instruments.5Volatile odorants dissolve on the mucous layer that overlies receptors on the epithelial surface. Odours then bind to receptor cells, initiating G protein-coupled transduction by opening nucleotide-gated ion channels. Signal transduction occurs by altering the resting potential of olfactory cells and transmitting signals via action potential propagation to the olfactory bulb and onwards to the olfactory cortex. Neural projections then reach the other areas of the brain including the limbic system, where signals are integrated and interpreted, influencing behaviour and emotions.6
Most vertebrates, including humans, detect odours as well as pheromones. The latter is a term that was introduced by Peter Karlson and Martin Lüscher in 1959 to describe a volatile substance that is secreted and produces a specific behavioural response in another animal.7The term 'pheromone' is derived from the Greek pherein ('to convey') and hormÅn ('to impel, set in motion'). Pheromone detection in animals is mediated by the vomeronasal organ (or Jacobson's organ), which was identified in snakes by Frederik Ruysch prior to 1732 and then as a pit on the inferior nasal septum in humans...