Olfactory Function
The perception of a certain flavor results from the interplay between three sensory systems. (1) The olfactory nerve mediates olfactory sensations, e.g., the pleasant odor of vanillin or the rotten smell of hydrogen sulfide, (2) the gustatory system mediates basic tastes, e.g., sweet, salty, sour, and bitter 13, and (3) the trigeminal nerve mediates, for example, the burning and stinging of chilli peppers, or the cooling of menthol 14. In addition, both the vagal and glossopharyngeal nerve (innervating areas in the back of the throat) may also contribute to these sensations 15. The physiological significance of still other systems like the vomeronasal organ is currently under investigation 16. Owing to this diversity, disturbances of chemosensory perception may have a large variety of causes. By far the most frequent causes, however, are disturbances related to the olfactory system 17.
The olfactory neuroepithelium is found in the roof of the nasal cavity 18. Central processing of olfactory dysfunction involves structures like the amygdala and the hippocampus 19-22, both of which are involved in the processing of emotions and memories. In fact, within the olfactory pathway only two synapses are needed to mediate olfactory information to these "limbic structures" (for a discussion of this term see 23). The intimate anatomical relation between these brain structures and the olfactory system explains - at least partly - the notion that odors so easily evoke memories and emotions 24.
Major determinants of olfactory sensitivity are gender and age. It is well known that women outperform men in almost all aspects of olfactory sensitivity. Although the exact reason for this difference remains unclear, social, hormonal, or genetic factors are thought to be involved 25. Using different measures, studies have shown a decrease of olfactory sensitivity past the age of 60 26-30. In fact, in terms of odor identification more than 50% of 80-year old people exhibit olfactory disturbances 8, 31. Other determinants of olfactory sensitivity include smoking which leads to a mild but significant decrease of olfactory function. This decrease has been shown to be related to the number of cigarettes smoked, but can be reversed after cigarette smoking has terminated 32, 33.
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