Ageing and Chemical Senses

Olfaction

Olfactory receptors are found on primary olfactory receptor neurons (ORNs) located in a relatively small patch of tissue high in the nasal cavity. These neurons extend cilia along the epithelial surface and, thus, are uniquely exposed to the external environment and subject to a constant barrage of potentially toxic chemicals and particulates, as well as being susceptible to direct injury from microbes. Although ORNs are also highly unusual in that there is ongoing replacement of these neurons throughout life, this is a complex process requiring reinnervation of the olfactory bulb (the first brain relay in the olfactory pathway), and it is often imperfect. Degeneration of the olfactory neural epithelium, and patchy replacement by respiratory epithelium, is seen even in young adults and becomes more pronounced with aging.

Olfactory function is most often assessed via tests of threshold sensitivity (the lowest concentration of an odorant that can be detected) or of the ability to identify suprathreshold concentrations of odors (e.g., is this orange, licorice, grass, or banana?); less frequently, ratings of the perceived intensity of suprathreshold odors are obtained. Almost uniformly, studies using all of these measures have shown a significant decline with age, typically beginning in the seventh or eighth decade of life. Age-related olfactory loss appears to develop gradually and is rarely complete, except in extreme old age. Nonetheless, it is often of sufficient magnitude to render older people vulnerable to chemical hazards such as gas leaks and to greatly diminish olfactory food flavor perception, reducing food enjoyment.

Despite extensive documentation of age-related decline in average olfactory sensitivity, debate continues regarding the uniformity of that decline, both across different odors and across individuals. Although there is little variation in the degree of loss reported for different odors, most studies have simply contrasted the performance of an elderly group with that of a group of young adults. As a result, possible differences in the onset or rate of decline in sensitivity to specific odors have not been examined. In addition, at the individual level, extreme differences among elderly subjects in olfactory abilities have frequently been noted, with some older individuals performing as well as the average young person. However, specific genetic, medical, and environmental factors that underlie this variation have not been identified, and some have questioned the reliability of apparent overlaps between the young and elderly in measures of olfactory ability.

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