The olfactory system adapts quickly to a sensory stimulus, and for sensation to be sustained, there must be repeated stimulation. The olfactory sense differs from other senses in yet another way. It is common experience that an aroma can restore long-forgotten memories of complex experiences. That olfactory and emotional stimuli are strongly linked is not surprising in view of their common roots in the limbic system. Yet, paradoxically, the ability to recall an odor is negligible in comparison with the ability to recall sounds and sights. As Vladimir Nabokov has remarked, “Memory can restore to life everything except smells.” It is also interesting that dreams do not embody olfactory experiences.
The remarkable evolutionary role of olfactory receptors can be appreciated by the fact that about 2 percent of the human genome exists to express unique odorant receptors (over 400 distinct functional genes). The wide diversity of these transmembrane proteins permits subtle differentiation of thousands of different odorant molecules, as delineated by Young and Trask and the genetic basis for which Buck and Axel were awarded a Nobel Prize.
This specificity for molecules is encoded neuroanatomically. Different odorant molecules activate specific olfactory receptors. Each olfactory neuron expresses only one allele of one receptor gene. Moreover, each olfactory glomerulus receives inputs from neurons expressing only one type of odorant receptor. In this way, each of the glomeruli is attuned to a distinct type of odorant stimulus. Presumably, this encoding is preserved in the olfactory cortex.
Something is to be learned from a second, distinct olfactory system in many animals (the vomeronasal olfactory system or organ of Jacobson), in which the repertoire of olfactory receptors is much more limited than in their main olfactory system. This functionally and anatomically distinct olfactory tissue is attuned to pheromones and thereby importantly influences menstrual, reproductive, ingestive, and defensive behavior (see review of Wysocki and Meredith). The vomeronasal receptors employ different signaling mechanisms than other olfactory receptors and project to the hypothalamus and amygdala via a distinct accessory olfactory bulb.
Clinical Manifestations of Olfactory Lesions
Disturbances of olfaction may be subdivided into four groups, as follows: 1. Quantitative abnormalities: loss or reduction of the sense of smell (anosmia, hyposmia) or, rarely, increased olfactory acuity (hyperosmia) 2. Qualitative abnormalities: distortions or illusions of smell (dysosmia or parosmia) 3. Olfactory hallucinations and delusions caused by temporal lobe disorders or psychiatric disease 4. Higher-order loss of olfactory discrimination (olfactory agnosia)
Anosmia, or Loss of the Sense of Smell (Table 11-1)
This is the most frequent clinical abnormality of olfaction and, if unilateral, usually is not recognized by the patient. Unilateral anosmia can sometimes be demonstrated in the hysterical patient on the side of anesthesia, blindness, or deafness. Bilateral anosmia, on the other hand, is a common complaint, and the patient is usually convinced that the sense of taste has been lost as well (ageusia).