A Comparison of the Moth and the Human Olfactory system
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- Institutt for psykologi 
The aim of this thesis was to review the olfactory system of moths and humans in a comparative perspective. The basic motivation for performing such a work was to increase the understanding of the human olfactory system. A comparison is assumed to contribute in this regard because both differences and similarities across species speak about how the evolutionary challenge of detecting airborne chemicals has been solved. In addition, this kind of comparative approach contributes to improved awareness of the distinct roles olfaction play in different species. As mentioned previously, taking into consideration the different roles of olfaction for moths and humans, one might expect that their chemosensory systems should be quite different. While moths depend heavily on olfaction, humans relay – at least apparently – substantially less on this sense. Moths, which are often nocturnal, use olfaction to navigate the world, for example. Humans, on the other hand, use vision. However, the role of olfaction in humans is probably underestimated. This might be because it is not as connected to conscious perceptions as other senses (Distel et al., 1999) and because of a general lack of knowledge about this sense. When looking at the greater picture, olfaction in humans and moths is involved in a lot of the same evolutionary important functions, but in somewhat different ways. When comparing the systems, it is useful to think about how these similarities have emerged. One way this can happen is through homology, where the similarities are due to a shared inheritance. Another way is convergence, which means that there are similarities, but that they arose due to constraints or adaptations (also referred to as analogy). Knowing which one is involved in olfaction can give insight into the phylogeny of species and how the environment or developmental and genetic processes constraint the evolution of the olfactory system. The main similarities pointed out in this thesis include the OSNs, the organization of glomeruli in the AL and OB, the different neuron types found in the primary olfactory center, the role of OBPs and the link between higher olfactory centers and memory formation. These similarities indicate that there are optimal solutions to the challenge of detecting and processing odor input. Thus, the same kind of arrangements have evolved independently of each other to meet the common problem of detecting odor stimuli. OBPs have not been found in aquatic animals, which may support the notion that they are a solution for detecting odorants in air (Eisthen, 2002). Higher up in the olfactory pathway both insects and humans possess brain areas involved in learning and memory. One main difference in the olfactory system of insects and humans/mammals the ORs. Thus, the ORs in humans and moths are not homologous (Wistrand et al., 2006). This might be due to differences in the demands on humans’ and moths’ olfactory systems. For moths it is essential to be able to respond to olfactory stimuli quickly. This to navigate after odors during flight. By using a ligand-gated ionotropic channel this is made possible. The ionotropic part takes care of the rapid transduction, while the ligand makes prolonged odor signaling possible as well (Wicher et al., 2008). For humans there is no corresponding demand on speed and as such, it is sufficient with a slower system including a classical metabotropic channel. In addition to the ORs a prominent difference between the two systems are the presence of multiglomerular PNs and three main olfactory tracts in the moth olfactory system. For the field of psychology, improved exploration of the olfactory system is important to better understand how it affects cognitive and affective functions. As olfactory function is found to be correlated to several psychological and neurological disorders such knowledge might contribute to improvements in treatment and diagnosis. Knowledge about the close connection between olfaction and memory might also be utilized for performances in cognitive tasks, if better understood. There are several difficulties in studying olfaction in humans. This is due to ethical restrictions, as well as the problem of accessing some anatomical structures because of their placement. The moth is useful as a model animal as it has a simpler brain and identified stimuli associated with well-defined behaviors. Research on model animals can also lead to new methods which can be used in the field. Better methods might make it possible to do more studies on humans, but for now model animals play an invaluable role in understanding the olfactory system better. Studies on the moth and the human olfactory system have so far been very fruitful. Probably, these two research approaches will continue to be mutually beneficial in the future. The comparison gives an insight into the somewhat remarkable similarities that exist in the two species’ olfactory systems. This despite the differences in how and when the olfactory system is used. The differences found can be explained as solutions to different demands, but sometimes it is not obvious whether this is the case, or if it is simply different ways of solving similar challenges.