Assessment of direct knowledge of the human olfactory system

Abstract

The olfactory system is highly conserved across mammalian species (Ache and Young, 2005; Gelperin, 1999; Hildebrand and Shepherd, 1997; Laurent, 2002). In particular, the human and rodent olfactory systems share numerous common traits, including the basic organization of the olfactory central nervous system, aspects of odor guided behaviors, the nature of odorant receptor proteins, and processing at the molecular and synaptic levels (Fig. 1). These shared characteristics make rodents an excellent model for use in understanding the human olfactory system and has led to widespread use of rodents in olfactory studies. As a result, knowledge of the rodent olfactory system is expansive, and has been the primary guide to human olfactory research for decades. Though the olfactory systems of the two species are highly similar (Ache and Young, 2005), some differences are apparent (Mainland et al., 2014; Maresh et al., 2008; McGann, 2017; Trimmer et al., 2019), and direct data from humans is lacking. Our current understanding of the human olfactory system relies substantially on inferences from direct knowledge obtained in rodents. Inferring from rodents is well justified, and has moved the field of human olfaction forward. However, within the growing human olfaction literature, it is often unclear which statements are inferred from rodent work, and which are directly from human data. In a field dominated by rodent studies, and in order to avoid confusion and compounded misstatements, there arises a periodic need to assess the state of direct knowledge of the human olfactory system. Our goal here is to provide a thorough review of olfactory literature in order to assess and clarify our current direct knowledge of the human olfactory system, as compared to the rodent olfactory system. Open in a separate window Figure 1: Overview of the human and rodent olfactory systems. The major targets of olfactory bulb efferents are illustrated for humans (red) and rodents (blue and purple). MOB, main olfactory bulb; AOB, accessory olfactory bulb; AON, anterior olfactory nucleus; vTT, ventral taenia tecta; dTT, dorsal taenia tecta; IG, indusium griseum; OT, olfactory tubercle; APC, anterior piriform cortex; PPC, posterior piriform cortex; pirF, frontal piriform cortex; pirT, temporal piriform cortex; BNST, bed nucleus of stria terminalis; MeA, medial amygdala; PAC, periamygdaloid cortex; ACo, anterior cortical nucleus of the amygdala; PlCo, posterolateral cortical amygdala; PmCo, posteromedial cortical amygdala; NLOT, nucleus of the lateral olfactory tract; lENT, lateral entorhinal cortex; mENT, medial entorhinal cortex; SO, supraoptic nucleus. Dashed lines indicate that these projections have not been definitively shown in humans but are likely based on strong indirect evidence. *, it is not known whether this area receives direct projections from the main olfactory bulb in humans.