Multiple Olfactory Subsystems Convey Various Sensory Signals

To detect a myriad of chemical cues signaling potential food, mates, and danger, most species (from worms, insects to mammals) develop sophisticated chemosensory systems. In mammals, the olfactory, gustatory, and trigeminal systems, which are primarily responsible for smell, taste, and somatosensation, respectively, are all involved in chemical senses. The nose, a seemingly unitary organ, consists of multiple olfactory apparatuses, among which the main olfactory epithelium (MOE) and the vomeronasal organ (VNO) have been extensively studied. Both systems comprise several subtypes of sensory cells with specialized morphological, molecular, and/or functional features. The MOE contains ciliated olfactory sensory neurons (OSNs) and microvillar cells. Most ciliated OSNs express G-protein-coupled odorant receptors (GPCRs) (Chapter 7) and employ the canonical cyclic adenosine monophosphate (cAMP) cascade to transform chemical energy into electrical signals (Chapter 8). Some of the ciliated OSNs express distinct chemoreceptors or noncanonical signal transduction machineries and project to specific regions in the olfactory bulb (OB) (see below). Likewise, the VNO contains at least two subsystems (the apical and basal compartments), which express two classes of vomeronasal receptors (V1Rs and V2Rs, respectively) and project to different portions of the accessory olfactory bulb (AOB) (Chapter 6). Additionally, some species (e.g., rodents) develop two spatially segregated clusters of sensory cells in the nasal cavity, forming the septal organ (SO) of Masera and the Grueneberg ganglion (GG) (Figure 9.1A). These chemosensory subsystems detect distinct but overlapping olfactory cues and some neurons may convey other sensory modalities transmitted by mechanical and thermal stimuli. This chapter covers several subsystems within the MOE as well as the SO and the GG. The key features of each subsystem will be discussed, including chemoreceptors, signal transduction cascades, central projections, and functional roles.