ABSTRACT
The olfactory system detects airborne chemicals as odorants. While visual and auditory sensory stimuli are continuous physical quantity, odorants detected by the olfactory system are extremely diverse and discontinuous in nature. The olfactory system utilizes a large repertoire of odorant receptors (ORs) to detect and discriminate a diverse kind of odorants in the environments. After the discovery of ORs in 1991, there has been progress in understanding of the olfactory system. ORs are found in most of vertebrate species and chordates, while there are considerable species-specific variations. In addition, specific additional receptors are known to exist for olfactory subsystems in some species. In mice, there are ∼1,000 types of functional OR genes in the genome, and each olfactory sensory neuron (OSN) expresses just one type of OR out of ∼1,000. OSNs expressing a given type of OR converge their axons to a specific set of glomeruli in the olfactory bulb. The glomerular map formed in the olfactory bulb is the basis for odor discrimination. In glomeruli, sensory inputs are relayed to second-order neurons, mitral and tufted (M/T) cells, which project axons to the olfactory cortex. The olfactory system mediates not only stereotyped innate behaviors, such as attraction and aversion, but also more flexible learned behaviors. For this purpose, projections from the olfactory bulb to the olfactory cortex are stereotyped in some, but divergent in other areas. For example, neurons in the piriform cortex receive divergent inputs from multiple glomeruli and mediate learned odor recognition, whereas neurons in the cortical amygdala receive inputs from specific glomeruli and mediate innate odor responses. Olfactory information is further conveyed to the orbitofrontal cortex, insula, and hippocampus, where it interacts with non-olfactory information.
