Odour perception: A review of an intricate signalling pathway

The perception of odours is the result of the complex processing of a signal, which initiates at peripheral receptors and ends in the brain. Along this pathway, olfactory signal processing proceeds through several steps; each step possesses its own complexity, and all steps are also intricately connected. This review aims to describe the main intricate steps of olfactory processing in mammals, some of which remain unclear, and the close associations and overlapping nature of these steps. The causes of both the complexity and the variability of olfactory signals are examined: the nature of olfactory receptors, involving the diversity of the genome; the spatial organization of the olfactory epithelium (OE) and the olfactory bulb (OB); connections in the OB and from the OB to the brain; integration and processing in the brain, which leads to the final perception of odours; and odour recognition and odour identification, which is associated with the difficulty to verbalize a reliable description of the perception in humans. Finally, the last part of this review encompasses recent progress made to decipher and understand olfactory coding and focuses on computational approaches