Olfaction

Abstract Olfaction, the ability to recognise and discriminate myriad airborne molecules with great accuracy and sensitivity, is one of the most remarkable, but least understood senses. It permits continuous monitoring of the surroundings for small volatile molecules, including chemical signals that identify territories, food, predators and mates; thus, olfaction plays a key role in survival and adaptation in the animal world. Recent work suggests that in mammals, odour perception begins through volatile molecules binding to a subset of several hundred G‐protein‐coupled receptors on the olfactory sensory neurons in the nose. The identity of the odorant is encoded by a spatiotemporal pattern of activity within the first central relay, the olfactory bulb. The olfactory cortex then functions as a combinatorial array that allows recognition of those odorant‐specific patterns and forms synthetic odour percepts. This pattern recognition process is experience‐dependent, and thus odour discrimination and perception can be affected by a variety of memory and cognitive disorders. Key Concepts Odorous stimuli are transduced by olfactory sensory neurons through a large family of G‐protein‐coupled receptors, with individual sensory neurons expressing a single type of receptor. Olfactory sensory neurons project axons directly into the mammalian forebrain targeting the olfactory bulb and are continually replaced throughout life. Odorant identity is initially encoded by odour‐specific spatiotemporal patterns of activity in the olfactory bulb which emerges due to highly specific sensory neuron axonal projections. In contrast to other sensory systems, the direct projection to the primary olfactory cortex does not pass through the thalamus, although there is an olfactory thalamocortical pathway. Encoding of odour quality in the piriform cortex is performed by populations of highly distributed neurons, with no known odour‐specific spatial topography. Information about internal state, emotion, expectation and past experience can influence odour processing as early as the first central synapse, and odour learning can influence processing through the entire olfactory pathway. Given that most natural odours are mixtures of many volatile components, odour perception is based on an experience‐dependent combinatorial process resulting in the perception of synthetic odour objects. Odour habituation is a central phenomenon involving changes in the strength of connections between specific neurons within the olfactory bulb and cortex. Through extensive interaction with other sensory systems, olfaction plays a critical role in the perception of flavour and food perception. Impairment in olfactory perception is a prevalent problem which is associated with a wide variety of neurological disorders.