ORNs

Olfactory receptor neurons (ORNs) are the primary sensory neurons of the olfactory epithelium, responsible for detecting volatile odorants and initiating the sense of smell. They are bipolar neurons with dendritic processes that extend to the mucosal surface, bearing cilia or microvilli where odorant receptors reside, and an unmyelinated axon that projects through the cribriform plate to the olfactory bulb.

Each ORN expresses a single odorant receptor gene from a large receptor gene family, a member of the G protein-coupled receptor (GPCR) superfamily. When an odorant binds its receptor, a G protein activates adenylyl cyclase III, increasing cyclic AMP, which opens cyclic nucleotide-gated channels. The ensuing cation influx depolarizes the neuron and generates action potentials. This signaling supports combinatorial coding, allowing discrimination among many odorants.

In the olfactory bulb, ORN axons expressing the same receptor converge onto specific glomeruli, creating a receptor-type–specific map. Mitral and tufted cells relay this information to higher brain regions, including the piriform cortex, amygdala, and other areas involved in odor perception and associated memories and emotions.

ORNs continually regenerate in adult mammals from basal stem cells within the olfactory epithelium. This turnover enables recovery after injury and contributes to plasticity in odor perception. Disruption of ORN function or density can lead to smell disorders, highlighting their importance in sensory biology.

Research on ORNs informs understanding of olfactory coding, neural regeneration, and potential applications in biosensing and olfactory prosthetics.