Olfaction

Olfaction, or the sense of smell, is the detection and interpretation of volatile chemical compounds. In humans and many vertebrates, odorants are detected by olfactory receptor neurons in the olfactory epithelium, a specialized tissue in the roof of the nasal cavity. Each olfactory receptor neuron expresses only one type of odorant receptor, and humans have hundreds of receptor gene families. Odorants bind to receptors based on chemical structure; because the same odorant can activate multiple receptor types and each receptor can respond to several odorants, odors are encoded by patterns of receptor activation—a combinatorial code that allows a relatively small set of receptors to detect a vast number of smells.

When an odorant binds, the olfactory receptor activates a G protein (Golf), stimulating adenylyl cyclase III and raising cyclic AMP. This opens cAMP-gated ion channels, depolarizing the neuron and triggering action potentials. The axons of olfactory receptor neurons project directly to the olfactory bulb, where they converge on glomeruli. Each glomerulus receives input from neurons expressing the same receptor type, creating a spatial map that is transformed by mitral and tufted cells to higher brain areas.

Olfactory information reaches the piriform cortex (primary olfactory cortex), orbitofrontal cortex, amygdala, and hippocampus, linking smell to perception, memory, and emotion. Perception is influenced by learning, context, and expectation; retronasal olfaction contributes to flavor during eating. Common clinical topics include anosmia (loss of smell) and hyposmia. Causes include viral infections (including COVID-19), nasal obstruction, head trauma, and neurodegenerative diseases. Testing often uses threshold, discrimination, and identification tasks, and treatments may include addressing the cause and olfactory training. Olfaction is ancient and widespread among animals, with evolutionary pressure from nutrition, danger detection, and social signaling.