Smell

Smell is the sense by which volatile compounds are detected and identified. In humans, odor detection begins when odorants enter the nasal cavity and dissolve in the mucus, stimulating chemosensory neurons in the olfactory epithelium.

The olfactory epithelium contains olfactory receptor neurons, each expressing a type of G-protein-coupled receptor. Humans have about 400 functional odorant receptor genes, enabling combinatorial coding where a single odorant activates multiple receptors and a receptor can respond to multiple odorants. Signals from receptor neurons converge to olfactory bulb glomeruli, producing spatial patterns that the brain interprets as odor identity.

The olfactory pathway projects to primary olfactory cortex areas, including the piriform cortex, and to limbic regions such as the amygdala and orbitofrontal cortex, supporting odor perception, memory, and emotion. Unlike other senses, olfactory signals reach higher centers without a thalamic relay, enabling rapid associations with memory and affect. Odor perception also depends on concentration, receptor expression, and adaptation to persistent odors.

In daily life, smell contributes to flavor perception, safety (smelling smoke or gas), and social communication, though evidence for human pheromones is limited. Smell dysfunction is common and ranges from anosmia (loss) to hyposmia (reduced sensitivity); causes include infections, head injury, and neurodegenerative disease. Testing tools include standardized smell tests and odor identification tasks.

Mixtures of odorants can interact in complex ways, sometimes producing novel perceived qualities. Overall, smell is a richly interconnected sense that enables detection and interpretation of volatile cues critical to eating, environment, and memory.