Olfactory

Olfactory, or the sense of smell, is the detection and perception of volatile chemical compounds called odorants. Odorants interact with receptors in the nasal cavity, triggering neural signals that are interpreted by the brain. The term derives from Latin olfactus, meaning “smelling.”

Anatomy and receptors are central to olfaction. The olfactory epithelium lines the upper part of the nasal cavity and contains olfactory receptor neurons, supporting cells, and basal cells. Each receptor neuron expresses a single type of olfactory receptor protein, a G protein–coupled receptor. Neuron axons bundle together to form the olfactory nerve (cranial nerve I) and pass through the cribriform plate to the olfactory bulb, where they synapse in structures called glomeruli. The olfactory system also involves the olfactory bulb’s local circuits and higher-order processing centers.

Transduction and signaling begin when odorants bind to receptors, activating a G protein and increasing intracellular cAMP. This opens ion channels, generating receptor potentials and action potentials that travel to the olfactory bulb. Mitral and tufted cells relay information from the bulb to higher brain regions, preserving a chemotopic organization that reflects odorant properties.

Central processing involves primary olfactory cortex areas such as the piriform and parts of the entorhinal cortex. From there, signals reach the amygdala and orbitofrontal cortex, among others. Olfaction often engages limbic structures, contributing to emotional responses and memory. Some pathways also route through the thalamus before reaching frontal regions, supporting conscious perception and discrimination.

Variation and clinical relevance include substantial individual differences in receptor gene repertoires, influencing sensitivity and odor perception. Olfactory dysfunction (anosmia or hyposmia) can result from nasal obstruction, injury, infections, or neurodegenerative diseases and may serve as an early clinical indicator.