Smells

Smell, or olfaction, is the sense by which volatile chemical compounds are detected by receptors in the nasal cavity. Odorants are typically small, volatile molecules that interact with olfactory receptor proteins on the cilia of sensory neurons in the olfactory epithelium. Each receptor can bind multiple odorants, and each odorant can bind multiple receptors, creating combinatorial patterns that encode odor identity.

Signals are transmitted to the olfactory bulb and then to higher brain regions such as the piriform cortex, amygdala, and orbitofrontal cortex. Unlike most senses, olfactory signals reach the cortex directly with only indirect thalamic involvement, though thalamic relays and integration occur downstream.

Perception includes quality, intensity, and hedonic value. Odors are described with terms like floral, fruity, or smoky. Perception is highly subjective and shaped by experience, context, culture, and even individual differences in receptor expression and neural processing.

Measurement and health aspects: detection thresholds vary widely among odorants; olfactometry and other methods estimate sensitivity to specific compounds. Environmental odor assessments use descriptors and, in some cases, odor units to gauge perceived intensity. Anosmia (loss of smell) and hyposmia (reduced smell) can result from injury, infection, chronic conditions, or aging and can affect flavor perception, safety, and quality of life.

Chemistry and ecology: Odorants occupy diverse chemical spaces, including many aromatic and aliphatic compounds. Most smells arise from complex mixtures, and perception reflects neural processing of activation patterns across receptor types. The trigeminal nerve can convey chemesthetic sensations such as irritation or cooling that accompany some odors.