Hajuaistin

Hajuaistin, or sense of smell, is the ability to detect volatile chemical compounds called odorants. It is one of the traditional five senses and contributes to flavor perception, hazard detection, emotional memory, and social communication.

The detection occurs in the olfactory epithelium in the roof of the nasal cavity, containing olfactory receptor neurons, supporting cells, and basal cells. Odorant molecules bind to receptors on the cilia of these neurons, triggering a signal transduction cascade that converts chemical signals into electrical signals.

These signals are carried to the olfactory bulb, where olfactory receptor neurons converge on glomeruli. Output neurons (mitral and tufted cells) relay information along the olfactory tract to primary olfactory cortex areas in the piriform and entorhinal cortices, and to orbitofrontal cortex and limbic regions, linking smell to memory and emotion.

Humans possess a large gene family encoding olfactory receptors; only a subset is functional. Olfactory receptor neurons continually regenerate. The sense can decline with age or be affected by nasal disease, head trauma, or neurodegenerative conditions.

Clinically, impaired olfaction is called hyposmia or anosmia. Diagnostic tests include smell identification tests such as UPSIT and Sniffin’ Sticks to quantify odor detection and identification ability.

Understanding hajuaisti informs fields from neurology and psychology to fragrance science, and research investigates odor coding, neural plasticity, and flavor perception.