hajuaisti

Hajuaisti, in Finnish, refers to the sense of smell, or olfaction. It is the ability to detect and identify volatile chemical compounds in the air, enabling the perception of odors. Olfaction plays a key role in flavor perception, hazard detection, social signaling, and memory.

Anatomy and physiology: The olfactory system begins in the nasal cavity with the olfactory epithelium, where specialized olfactory receptor neurons reside on the mucous membrane. Odorant molecules bind to receptors on the cilia of these neurons, triggering a signal that is transmitted via their axons to the olfactory bulb. Within the bulb, signals project to mitral and tufted cells and then to higher brain regions, including the piriform cortex (primary olfactory cortex), the entorhinal cortex, and the amygdala. Unlike other senses, olfactory pathways often reach limbic areas directly, contributing to the emotional and memory associations of odors.

Function, development and aging: Olfaction relies on a large family of odor receptors, with humans possessing hundreds of different receptor genes. Olfactory receptor neurons undergo lifelong turnover, regenerating approximately every month. Sensitivity to smells can decline with age and may be affected by health conditions, certain medications, viral infections, and environmental factors. Olfaction interacts closely with taste and can be altered by recent exposure, adaptation, or nasal obstruction.

Disorders and testing: Conditions affecting hajuaisti include anosmia (loss of smell), hyposmia (reduced smell), dysosmia (distorted smell), and phantosmia (phantom smells). Causes range from nasal disease and trauma to neurodegenerative disorders. Clinical assessment uses standardized tests such as smell identification tests and threshold/discrimination tests, for example UPSIT or Sniffin’ Sticks, to evaluate olfactory function.

See also: olfactory system, sensory perception.