Smell physiology and perception

Odors are important for several biological purposes, which includes detecting prey and predators, selecting potential mates, and judging whether food is safe to eat. The olfactory receptor neurons lie in the roof of the nasal cavity, covering an area of 2 to 4 cm$^2$. There are around $6$ million receptors, which are believed to span $500$ to $1000$ different types depending on their responsiveness to specific chemical compositions [206]. Airborne molecules dissolve into the olfactory mucus, which triggers detection by cilia (small hairs) that are part of the receptor. The olfactory receptors are constantly regenerating, with an average lifespan of about $60$ days. In addition to receptors, some free nerve endings lie in the olfactory mucus as well. The sensory pathways are unusual in that they do not connect through the thalamus before reaching their highest-level destination, which for smell is the primary olfactory cortex. There is also a direct route from the receptors to the amygdala, which is associated with emotional response. This may help explain the close connection between smell and emotional reactions.

In terms of perception, humans can recognize thousands of different smells [291], and women generally perform better than men [36]. The discrimination ability depends on the concentration of the smell (in terms of molecules per cubic area). If the concentration is weaker, then discrimination ability decreases. Furthermore, what is considered to be a high concentration for one odor may be barely detectable for another. Consequently, the detection thresholds vary by a factor of a thousand or more, depending on the substance. Adaptation is also important for smell. People are continuously adapting to surrounding smells, especially those of their own body or home, so that they become unnoticeable. Smokers also adapt so that they do not perceive the polluted air in the way that non-smokers can.

It seems that humans can recognize many more smells than the number of olfactory receptors. This is possible because of combinatorial encoding. Any single odor (or chemical compound) may trigger multiple kinds of receptors. Likewise, each receptor may be triggered by multiple odors. Thus, a many-to-many mapping exists between odors and receptors. This enables far more odors to be distinguished based on the distinct subsets of receptor types that become activated.