1. Olfactory receptor neuron:
In our nasal cavity there are about 40 million olfactory neuron receptors waiting to catch different odours to send them on to the olfactory bulb. Individual ORNs have a cycle of about 40 days. So there is a constant turnover of new cells occurring in the nasal passage.
2. Olfactory bulb:
As the middleman between your nose and your brain it forever transfers smell information helping you distinguish key odours from other smells that occur at the same time. Therefore it's an indispensable part of our survival instinct as humans, helping us source edible food and allowing us to detect those phantom dog smells.
3. Nasal cavity:
A large part of it has absolutely nothing to do with smell. Its primary function is to purify the air we breathe before it reaches our lungs. Therefore it's the moisture in the nasal cavity that dissolves odour particles into liquid form so that the olfactory neurons can detect them.
4. The limbic system:
Long before we can speak we develop this set of structures as children. It is an emotional memory bank – storing all our happiness, sorrow, fear, pleasure and anger – assigning feelings to different stimuli. And since scent can tap straight into the limbic system, it can evoke a certain emotion from our past without our permission.