Our sense of smell is about 10,000 times more sensitive than taste. We often overlook the importance smell plays in alerting us to danger, with fire being one example and health-threatening substances like poisons another.
With 40 million smell receptors, humans have a poor sense of smell compared to some other animals. Rabbits have 100 million and dogs a staggering 1 billion smell receptors!
The human nose is structured to inhale, warm and filter air before it enters the lungs and to give us the sense of smell.
Air is sucked into your nose and over the turbinates – bony ridges increasing surface area. This is called ‘orthonasal olfaction’, differing from ‘retronasal olfaction’ where an odour is detected as it’s released from food in the mouth as you chew or swallow.
Once inside the nose, air travels over millions of olfactory receptors inside your nasal cavity, stimulating or inhibiting them.
Dangling from the olfactory receptors are tiny hairs or cilia attached to nerve fibres. The cilia are covered in mucus, which bonds with fat and water soluble odorants. There are thought to be about 400 different olfactory receptor types, and each odorant bonds with a unique set of olfactory receptors.
A series of biochemical reactions follows, resulting in a nerve impulse travelling along its axon (length of nerve fibre) to the olfactory bulb of the brain. This then relays odour information to many other brain regions.
Know your nose!
People have differing sensitivity to some odorants, with these genetic variations explained by the fact that the proteins expressed in the olfactory receptors will depend on a person’s genetic code.
We generally like or dislike smells/aromas, and rarely feel neutral towards them. Aroma preferences are learned through experiences – bad smells appear to be remembered more easily – and a lasting impression is imprinted upon the memory. The smell of the sea might conjure up dreams of a holiday, or the aroma of a fruit cake might recall your mother’s baking.
The particular smell of a man can give a woman clues to their genetic makeup and their ability to be a healthy mate. In choosing a mate, we appear to unconsciously look for someone with a different immune system to our own – leading to healthier offspring who are more disease resistant with improved chances of survival.
Inability to smell
‘Anosmia’ is an inability to smell (temporary or permanent). This disorder can impact upon an individual’s ability to enjoy food. It can be caused by something as simple as a cold or as serious as brain injury. People who can’t smell some compounds like urine or mint have ‘specific anosmia’, caused by differences in the genetic expression of some olfactory receptor proteins.
View this BBC animation showing how smell works.