The Sense of Smell

Although our sense of smell is quite poor when compared to many animals, it is still highly sophisticated and humans can identify up to 10,000 different odour molecules, sometimes at a concentration of only one part to several billions parts of air.

The sense of smell is often described as the sense tied most closely to memory, with the ability to profoundly influence people’s ability to vividly recall past events and experiences from as long ago as our childhood.

The knowledge that smells can have a powerful effect on the mind has been known since ancient times. Precisely how this comes about however, is still not fully understood despite a vast amount of modern research. Have you ever wondered how the nose can distinguish between the pleasant aroma of food cooking and the danger warning of something burning?

Smell is the most basic and most primitive of all of our senses and has always been of great importance. To pre-historic man, the sense of smell was just as important as vision in the battle for survival. Modern man no longer depends upon odours as a means of survival and we can now see the sense of smell actually being repressed.

Just think about the way we try to mask any smells in daily life that we feel are unpleasant and unsociable. Smell is still very important to us in the modern world except now we seem to have become tolerant to only pleasing odours.

How does the sense of smell work?

The process of smelling is called olfaction and is incredibly complicated, taking place in several areas of the brain including the limbic system which itself has approximately 34 structures and 53 pathways. The limbic system is linked to the perception of odour, sensations of pleasure and pain, emotions like rage, fear, sadness and sexual feelings.

When we sniff to distinguish a particular odour, molecules emitting from the substance travel through the air and enter the nose. There are 50 million smell receptors located at the top of the nasal cavity occupying an area the size of a small postage stamp.

These slender nerve receptors are called ‘cilia’ and are connected to an organ called the olfactory bulb (bulbus olfactorius), which in turn is connected directly to the brain.

The science bit

On the surface of each cilia lies a 1,000 different types of sensors known as odorant receptors. Each of these receptors can recognise multiple odorants, and a single odorant can be recognized by multiple receptors. Odorants are simply the molecules that we and animals perceive as odours, and different odours are identified by different combinations of receptors. This is how only 1,000 types of sensors are able to identify up to 10,000 different odours.

Each receptor is used over and over to define different odours, just like letters are used over and over again to spell different words. This ingenious system greatly reduces the number of sensors (letters) needed to code for smells (words). The way that different and overlapping combinations of letters can spell ‘red,’ ‘read,’ or ‘reed,’ similar combinations of sensors can identify for example, neroli, jasmine, or rose.

When an odour molecule has penetrated into the receptor there is an effect on the nerve cell belonging to this cilium. This nerve now sends a signal to the olfactory bulb which is a major structure in the limbic system and is where the olfactory impressions are first processed. From there, nerve cell responses are sent in the olfactory stalk to the limbic system. These receptor cells are in effect, an extension of the brain into the outside world!

Despite all the continual research it is still not known exactly how the brain converts these odour molecules into the smell that we perceive. However, scientists have determined that the human brain can easily identify subtle differences between thousands of odours which proves that the sense of smell is one of the most remarkable assets of the human body.

Sixth sense

Researchers at Harvard University believe that the remnant of an organ located in the human nose was once responsible for detecting chemical signals given off by other humans. However, this sixth sense used by our distant ancestors is no longer present in modern humans since it was lost due to the effects of genetic mutation.

Located just behind our nostrils are two tiny pits known as the Vomeronasal organ (VNO), but this organ no longer appears to function in humans. For a period of time during the development of an embryo the VNO exists with pits and tubes together with nerve endings which connect to the brain. However, several weeks later the organ ceases developing and begins to deteriorate.

Fake desire

In animals, the organ contains nerves that respond to chemicals called pheromones that are secreted by many other animals. In many creatures, pheromones trigger a variety of instinctive behaviours such as aggression and mating. However, whether or not humans have the ability to detect pheromones is still hotly debated.

Professor Catherine Dulac at Harvard University’s Department of Molecular and Cellular Biology believes that humans no longer have this ability, ‘It’s lost to evolution, – even apes and gorillas, our evolutionary first cousins, no longer rely on it to find mates.’, she claims. So much for those expensive perfumes claiming to contain human pheromones that will attract the perfect mate!

Spend, spend, spend

Research by scientists and universities is being conducted into understanding the way that fragrances influence our emotions. Sadly, very little is ever heard of the results of such research, and this is most likely because the knowledge will be used in some revolutionary new application or product. Certainly, some of the knowledge gained from this type of research has begun to quietly filter through and is being used in our daily lives, even though we may not notice it.

For example, special fragrances are already used in some stores, factories and office environments to encourage us all to spend more money or work more efficiently and productively. Goodness knows what some boffin will think up next! But whatever it may be, it is most unlikely we will be told in advance of it being put on the market!

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