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We often do not realise how much we use our senses in day-to-day life until one of them stops working. (Image: FAU/David Harfiel)

In the stone age, human beings were equipped with the sensory organs that they need for survival. Today, these fantastic organs are still relevant for us in navigating the challenges of urban environments.

By Roland Knauer

Many of us still believe that other species outperform human senses. Impressive figures seem to confirm this widespread assumption. The human nose has around twelve million sensory cells, while a German shepherd’s has around a billion.

These kinds of figures alone tell us nothing

‘These kinds of figures alone tell us nothing,’ says Prof. Dr. Hermann Handwerker from FAU’s Institute of Physiology and Pathophysiology, shaking his head. While our sense of smell may be weak in comparison with a dog’s, we can still distinguish one trillion different smells. If our noses are able to tell the difference between one thousand billion scents, from enchanting fragrances to horrible stenches, they can’t be too bad.

Different, but not worse

The human nose is no worse than a dog’s – at least not in the areas in which it is normally used. However, these vary at different stages in life. ‘The nose is much more important for a baby than for an adult, which is why a baby’s sense of smell is much more well developed,’ explains Hermann Handwerker. Later in life, other sensory organs become more important. Sight in particular requires a large proportion of the brain’s resources.

The nose is much more important for babies

Despite this, the nose remains important. ‘The fact that fragranced care products bring in 25 billion dollars a year in the USA alone is proof of this,’ says Hermann Handwerker, referring to an economic aspect that highlights the importance of the sense of smell for us human beings.

Scent molecules float through the air

As with so many functions of the human body that seem simple at first glance, some rather complicated processes are behind the performance of the sensory organs. With the sense of smell, this chain reaction starts with tiny scent molecules that float through the air. When we breathe in, some of them are directed to the olfactory mucosa in the nose which, with an area of ten square centimetres, is around three times the size of a thumb nail.

In humans, around twelve million olfactory cells are located here. The olfactory mucosa of a dog, by contrast, is five times larger and contains one billion of these cells. These cells have small areas which scent molecules with specific chemical properties can bind to. There are around 400 different types of these olfactory receptors in the olfactory mucosa of humans, compared with around one thousand – more than twice as many – different types in a dog’s nose.

A chain of biochemical reactions

When these bonds are created, a chain of biochemical reactions is started, finally leading to an electrical impulse that is triggered in a neuron and sends information to the olfactory centre in the brain telling it that a specific scent has just been smelt. As only a small amount of the air that we breathe flows past the olfactory mucosa, the brain usually only receives a kind of background information passed on from rather weak stimuli.

If the brain registers smells that could be interesting, we start sniffing – just like many other animals do. This directs much more air past the olfactory mucosa, meaning that the cells there register many more scent molecules which can then be analysed more closely.

It all starts with a stimulus

The other sensory organs in humans and animals function in a similar way. A chain of biochemical reactions transforms a stimulus that reaches the sensory organ into an electrical nerve impulse which is then analysed in specialised areas of the brain. The differences are found only at the start of these reactions, where the receptors in different sensory organs are responsible for different stimuli. In the same way as the cells in the nose, a large number of cells on the tongue react to certain molecules which we register as ‘taste’.

In the second part of ‘Ready to receive on multiple channels’, you can find out which information is provided by our sense of taste and why we are able to see in 3D.

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