How Does the Sense of Smell Work in Humans

The sense of smell is one of the most powerful senses we have, as it enables us to distinguish between several hundred odors. This sense of smell is referred to as olfaction. Olfaction is often linked to memory.

Your brain can associate between an object and its odor. That's how you 'remember' smells and can also distinguish them from one another. For instance, when you smell a cologne that your partner uses, don't you start thinking about him?

There are thousands of different smells in the world. There is a mechanism in the human body by which we are able to distinguish between these smells.

Before I start with the mechanism, I would be telling you a few things about the internal structure of the human nose, which will help you understand the mechanism behind the sense of smell.

The innermost part of the nose contains special neurons. They are called olfactory receptor neurons. They can come in direct contact with the air. These neurons have cilia, which increases their surface area. Cilia are projections similar to hair.

The cilia are surrounded by a layer of mucus. These olfactory receptors are connected to the olfactory bulb, which is connected to the orbitofrontal cortex.

Now let's take an example to understand how the human sense of smell works. Consider you have a banana on your table. The volatile molecules from the surface of the banana get diffused in the air. These molecules are also called odorant molecules. This air reaches your nose when you inhale.

The odorant molecules in the air mix with the mucus and come in contact with the cilia. This triggers the olfactory receptor neurons, which in turn send a signal to the olfactory bulb. The olfactory bulb transmits this impulse to the orbitofrontal cortex (OFC).

The OFC analyzes the pattern of the impulse and identifies the substance whose molecule has struck the cilia. This information is then transmitted to the brain. This is how you identify a substance by smelling it.

There are some smells that you like while there are others that disgust you. Your perfume smells good while carcass of an animal smells awful. We often classify smells as good or bad, but scientifically speaking, there is no distinction between smells in terms of good and bad.

The olfactory receptors and the OFC work in the same way regardless of the substance being smelled.

If the working of the receptors and the OFC does not vary for each substance, then how are we able to distinguish between the smells of different substances? The answer is simple. It is believed that there are a large number of olfactory receptor neurons, each of them encoded with a gene.

Each gene corresponds to a particular odor. And that's how you can distinguish between several odors. So when you can smell something, it's because you have the gene for it.

You might have come across people who cannot smell certain things. This is because, the gene which corresponds to the specific smell is either missing or damaged in these individuals. The smelling sense also depends on the olfactory nerves which carry out the transmission of signals.

The ability to smell can also be hampered if there is damage to these nerves. Damage occurs in case of accidents or respiratory infections. Loss of the sense of smell is called anosmia. The good news is that anosmia can be cured.

Recovering from anosmia is possible because the sense of smell is regenerative, which means that the olfactory nerves can grow back again into the corresponding receptors. Scientists believe that the receptor cells have a lifespan of 3 weeks. Once these cells die, new receptor cells replace them.

The rate of regeneration of the receptor cells may slow down after the age of 45, thus hampering one's smelling sense.

• The receptors tend to reduce the intensity of the stimuli. This is useful when we experience foul smells which lead to our repugnance. Scientists have discovered that calcium cations are responsible for this lowering of the intensity of stimuli.

• Researchers also believe that people have a dominant nostril, either left or right, similar to the dominance of one hand over the other. Yes, smells are better sensed through one's dominant nostril.

• Few researchers also correlate the number of olfactory receptors to the size of the olfactory bulb. They say, the larger the size, the better is your smelling sense.

• Any substance with no volatile molecules cannot be smelt. That is why you cannot smell gold or steel.
• You can distinguish between several hundred odors. Such is the capacity of the olfactory system of human beings.

• Sometimes you come across odors which only you can smell. Such odors, which only you can sense and are mostly unpleasant, are called phantom odors. Phantosmia is a condition wherein an individual senses a certain smell even when there is not stimulus generating it. This condition is caused by damage to one's temporal lobe.

• Though human beings can differentiate between several hundreds of smells, their smelling sense is not as acute as that in other animals.

Olfaction is one of the most amazing senses in human beings. Interestingly, it is related to the sense of taste and closely associated with memory. You sense smells, you remember them, and can differentiate between hundreds of them.

But you know something? You cannot smell everything around you. Yes, you might be able to identify a hundred different scents, but there are a thousand others which your olfactory system cannot sense.