Posted by Irina
On March 8 2014

Introduction to the science of smell

The fundament of this platform is the science of smell. So what better subject for our first blog post than a science of smell 101? This exciting field covers a broad span of many different interests and subjects. From physiology to economics, from medical disabilities to quantum physics.

Let us take you on a journey up the nose through a short overview of some of the most influential scientific works. Within this blog we hope to expand on each one in due time. Our goal is to offer a comprehensible glossary of this breathtakingly fascinating sense.

Fun Facts

  • Smelling is the first sense we use the moment we're born
  • Our olfactory epithelium is pigmented just like the rest of our skin.
  • The intensity of the odors we smell depends on the temperature and humidity of the inhaled air.
  • Some odors can also cause tactile sensations like cold, warmth and sting. This is because the olfactory epithelium contains pain receptors (also called trigeminal nerve fibers).
  • Odor receptors are renewed each 4 to 8 weeks, something that is very rare among mature neurons.
  • Physiological studies concentrate on ascertaining: how do we smell?

    The highest awarded studies were those conducted by Linda B. Buck and Richard Axel. They won the Nobel Prize for Physiology or Medicine in 2004 for their scientific breakthroughs on how the sense of smell (or olfaction) works.

    In essence, when inhaled, smelly molecules reach a small part of the inside of the nose (the olfactory epithelium). The epithelium contains a large number of special neurons: odorant receptors cells. These cells detect smells with the help of odorant receptors. Then the neurons fire electrical impulses through the olfactory tract to the olfactory bulb in our brain.

    Both Buck and Axel made an exciting discovery: each olfactory receptor cell only carries one type of olfactory receptor. Also each receptor can only detect a specific number of odors. Like a key in a lock, a certain smell molecule fits by its specific shape to a certain receptor. In popular literature this study adds on 'the shape theory of smell’.

    The brain translates the different information from different receptors into a ‘pattern’. These patterns are further analyzed by several parts of the brain. Thus explaining why the nose is able to pick up a huge amount of different smells.

    Smell on the frontiers of neuro and behavioral science

    The olfactory bulb changes sensation in to perception. In general we do not deliberately focus on smell, it happens automatically during the inhalation process. With perception what we smell becomes integrated into how we experience our daily life, just like we see, hear, touch and taste.

    Olfactory Recepter Cells The olfactory bulb is part of the limbic system along other brain structures like the amygdala and the hippocampus. This system is also called 'the emotional brain' as its main function is processing emotion and memory.

    But there are more brain structures involved in the olfactory system. For example the thalamus and the piriform, orbitofrontal and motor cortex, they all bring something special to the table. Collectively they determine a couple of things: what exactly do we smell, how can we tell the difference between smells (is it lilacs or coffee?) and how and if we should act upon that information.

    In everyday life we walk about mostly unaware of what we smell. But we sniff out something new or out of place, in a matter of seconds. Our nose knows if our usual brand of detergent changed its fragrance. We toss out food that smells off right away.

    Upon sniffing the aroma of warm apple pie, we salivate. And like a smell-time-machine the memory of sitting at our grandmother's kitchen table rushes us back to our childhood.

    Genetic studies research how much of our ability to smell depends on our genes

    Gene Expression Patterns in Olfactory epithelium Buck and Axel’s research also made path for deeper inquires into the genetics of olfactory receptors. Our olfactory bulb comprises of around 400 different receptors. Which means that one out of 50 of our genes (around 3%) are olfactory receptor genes. Human and animal studies reveal a lot of individual variation (between 30 and 40%) on how active these receptors are. This depends not only on innate sensitivity, but also on how often we challenge our scentual sensibilities. The renewal of olfactory receptors results in the plasticity of our olfactory system.

    And that explains why some of us love Brussel sprouts and others hate them. But also why some people cannot smell certain odors like asparagus pee, parma violets candy or even garlic.

    In short: different people experience the world in different shades of smell.

    Genetic immunological studies on how our body odor depends on our genes and their social ramifications

    The olfactory genes are almost as variable as our immune system. The cool link between our immune system and smell is our body odor. Our body odor is as unique as a fingerprint. It is a smelly manifestation of our individual immune system gene cluster, also called MHC: major histocompatibility complex.

    The search for the scent of desire... Enter the (in)famous “T-shirt experiments”. Men had to wear and sleep in the same T-shirt for at least 48 hours, while not wearing any fragrance or using any fragranced products. Then women were to chose blindly between those T-shirts whose smells they found to be most appealing. Women rated the T-shirts that belonged to men who had immune systems that were different from their own, as the most sexy and pleasant.

    Thus the search for the scent of desire began...

    But wait! There is more...
    Let the following smelly podcasts and video’s entertain you, till the next blog reveal!

    Rose Eveleth : How do we Smell

    Julie Gould : Science of Smell
    » Twitter @JuliePCGould

    Naked Scientists
    » Twitter @NakedScientists

    Smell you later!

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