What do isochrone maps tell us

Nature Science Blog

The winter time with Christmas, tea and mandarins offers many typical scents. We smell “cinnamon” and think of Christmas with cookies, a Christmas tree and gifts. Smells seem to have a direct line to our memories.

For example, a cup of tea (of course with winter spices 😉) puts me in a good mood. I like the smell when I hold the cup in my hand, but of course also the taste when I drink the tea. As a result, I smell and taste the tea. If I look at the tea beforehand, it's almost like a tea wine tasting. 😉

Tea with spices, photo: R. Schügner

As we will see in a moment, smell and taste are closely related, since the tongue provides only part of the information for our taste.

On the tongue we can only distinguish five flavors, sweet, sour, salty, bitter and umami. Umami reacts to glutamic acid in food. Glutamic acid occurs naturally as an amino acid in protein foods such as meat and cheese. Artificial glutamate (the salt of glutamic acid) is used as a flavor enhancer.

For an apple or cinnamon taste, the information provided by the tongue is not enough. The aromas of the food ingested are transported "from behind" via the pharynx into the nasal cavity and to the odor receptors (binding point of the aroma / scent molecules) in the nose. Smell is perceived at the olfactory receptors in two different ways: “from the front” through the nose (orthonasal) and “from behind” through the mouth (retronasal). Chewing or saliva in the mouth can cause the smell or taste of the food to differ from the smell that is perceived through the nose.

Smell and taste on the way to the olfactory mucous membrane in the nose, image: Ricarda Schügner

The olfactory sensory cells with the olfactory receptors are embedded in the olfactory mucous membrane (Regio olfactoria) of the nose. From the sensory cells, hairs (cilia) protrude into the nasal mucous membrane. These “hairs” enlarge the surface of the cells. The receptors (docking points) for the odor molecules are located in the cilia.

The receptors are large proteins (protein molecules) that are firmly located in the cell membrane. Humans have more than 350 different types of olfactory receptors - and can perceive over 10,000 different smells, even in low concentrations. Researchers Linda Buck and Richard Axel investigated the exciting question of how so many smells can be recognized with “so few receptors”. They were awarded the Nobel Prize in Physiology or Medicine in 2004 for their research on this topic.

How does a "detect smell" work?

The odor molecules are inhaled and reach the receptors. There a molecule binds to the appropriate receptor. Docking triggers a chain reaction. The receptor molecule activates an enzyme that produces a messenger substance that migrates to the cell membrane and opens cell channels there. Positively charged sodium ions flow through the channels into the cell. The cell is "electrically charged". 😉 Due to the difference in charged particles, more precisely, an action potential has developed. The - now electrical and clearly amplified - information is passed on via the olfactory threads in the olfactory bulb (yes, that really means that) (olfactory bulb). There, information from receptors for the same molecules converges in glomeruli (nerve balls). From there it goes on to different brain areas, such as the amygdala (limbic system), the hippocampus and the hypothalamus.

Smell information in different areas of the brain

The amygdala (almond kernel) belongs to the limbic system, which is also responsible for processing emotions. This gives the smell a fairly quick access to our emotions.

Via the hippocampus, new information - including about smells - is transferred to short and long-term memory. This is the access of smells to our memories.

The olfactory information is also sent to the hypothalamus, the main control system of the autonomic nervous system. The autonomic nervous system is responsible for body functions that run automatically. Examples of this are the regulation of body temperature, blood pressure, food and water consumption, sleep and the control of reproductive behavior.

As can be seen from the three selected examples, smells actually have direct access to emotions, memories and it is no wonder that we get hungry when we smell delicious food. 😉

Nobel Prize 2004: How few receptors recognize many different smells

The fact that a type of receptor passes on the information for a certain odor does not work, or at least not so easily. On the one hand, this is due to the fact that an odor is usually a mixture of different odor molecules in different concentrations. This can best be illustrated with perfumes made from a mixture of different substances. If a perfume is known, the smell / fragrance is also recognized.

The clue for recognizing the variety of different smells is the combination. It is a code that allows us to recognize over 10,000 smells despite a relatively small number of receptors.

Here I have summarized the information about the olfactory code:

  • A smell (an olfactory substance) is usually a mixture of several odor molecules that can bind to different types of receptors.
  • A sensory cell develops only one type of receptor type, but it has many individual receptor proteins of the same type distributed in the cilia.
  • One type of receptor binds “a chemical structure” and not just a specific molecule. This means that similar molecules are bound. However, they trigger signals of different strengths.
  • The signals from the receptor cells of the same type converge in the glomeruli.
  • The different types of receptors form a signal pattern that conveys the unique smell impression. With the diverse combinations of more than 350 types of receptors, thousands of smells can be identified in this way. Similar to how the overall picture of a puzzle becomes visible by putting the pieces together, the smell is only perceived when the individual pieces of information are put together.


The scent molecules bind to the receptor cells, image: Ricarda Schügner

Smell and memories

As shown above, the smells have a “direct line” to our emotions and memory. That's why the smells of cinnamon, cloves, oranges / tangerines and fir trees can take us back to childhood at Christmas. Or they transport us to many other places: the smell of freshly baked bread, freshly brewed coffee - maybe it reminds you of a nice Sunday breakfast on vacation.

Unfortunately, the memory of smells also works “the other way around”. Smells are also able to remind us of unpleasant or bad situations. A simple example is food that is not good for you when you eat it. A smell of this food can certainly cause nausea.

I selected the YouTube video “Biosensor Nose” from MaxPlanckSociety for you as a summary on the subject of “smelling”.

I like it:

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